Invasive Species Compendium

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Datasheet

Cytisus scoparius
(Scotch broom)

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Datasheet

Cytisus scoparius (Scotch broom)

Summary

  • Last modified
  • 27 September 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Host Plant
  • Preferred Scientific Name
  • Cytisus scoparius
  • Preferred Common Name
  • Scotch broom
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • C. scoparius is a perennial shrub that has been widely commercialized as an ornamental in temperate and subtropical regions of the world. It is a prolific seeder that escaped from cultivation and has become an...

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Identity

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Preferred Scientific Name

  • Cytisus scoparius (L.) Link

Preferred Common Name

  • Scotch broom

Other Scientific Names

  • Genista scoparia Monnet de la Marck
  • Sarothamnus bourgaei Boissier
  • Sarothamnus oxyphyllus Boissier
  • Sarothamnus scoparius (L.) W.D.J. Koch
  • Sarothamnus vulgaris Wimmer
  • Spartium scoparium L.

International Common Names

  • English: broom; common broom; English broom; European broom; Irish broom; Scots broom; Scottish broom
  • Spanish: escoba negra; retama; retama de escobas; retama negra
  • French: genêt à balai
  • Chinese: jin que hua
  • Portuguese: giesta; giesteira das vassouras

Local Common Names

  • Australia: English broom; Scottish broom
  • China: jin que er shu
  • Denmark: gyvel
  • Dominican Republic: citiso; gandulillo
  • Finland: Jänönvihma
  • Germany: Besenginster
  • Italy: amaracciole; emero scornabecco; ginestra dei carbonai
  • Netherlands: bezemstruik; brem
  • New Zealand: wild broom
  • Norway: gyvel
  • Russian Federation: zharkovetz metelchatyi
  • South Africa: Skotse brem
  • Sweden: gyvel; har-ris

EPPO code

  • SAOSC (Cytisus scoparius)

Summary of Invasiveness

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C. scoparius is a perennial shrub that has been widely commercialized as an ornamental in temperate and subtropical regions of the world. It is a prolific seeder that escaped from cultivation and has become an invasive species and a serious weed in temperate areas of the United States, Canada, Hawaii, Chile and Argentina, the eastern halves of both islands of New Zealand, Australia (including Tasmania), India, Iran, Japan and South Africa (Holm et al., 1979; Parsons and Cuthbertson, 1992; Hosking et al., 1998; Peterson and Prasad, 1998; Isaacson, 2000). C. scoparius is an aggressive fast-growing invader with the capability to grow forming dense impenetrable monospecific stands that degrade native grasslands, forests, rangelands, and agricultural lands; prevent the regeneration of natural forests and prairies; and create fire hazards (Syrett et al., 1999; USDA-NRCS, 2016). Because of its association with nitrogen fixing bacteria, it is very competitive in areas with poor soils and can alter the nutrient cycling of invaded areas (Peterson and Prasad, 1998).

C. scoparius is also very common and widespread within its native range and reaches densities where it is considered a weed (Maury, 1963; Engel, 1964). Consequently, in many European countries (within its native range) it has been included in national lists of invasive species (DAISIE, 2016). However, treating this species as invasive in areas within its native distribution range is controversial as it has been present in the European flora for centuries and in many countries where it is now listed as invasive it was previously listed as native (Rosenmeier et al., 2013; DAISIE, 2016). 

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Fabales
  •                         Family: Fabaceae
  •                             Subfamily: Faboideae
  •                                 Genus: Cytisus
  •                                     Species: Cytisus scoparius

Notes on Taxonomy and Nomenclature

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Fabaceae is one of the largest families of flowering plants including about 745 genera and 19,500 species of herbs, lianas, shrubs, and trees growing in a great variety of climates and habitats (Stevens, 2012). The genus Cytisus (subfamily Faboideae) comprises about 60 species distributed from northern Africa (Morocco) to Europe, western Russia, the Black Sea and Turkey. The highest species diversity is observed around the Mediterranean Sea (Cristofolini and Troìa, 2006). 

Various synonyms for C. scoparius are used in older literature, mainly Sarothamnus scoparius and Spartium scoparium (Peterson and Prasad, 1998). There are three recognized subspecies associated with C. scoparius and the subspecies “C. scoparius scoparius” is considered to be the taxon that has been widely introduced throughout the world (Bolos and Vigo, 1984; Hosking et al., 1998). However, many new infestations come from local garden escapes of horticultural varieties and so may exhibit quite significant associated variation in morphology and tolerances (Smith, 2000). This weed is the commonest of the exotic brooms worldwide (Syrett et al., 1999) but identification is complicated as there are at least another 15 species, from six genera, and several hybrids variably described under 'brooms', which are exotic throughout the world. Invasive C. scoparius frequently hybridizes with native genotypes, introducing more vigorous growth and invasive traits (Nielsen et al., 2016). 

 

Description

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C. scoparius is an unarmed leguminous shrub, having several erect or ascendant stems which can later collapse to become prostrate where crushed by snow (Hosking et al.,1998). Plants grow to 4 m high, and often form dense thickets in cooler areas. Branches are green, five-angled and mostly glabrous. Leaves are usually three-foliate, petiolate to subsessile, but one-foliate and sessile on young growth. Leaflets are narrow-elliptic to obovate, 5-20 mm long and 1.5-8 mm wide, with scattered hairs on the upper surface and numerous short hairs on the lower surface. Flowers are pedicellate, solitary or in pairs, and borne in the axils on 1-year-old stems. The calyx is glabrous, ca 6 mm long, two-lipped, upper lip with two teeth, lower lip with three teeth, all teeth usually much shorter than the lips. The corolla is golden yellow, 15-25 mm long. Fully developed pods are 2.5-7 cm long and 8-13 mm wide, oblong, dehiscent, strongly compressed, with brown or white hairs on the margin, otherwise glabrous, initially green then black at maturity. Plants are deciduous in winter in colder areas and in summer in areas with summer drought. The plant is most easily distinguished from other closely related species by its five-sided green stems, its yellow, pea-like flowers, and pea-like pods mainly 2.5-7 cm long with hairy margins (see Pictures). In the field, broom plants are conspicuous because of their dark green colour (compared to e.g. the grey-green colour of the more robust C. striatus) and especially their abundant large flowers at the peak of flowering.

Plant Type

Top of page Broadleaved
Perennial
Seed propagated
Shrub
Woody

Distribution

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C. scoparius is native to Europe, from Ireland to west-central Ukraine and from southern Spain to southern Sweden (Heywood and Ball, 1968; USDA-ARS, 2016).  It has been widely introduced as an ornamental plant and now it can be found naturalized in temperate areas of the eastern and western USA and Canada, Hawaii, Chile and Argentina, the eastern halves of both islands of New Zealand, south-eastern Australia including Tasmania, India, Iran, Japan and the western cape of South Africa (Holm et al., 1979; Parsons and Cuthbertson, 1992; Hosking et al., 1998; Peterson and Prasad, 1998; Isaacson, 2000). This includes, therefore, all habitable continents and several temperate island systems. Its distribution is less well documented in Eastern Europe and is considered doubtful in Turkey (Greuter et al., 1984).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasivePlantedReferenceNotes

Asia

ChinaPresentIntroduced Planted ILDIS, 2003
IndiaPresentIntroduced Invasive Planted ILDIS, 2003
-Arunachal PradeshPresentIntroduced Invasive Chandra, 2012
-AssamPresentIntroduced Invasive Chandra, 2012
-Himachal PradeshPresentIntroduced Invasive Chandra, 2012
-Jammu and KashmirPresentIntroduced Invasive Chandra, 2012
-ManipurPresentIntroduced Invasive Chandra, 2012
-MeghalayaPresentIntroduced Invasive Chandra, 2012
-MizoramPresentIntroduced Invasive Chandra, 2012
-NagalandPresentIntroduced Invasive Chandra, 2012
-SikkimPresentIntroduced Invasive Chandra, 2012
-TripuraPresentIntroduced Invasive Chandra, 2012
-Uttar PradeshPresentIntroduced Invasive Chandra, 2012
-UttarakhandPresentIntroduced Invasive Chandra, 2012
-West BengalPresentIntroduced Invasive Chandra, 2012
IranWidespreadIntroducedPeterson & Prasad, 1998; Holm et al., 1979
JapanWidespreadIntroduced1670 Planted Hotta et al., 1989

Africa

South AfricaWidespreadIntroduced Invasive Planted Invasive Species South Africa, 2016
Spain
-Canary IslandsRestricted distributionIntroduced Invasive Planted Heywood and Ball, 1968; USDA-ARS, 2016

North America

CanadaPresentPresent based on regional distribution.
-British ColumbiaRestricted distributionIntroduced1850 Invasive Planted Peterson & Prasad, 1998
-Nova ScotiaRestricted distributionIntroduced Invasive Planted Peterson & Prasad, 1998
-Prince Edward IslandPresentIntroduced Invasive Planted Peterson & Prasad, 1998
USAPresentPresent based on regional distribution.
-AlabamaPresentIntroduced Invasive USDA-NRCS, 2016
-AlaskaPresentIntroduced Invasive Planted Luken & Thieret, 1996
-CaliforniaWidespreadIntroduced1902 Invasive Planted Luken & Thieret, 1996
-ConnecticutPresentIntroduced Invasive Planted Luken & Thieret, 1996
-DelawarePresentIntroduced Invasive Planted Luken & Thieret, 1996
-GeorgiaPresentIntroduced Invasive Planted Luken & Thieret, 1996
-HawaiiWidespreadIntroduced Invasive Luken & Thieret, 1996; Holm et al., 1979
-IdahoPresentIntroduced Invasive USDA-NRCS, 2016
-KentuckyPresentIntroduced Invasive USDA-NRCS, 2016
-MainePresentIntroduced Invasive Planted Luken & Thieret, 1996
-MarylandPresentIntroduced Invasive Planted Luken & Thieret, 1996
-MassachusettsPresentIntroduced Invasive Planted Peterson & Prasad, 1998
-MontanaPresentIntroduced Invasive Planted Luken & Thieret, 1996
-New HampshirePresentIntroduced Invasive USDA-NRCS, 2016
-New JerseyPresentIntroduced Invasive Planted Luken & Thieret, 1996
-New YorkPresentIntroduced Invasive Planted Luken & Thieret, 1996
-North CarolinaPresentIntroduced Invasive Planted Luken & Thieret, 1996
-OhioPresentIntroduced Invasive Planted Luken & Thieret, 1996
-OregonWidespreadIntroduced1880 Invasive Luken & Thieret, 1996; Isaacson, 2000
-PennsylvaniaPresentIntroduced Invasive Planted Luken & Thieret, 1996
-South CarolinaPresentIntroduced Invasive Planted Luken & Thieret, 1996
-TennesseePresentIntroduced Invasive Planted Luken & Thieret, 1996
-UtahPresentIntroduced Invasive Planted Luken & Thieret, 1996
-VirginiaPresentIntroduced Invasive Planted Luken & Thieret, 1996
-WashingtonWidespreadIntroduced Invasive Planted Luken & Thieret, 1996
-West VirginiaPresentIntroduced Invasive Planted Luken & Thieret, 1996

Central America and Caribbean

Dominican RepublicPresentIntroduced Invasive Mir, 2012

South America

ArgentinaPresentIntroduced Invasive Burkart, 1952; IABIN, 2015Cultivated and naturalized
BoliviaPresentIntroducedJørgensen et al., 2015
BrazilPresentIntroduced Invasive Cordero et al., 2016Campos Sulinos grasslands, in the Campos de Cima da Serra, and in the Serra do Sudeste
ChileWidespreadIntroduced Invasive Planted Holm et al., 1979; Belov, 2013

Europe

AlbaniaPresentNative Not invasive Natural Heywood and Ball, 1968
AndorraPresentNative Not invasive Natural Heywood and Ball, 1968
AustriaWidespreadNative Natural Royal Botanic Garden Edinburgh, 2003
BelarusPresentNative Not invasive Natural Heywood and Ball, 1968
BelgiumWidespreadNative Natural Heywood and Ball, 1968; DAISIE, 2016
Bosnia-HercegovinaWidespreadNative Not invasive Natural Heywood and Ball, 1968
BulgariaPresentNative Not invasive Natural Heywood and Ball, 1968
CroatiaWidespreadNative Not invasive Natural Heywood and Ball, 1968
Czech RepublicWidespreadNative Not invasive Natural Heywood and Ball, 1968
Czechoslovakia (former)WidespreadNative Not invasive Natural Heywood and Ball, 1968
DenmarkWidespreadNative Not invasive Natural Heywood and Ball, 1968; Rosenmeier et al., 2013
EstoniaPresentNative Not invasive Natural Heywood and Ball, 1968; USDA-ARS, 2016
FranceWidespreadNative Natural Heywood and Ball, 1968; DAISIE, 2016
-CorsicaRestricted distributionNative Not invasive Natural Heywood and Ball, 1968
GermanyWidespreadNative Natural Heywood and Ball, 1968; DAISIE, 2016
GreecePresentNative Not invasive Natural Greuter et al., 1984
HungaryWidespreadNative Not invasive Natural Heywood and Ball, 1968
IrelandWidespreadNative Not invasive Natural Heywood and Ball, 1968
ItalyRestricted distributionNative Not invasive Natural Heywood and Ball, 1968
LatviaPresentNative Not invasive Natural Heywood and Ball, 1968
LiechtensteinPresentNative Not invasive Natural Heywood and Ball, 1968
LithuaniaPresent Natural
LuxembourgPresentNative Not invasive Natural Heywood and Ball, 1968
MacedoniaPresentNative Not invasive Natural Heywood and Ball, 1968
MoldovaPresent Natural
NetherlandsWidespreadNative Not invasive Natural Heywood and Ball, 1968
NorwayPresentNative Not invasive Natural Heywood and Ball, 1968
PolandRestricted distributionNative Not invasive Natural Heywood and Ball, 1968
PortugalRestricted distributionNative Not invasive Natural Heywood and Ball, 1968
-AzoresPresentIntroduced Planted Heywood and Ball, 1968
-MadeiraPresentIntroduced Planted Heywood and Ball, 1968; DAISIE, 2016
RomaniaRestricted distributionIntroduced1865 Invasive Heywood and Ball, 1968; Marusca et al., 1999
Russian FederationPresentPresent based on regional distribution.
-Central RussiaPresentNative Not invasive Natural Heywood and Ball, 1968
SerbiaPresentNative Not invasive Natural Heywood and Ball, 1968
SlovakiaPresentNative Not invasive Natural Heywood and Ball, 1968
SloveniaPresentNative Not invasive Natural Heywood and Ball, 1968
SpainRestricted distributionNative Natural Heywood and Ball, 1968
SwedenWidespreadNative Not invasive Natural Heywood and Ball, 1968
SwitzerlandWidespreadNative Not invasive Natural Heywood and Ball, 1968
UKWidespreadNative Natural Heywood and Ball, 1968
-Channel IslandsPresentNative Not invasive Natural Heywood and Ball, 1968
UkrainePresent Natural
Yugoslavia (former)PresentNative Not invasive Natural Heywood and Ball, 1968

Oceania

AustraliaPresentPresent based on regional distribution.
-New South WalesPresentIntroduced Invasive Weeds of Australia, 2016
-South AustraliaPresentIntroduced Invasive Weeds of Australia, 2016
-TasmaniaPresentIntroduced Invasive Weeds of Australia, 2016
-VictoriaPresentIntroduced Invasive Weeds of Australia, 2016
-Western AustraliaPresentIntroduced Invasive Weeds of Australia, 2016
New ZealandWidespreadIntroduced1872 Invasive Webb et al., 1988; Fowler and Syrett, 2000

History of Introduction and Spread

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Introductions of C. scoparius are associated with its horticultural value in eastern and western North America, Australia, New Zealand, South Africa, India and Japan (Hosking et al., 1998; Peterson and Prasad, 1998). It has been in Japan since 1670, where it was introduced for horticultural reasons and subsequently deliberately spread to improve soil fertility (Hotta et al., 1989; Nemoto et al., 1993). C. scoparius arrived in the USA in the early 1800s (Isaacson, 2000). In Australia it apparently arrived soon after 1800, when the Governor requested broom seeds that were to be grown and used as a substitute for hops (Waterhouse, 1988). Reasons for the initial importation of broom consisted, therefore, of more than simply interest in its flowers. In the western USA it was also transported in the empty holds of ships as ballast as it was easily obtainable around European ports and then dumped on arrival in the New World. This explains why many insects and pathogens closely associated with broom were also accidentally released in the USA (Waloff, 1966). C. scoparius was subsequently planted along roadsides in Canada (Peterson and Prasad, 1998) and for stabilizing dune systems along the Oregon coast, which led to landscape changes as coniferous woodland subsequently developed (Arnst, 1942; McLaughlin and Brown, 1942). In Oregon, C. scoparius is known to infest at least 120,000 ha of forests and to occur along 5500 km of roadsides (Isaacson, 2000), while in California broom infests 250,000 ha of rangeland (Bossard, 1991).

Spread has been associated with invasion following prairie fires in North America (Tveten and Fonda, 1999). In Australia, broom now infests more than 200,000 ha of land (Hosking et al., 1998). In this country, most  infestations started either in gardens or in mining communities at the top of catchments. C. scoparius was first recorded in New Zealand in 1872 and now infests 0.92% of farmable land on the South Island (Fowler and Syrett, 2000). It was reported for the first time in Brazil in 2016, in species-rich grasslands in the south of the country (Cordero et al., 2016). Naturalizing populations have been recorded here in natural habitats near human settlements.

Risk of Introduction

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As C. scoparius is still an important horticultural species, risks remain for significant further spread both through fresh introductions into countries through uninformed horticultural practices and illicit passage of seeds through the post ordered via the Internet. Many horticultural varieties of broom have shown no capacity for naturalization and spread. The difficulty of clear identification of the different varieties by the inexperienced will remain a significant risk; however, many official horticultural bodies are aware of the risks and discourage sales of varieties known to be invasive (Atkinson and Sheppard, 2000). Similarly, horticultural use requires assessment of the risks biological control agents pose for the profitability of sales of harmless varieties.

Modelling of the future potential distribution of C. scoparius suggests that places most at risk from range expansion include China, Australia, Argentina and North America, as C. scoparius is already present, but has not yet colonised all areas with apparently high climatic suitability. Climate change is likely to lead to a poleward shift in the range of C. scoparius and a contraction of areas of suitable climate for the species in southern Europe, central Africa, Australia, China, Brazil and the southern United States (Potter et al., 2009).

Habitat

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C. scoparius occurs most commonly in cool temperate areas, but can be found down to sea level in areas with Mediterranean climate where winter temperatures drop below zero (Bolos and Vigo, 1984). C. scoparius can be found growing along river systems, in native grassland and pasture, and in open woodland, including a wide range of disturbed and undisturbed communities (Hosking et al., 1998; Peterson and Prasad, 1998). In its native range it is characteristic on the sandy soils of mesic grasslands, heath and inland sandy areas, fallow land and pasture on acid soils, and gravel deposits associated with old river systems (Rousseau and Loiseau, 1982; Ellenberg, 1996). In the southern part of its range, it is more of a montane grassland and woodland understorey species, often occurring in a distinct altitudinal zone relative to other co-occurring Cytisus species (Polunin and Smythies, 1973; Syrett et al., 1999; Smith, 2000), and is associated with early succession after fires (Herranz et al., 1996).

Where it is invasive, broom typically forms dense, continuous understorey thickets, less frequently seen in the native range (Rousseau and Loiseau, 1982), particularly in all types of open woodland. C. scoparius occurs as a weed in Eucalyptus plantation forestry in Australia (Barnes and Holz, 2000), India (Rashmi et al., 1987) and Spain (Bolos and Vigo, 1984), and in pine plantations in Japan (Nemoto et al., 1993), UK (Nimmo, 1963), New Zealand (Richardson et al., 1996) and the USA (Isaacson, 2000). It is an important weed of the younger stages of all types (softwood or hardwood) of plantation forestry, where short plantation cycles allow broom to persist as seed between disturbances (Peterson and Prasad, 1998; Barnes and Holz, 2000). In such situations it quickly invades forestry tracks, powerline rights-of-way and road verges. C. scoparius also invades and persists in treeless vegetation such as upland, subalpine or high latitude grasslands and cleared pasture, where it is often associated with similar weeds such as gorse (Ulex europaeus) or blackberry (Rubus spp.) (Hosking et al., 1998; Peterson and Prasad, 1998). It can also appear as the dominant overstorey species in largely annual grassland communities. It will not grow in heavily shaded or swampy places, except where waterlogging is temporary. C. scoparius can also colonize grey dune systems, where it has been used as a dune stabilizer, and is a frequent invader along coastal strips on both sides of North America (Arnst, 1942; Peterson and Prasad, 1998; Redmon et al., 2000), suggesting some tolerance of ocean spray. It is also common on disturbed land in urban parks and low fertility, prairie soils (Parker, 2000). In Brazil it has been recorded in species-rich Campos Sulinos grasslands, and is viewed as a threat to the Atlantic Forest and Pampa biomes (Cordero et al., 2016).

Habitat List

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CategoryHabitatPresenceStatus
Littoral
Coastal areas Present, no further details Harmful (pest or invasive)
Terrestrial-managed
Disturbed areas Present, no further details Harmful (pest or invasive)
Managed forests, plantations and orchards Present, no further details Harmful (pest or invasive)
Managed grasslands (grazing systems) Present, no further details Harmful (pest or invasive)
Rail / roadsides Present, no further details Harmful (pest or invasive)
Urban / peri-urban areas Present, no further details Harmful (pest or invasive)
Terrestrial-natural
semi-natural/Cold lands / tundra Present, no further details Harmful (pest or invasive)
semi-natural/Natural forests Present, no further details Harmful (pest or invasive)
semi-natural/Natural grasslands Present, no further details Harmful (pest or invasive)
semi-natural/Riverbanks Present, no further details Harmful (pest or invasive)
semi-natural/Wetlands Present, no further details Harmful (pest or invasive)

Hosts/Species Affected

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C. scoparius is a significant weed of forestry, particularly in pine and eucalypt plantations around the world. It either smothers planted saplings or reduces their growth (Peterson and Prasad, 1998; Barnes and Holz, 2000). In some areas, C. scoparius can be beneficial in these situations as a nurse crop protecting samplings from frost damage and other types of exposure (Peterson and Prasad, 1998), but it can also be associated with higher levels of plantation diseases (Peterson and Prasad, 1998). Once the plantation species grow above plants of C. scoparius, the impact on tree growth is minimal. In native woodland situations broom can prevent natural regeneration by shading (Hosking et al., 1998) and allelopathy (Nemoto et al., 1993).

Biology and Ecology

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Genetics

The basic chromosome number of C. scoparius is 23 (2n = 46) (Heywood and Ball, 1968). Significant morphological variation is observed between infestations in different locations. Even within weedy material recognized as C. scoparius, flower colour ranges from pale yellow, through the deeper yellow of the European wild type to the yellow and crimson flowers that result from the escape of the C. scoparius horticultural cultivars 'Andreanus' and 'Andreanus Aureus'. These cultivars have become naturalized in several parts of Australia (Rowell, 1991) and North America (Peterson and Prasad, 1998). Not all horticultural varieties of broom, where one of the parents is considered to be C. scoparius, for example, Cytisus × praecox Beauverd, are invasive (Atkinson and Sheppard, 2000). There are recommendations against the movement of plant material between infested areas which might lead to examples of hybrid vigour (Smith, 2000). A recent analysis by Nielsen et al. (2016) in Denmark confirmed the presence of two gene pools: one native and the other invasive. The nuclear genome of the native types was highly introgressed with the invasive genome, and advanced-generation hybrids were observed, suggesting that hybridization has been occurring for several generations.

Physiology and Phenology

C. scoparius plants form large, sprawling plants, up to 6 m across. They can live up to 25-30 years but this is extremely rare as most mature plants die between the ages of 6 and 15 years (Rees and Paynter, 1997; Downey and Smith, 2000). Annual growth is such that the axillary meristems produced on the main axis grow up into lateral shoots at the same time as the extension of the main axis within the same growing season (Peterson and Prasad, 1998). As last year's main axis is where most seed production occurs within a season, the lateral shoots frequently overtop the main axis each season, giving C. scoparius its characteristic growth form.

The optimum temperature for photosynthesis, which occurs in the green stems and leaves, is 20-25°C (Peterson and Prasad, 1998). The young stems remain green for about 3 years, allowing photosynthesis and nitrogen fixation to occur throughout the growing season. C. scoparius stem photosynthesis results in approximately 200 mmol/m² daily carbon dioxide accumulation (Nilsen et al., 1993). Stem photosynthetic tissue has been shown to contribute approximately 40% of photosynthates in C. scoparius (Hosking et al., 1998). Green stems comprise 15-25% of total fresh live stem biomass. Dense C. scoparius stands have been recorded as supporting over 40 tones/ha of dry weight biomass in both the native (Rousseau and Loiseau, 1982) and exotic range (Bossard and Rejmánek, 1994), of which up to 26 tones biomass/ha can be the live green stem material (Hosking et al., 1998). Leaves are shed in periods of stress such as dry periods in the summer, cold periods in winter, or towards the end of the growing season in autumn.

C. scoparius is intolerant of heavy shade; seedlings usually die if germination occurs beneath parental or other relatively dense canopy cover. Nodules containing nitrogen-fixing Bradyrhizobium sp. occur on the roots of C. scoparius (Sajnaga and Malek, 2001), but it is relatively poorly nodulated compared to agricultural legumes and has low nitrogenase activity (Wheeler et al., 1979). Nevertheless, this nitrogen-fixing ability is probably important in allowing C. scoparius to grow on nitrogen-poor soils (Williams, 1981). C. scoparius is also considered to be drought tolerant, tolerating -2.59 bar without showing any signs of stress (Peterson and Prasad, 1998). Germination occurs mainly in the spring and autumn, and the relative importance of these two germination periods varies from year to year. In drier areas, spring seedlings rarely survive the summer unless protected by surrounding vegetation (Hosking et al., 1998). Big flushes of germination follow droughts and fires after rain and may occur at any time of the year (Sheppard et al., 2002).

Reproductive Biology

C. scoparius plants generally flower first between their third and fifth year (Sheppard et al., 2002). A few flowers can be found on plants at any time of the year in warmer climates, although most flowering occurs from March to June in the Northern Hemisphere (Heywood and Ball, 1968) and from October to December in the Southern Hemisphere (Williams, 1981; Parsons and Cuthbertson, 1992). Flowering reaches a peak after a few weeks and continues at declining levels over several months. Late frosts can stimulate a second burst of flowers by killing the developing pods. Only a proportion of C. scoparius flowers (2-58%) develop into pods (Smith and Harlen, 1991; Parker and Haubensak, 2002) and this varies with pollination rate and plant size (Suzuki, 2000), and between habitats (Waloff and Richards, 1977) and years (Smith and Harlen, 1991). C. scoparius requires insect pollination for which honey bees appear to be best suited (Stout, 2000). The ubiquitous presence of honey bees means that it always has a suitable pollinator, but pollinator limitation frequently occurs, particularly in shady habitats and this can lead to reduced seed set (Suzuki, 2000; Parker and Haubensak, 2002). A biennial cycle of relatively low and high flower and pod density has been observed (Waloff and Richards, 1977).

The pods ripen over the summer, releasing seeds explosively on sunny days when they develop torsional stresses as they dry out. Seeds are mostly shed from June to early September in the Northern Hemisphere and from December to early March in the Southern Hemisphere, the last seeds being released in winter. Seed production is highly variable, ranging from 28 to 8000 seeds/m², depending on the growing conditions and plant age (Smith and Harlen, 1991; Sheppard et al., 2000). Pods contain up to 22 green to yellowish-brown seeds, although over 99% of pods have fewer than 15 seeds (Smith and Harlen, 1991). The mean number of seeds per pod varies from 5 to 8 between plants and sites (Smith and Harlen, 1991; Sheppard et al., 2002), but fewer seeds are produced per pod in the native range due to insect predation (Paynter et al., 1998). Seed production per mature plant can range from several hundred to 25,000 and tends to be higher in the exotic range (Rees and Paynter, 1997; Peterson and Prasad, 1998). Most seed falls within 1 m of parent plants, although exceptionally dehiscence can fling seeds 4-5 m (Smith and Harlen, 1991). Like many legumes, C. scoparius is hard seeded and only a small proportion of seeds germinate at any time. Fresh seed is 69-98% viable, but >65% remains dormant until the seed coat is ruptured delaying germination for months or years. About 7-20% of buried or submerged seed remains ungerminated and viable after 3 years (Smith and Harlen, 1991; Bossard, 1993). Less than 1% of seeds were viable after 81 years in storage (Turner, 1934). Seed longevity contributes to large soil seed banks below C. scoparius throughout its range. In the native range, soil seed banks range from 30 to over 10,000 per m² (Rees and Paynter, 1997); in the exotic range, soil seed banks can reach over 30,000 per m² (Downey, 2001). In the absence of seed rain the seed bank declines by about 40% a year (Paynter et al., 1998; Sheppard et al., 2002).

Environmental Requirements

C. scoparius can be found growing on areas up to 2000 m altitude, particularly at lower latitudes (Hosking et al., 1998). Its high altitudinal and latitudinal limits are set by persistent low winter temperatures causing dieback of fresh growth and early death of plants (Peterson and Prasad, 1998), but it can still invade areas despite persistent periods of snow cover (Downey and Smith, 2000). Its low latitude limits may be associated with a temperate physiology i.e., it ceases growing in the winter months (Hosking et al., 1998). The capacity of adults to survive extensive drought periods permits this weed to persist in quite harsh climates where recruitment is at least occasionally favourable (Sheppard et al., 2002). It can, therefore, occur on vegetation-denuded slopes of even a very steep aspect, though plants in such situations are notably less vigorous (Fowler et al., 1996). C. scoparius seedlings require persistent moisture to survive, and so it recruitment may be episodic in areas of shallow soils prone to drought (Smith, 1994). The frequent even age of  C. scoparius stands has been documented allowing four distinct stand stages to be recognized (Smith, 1994). As such, a whole broom stand can appear to senesce (Peterson and Prasad, 1998) with delayed and less dense subsequent regeneration (Downey and Smith, 2000). In open areas, tussock grasses can provide the drought/grazing protection necessary for establishment (Hosking et al., 1998). In drier climates, it can also remain a highly invasive species associated with the banks or braided riverbeds of watercourses or along drainage lines (Hosking et al., 1998).

In its native range, C. scoparius is usually a calcifuge (Polunin and Smythies, 1973). However, in the exotic range, it occurs on a broader range of soil types derived from a wide variety of substrates, particularly river sand, schist, granite or basalt and including brunisols, podsols and regosols, but it does not flourish on calcareous soils (Hosking et al., 1998; Peterson and Prasad, 1998). C. scoparius grows and spreads fastest on moist, fertile and alluvium soils high in inorganic phosphorus (Williams, 1981) but can rarely be found on disturbed skeletal sandy soils. Plants establish best after soil or vegetation disturbance caused by e.g., animals, fire or herbicide treatments, but it can readily invade vegetation without major disturbance (Downey, 2001; Sheppard et al., 2002). C. scoparius can invade all types of woodland and forests where canopy cover is less than about 50% (Waterhouse, 1988).

Associations

Associations with vesicular-arbuscular mycorrhizal fungi have been reported in Japan (Harley and Harley, 1987). Nodules containing nitrogen-fixing Bradyrhizobium sp. occur on the roots of C. scoparius (Sajnaga and Malek, 2001),

Latitude/Altitude Ranges

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Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
60 36 0 0

Air Temperature

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Parameter Lower limit Upper limit
Absolute minimum temperature (ºC) -40 0
Mean annual temperature (ºC) 5 12
Mean maximum temperature of hottest month (ºC) 10 30
Mean minimum temperature of coldest month (ºC) -10 5

Rainfall

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ParameterLower limitUpper limitDescription
Dry season duration012number of consecutive months with <40 mm rainfall
Mean annual rainfall3003000mm; lower/upper limits

Rainfall Regime

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Summer
Uniform
Winter

Soil Tolerances

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Soil drainage

  • free
  • seasonally waterlogged

Soil reaction

  • acid
  • neutral

Soil texture

  • heavy
  • light
  • medium

Special soil tolerances

  • infertile
  • shallow

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Aceria genistae Herbivore Stems
Agonopterix assimilella Herbivore Leaves
Arytainilla spartiophila Herbivore Leaves
Asphondylia sarothamni Herbivore Fruits/pods
Botryosphaeria dothidea Pathogen
Bruchus villosus Herbivore Seeds
Exapion fuscirostre Herbivore Seeds
Fusarium tumidum Pathogen
Gibberella avenacea Pathogen
Gibberella baccata Pathogen
Gonioctena olivacea Herbivore Leaves
Hexopmyza sarothamni Herbivore Stems
Leucoptera spartifoliella Herbivore Stems
Phyllonorycter scopariella Herbivore Stems
Pleiochaeta setosa Pathogen
Polydrusus confluens Herbivore Roots
Protopirapion atratulum Herbivore Inflorescence
Uromyces sarothamni Pathogen Leaves/Stems

Notes on Natural Enemies

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The arthropod natural enemies of C. scoparius are well known and have been extensively studied (Waloff, 1968; Syrett et al., 1999). At least 243 species of insects and mites are associated with C. scoparius plants in Europe, and several other generalist species are now found on C. scoparius in its exotic range (Memmott et al., 2000). High numbers of the accidentally introduced psyllid Arytainilla spartiophila, twig-mining moth Leucoptera spartifoliella and seed beetle Bruchus villosus, have been recorded in the USA (Pfeiffer, 1986; Syrett et al., 1999), the latter species causing up to 80% seed loss to adult C. scoparius in the eastern states (Redmon et al., 2000). Many other specialised broom-feeding insects have established in North America accidentally (Waloff, 1966), but it is not known how damaging these are. L. spartifoliella, also accidentally introduced in New Zealand, reaches high population density and contributes to the early death of plants. Natural enemies that have caused noticeable damage sufficient to affect the growth and spread of C. scoparius are listed. In the native range the natural enemy community can have quite dramatic impacts on C. scoparius growth and survival (Waloff and Richards, 1977). Native cerambycid beetles use C. scoparius in Australia (Hosking et al., 1998) and pyralid moths of the genus Uresiphita have started using this species since its introduction in both Australia and the USA (Smith, 2000). A range of pathogens has also been recorded from scotch broom, both from the native and exotic range (Guyot and Massenot, 1958; Johnson et al., 1995; Peterson and Prasad, 1998).

Means of Movement and Dispersal

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All movement and spread of C. scoparius occurs through the movement of seeds. Most widespread movement of C. scoparius has been considered to occur rapidly due to poor human practice or through deliberate planting (Hosking et al., 1998; Peterson and Prasad, 1998). Genetic analysis of the spread of invasive C. scoparius in Denmark suggests that seeds spread naturally over relatively short distances and that gene flow over longer distances is mainly facilitated by pollen dispersal (Nielsen et al., 2016).

Natural Dispersal (non-biotic)

Several cases of C. scoparius spread have been the result of movement from the top of water catchments, down river systems and out into the surrounding landscape, particularly during flood conditions (Williams, 1981; Smith and Harlen, 1991).

Vector Transmission (biotic)

Wild or feral animals are considered to be important agents of short-distance dispersal in pasture and upland areas, both by carrying seeds and by creating disturbance which assists germination and recruitment (Hosking et al., 1998). Some secondary local dispersal may result from ants (Bossard, 1991; Smith and Harlen, 1991; Nierhaus-Wunderwald, 1995).

Agricultural Practices

C. scoparius spread is known to have occurred via the movement of farm equipment, and through the activities of grazing livestock through semi-natural vegetation systems (Hosking et al., 1998). Cattle grazing can increase the rate of C. scoparius establishment (Sheppard et al., 2002).

Accidental Introduction

C. scoparius was introduced into western North America in discarded ships ballast (Waloff, 1966) and transported in seed-contaminated gravel moved in road maintenance (Peterson and Prasad, 1998). Dispersal may also occur by the movement of seed in mud attached to all-terrain vehicles and ramblers' boots (Hosking et al., 1998; Peterson and Prasad, 1998).

Intentional Introduction

C. scoparius was introduced into most countries for its floral interest and its value for cultural uses (Peterson and Prasad, 1998). Many infestations initially result from garden escapes, followed by other biotic or non-biotic factors assisting spread (Hosking et al., 1998).

Pathway Causes

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CauseNotesLong DistanceLocalReferences
Botanical gardens and zoosWidely introduced as ornamental Yes Yes USDA-ARS, 2016
DisturbanceOften common along roadsides and in disturbed sites Yes Yes USDA-NRCS, 2016
Escape from confinement or garden escapeFruits and seeds escaped from cultivation Yes Yes USDA-NRCS, 2016
Garden waste disposal Yes Yes
Hedges and windbreaksOften planted as hedge plant in gardens Yes Yes USDA-NRCS, 2016
Ornamental purposes Yes Yes USDA-ARS, 2016

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Clothing, footwear and possessionsSeeds Yes Peterson and Raj Prasad, 1998
Land vehiclesSeeds Yes Yes Peterson and Raj Prasad, 1998
LivestockSeeds Yes Yes Peterson and Raj Prasad, 1998
MailSeed catalogues, internet. Yes
Ship ballast water and sedimentIntroduced into USA in discarded ship ballast Yes USDA-NRCS, 2016
Soil, sand and gravelRiver sand carrying seeds. Yes Yes Peterson and Raj Prasad, 1998
WaterSeeds Yes Yes Peterson and Raj Prasad, 1998
Wind Yes Peterson and Raj Prasad, 1998

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
Fruits (inc. pods) seeds
Growing medium accompanying plants seeds
True seeds (inc. grain) seeds
Plant parts not known to carry the pest in trade/transport
Bark
Bulbs/Tubers/Corms/Rhizomes
Flowers/Inflorescences/Cones/Calyx
Leaves
Stems (above ground)/Shoots/Trunks/Branches
Wood

Impact Summary

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CategoryImpact
Animal/plant collections None
Animal/plant products None
Biodiversity (generally) Negative
Crop production Negative
Environment (generally) Negative
Fisheries / aquaculture None
Forestry production Negative
Human health None
Livestock production Negative
Native fauna Positive and negative
Native flora Negative
Rare/protected species Negative
Tourism Negative
Trade/international relations None
Transport/travel None

Economic Impact

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The economic impact of C. scoparius in western North America has been estimated as at least US$ 11 million (Isaacson, 2000). No systematic economic assessment of C. scoparius has been carried out in other areas due to the low or underestimated conservation value of infested National Park land. Spraying broom has been estimated to cost ca US$11 per hectare in Australian pasture (Clark, 2000), and containing Australia's largest C. scoparius infestation of 10,000 ha cost about US$ 25,000 (Schroder and Howard, 2000). Mulching costs about US$ 500 per hectare (Talbot, 2000).

Environmental Impact

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C. scoparius retards the establishment and spread of rare and endemic woody species in the natural ecosystems it has invaded and also smothers the herbaceous layer including the rare species within (Downey and Smith, 2000). C. scoparius can permanently alter the regular nature of tussock and high altitude grassland. Nitrogen fixation is thought to enhance soil fertility sufficiently in some low fertility native ecosystems to allow invasion by other alien species (Smith, 2000). The presence of C. scoparius may alter fire, nutrient and water cycling regimes that may have dramatic impacts on ecosystem function (Hosking et al., 1998; Downey and Smith, 2000). C. scoparius stands can also encourage other exotic feral animals and birds (Downey and Smith, 2000; Smith, 2000).

While C. scoparius can have general impacts on the habitats it invades, changing nutrient and water cycling, it also has direct impacts on species diversity. Heinrich and Dowling (2000) list 30 species of native flora under threat from C. scoparius invasion in Australia, with somewhat similar threats likely in Canada (Peterson and Prasad, 1998) and New Zealand (Fowler and Syrett, 2000). C. scoparius can also out-compete native vegetation and alter successional patterns in native grasslands and forests (Fogarty and Facelli, 1999; USDA-NRCS, 2016).

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Eremophila alpestris strigata (streaked horned lark)USA ESA listing as threatened species USA ESA listing as threatened speciesOregon; WashingtonEcosystem change / habitat alterationUS Fish and Wildlife Service, 2013
Lupinus oreganus var. kincaidii (Kincaid's lupine)NatureServe NatureServe; USA ESA listing as endangered species USA ESA listing as endangered speciesOregon; WashingtonCompetition - smotheringUS Fish and Wildlife Service, 2006
Packera layneaeUSA ESA listing as threatened species USA ESA listing as threatened speciesCaliforniaCompetition - monopolizing resourcesUS Fish and Wildlife Service, 1996
Speyeria zerene hippolyta (Oregon silverspot butterfly)USA ESA listing as threatened species USA ESA listing as threatened speciesCalifornia; OregonEcosystem change / habitat alterationUS Fish and Wildlife Service, 2001
Streptanthus glandulosus subsp. niger (Tiburon jewelflower)USA ESA listing as endangered species USA ESA listing as endangered speciesCaliforniaCompetition - stranglingUS Fish and Wildlife Service, 2010

Social Impact

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C. scoparius is a weed that infests areas of natural beauty and National Parks (Hosking et al., 1998) and is therefore likely to have a negative effect on the aesthetic value of certain sites and perhaps their frequency as destinations for tourism. C. scoparius infestations on farms are likely to affect land values and potential returns on the land, thereby affecting the livelihood of resident farmers. C. scoparius is also toxic to some livestock (Peterson and Prasad, 1998).

Risk and Impact Factors

Top of page Invasiveness
  • Invasive in its native range
  • Proved invasive outside its native range
  • Has a broad native range
  • Abundant in its native range
  • Highly adaptable to different environments
  • Is a habitat generalist
  • Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
  • Pioneering in disturbed areas
  • Highly mobile locally
  • Benefits from human association (i.e. it is a human commensal)
  • Long lived
  • Fast growing
  • Has high reproductive potential
  • Has propagules that can remain viable for more than one year
Impact outcomes
  • Altered trophic level
  • Damaged ecosystem services
  • Ecosystem change/ habitat alteration
  • Increases vulnerability to invasions
  • Modification of hydrology
  • Modification of nutrient regime
  • Modification of successional patterns
  • Monoculture formation
  • Negatively impacts agriculture
  • Negatively impacts forestry
  • Negatively impacts tourism
  • Reduced amenity values
  • Reduced native biodiversity
  • Threat to/ loss of native species
Impact mechanisms
  • Allelopathic
  • Competition - monopolizing resources
  • Competition - smothering
  • Competition - strangling
  • Poisoning
  • Rapid growth
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Highly likely to be transported internationally deliberately
  • Difficult to identify/detect as a commodity contaminant
  • Difficult/costly to control

Uses

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By far the most widespread use of C. scoparius is in the horticultural industry; it is attractive because of its large, coloured flowers and dark-green foliage (Peterson and Prasad, 1998). Commercial production of C. scoparius occurred in southwest France after the Second World War, when it was deliberately planted within Pinus pinaster plantations over large areas and harvested for textile fibres (Simon, 1950). C. scoparius is also used as a nurse plant in forestry to protect saplings from frosts and to prevent soil erosion, particularly following controlled burns (Nimmo, 1963; Nemoto et al., 1993; Peterson and Prasad, 1998). In Europe, it has been sown to restore soil fertility after the last hay crop and the wood generated used to fuel bakery ovens (Rousseau and Loiseau, 1982). C. scoparius shoots have also been used historically for making brooms, thatching and screens and as a substitute for hops in beer making (Polunin and Smythies, 1973; Peterson and Prasad, 1998). In the nineteenth and early twentieth centuries, the seeds were roasted and used as a hot drink in Canada (Peterson and Prasad, 1998). A drug obtained from the twigs is used medicinally for heart and respiratory conditions and the bark has been used for rope and tanning (Polunin and Smythies, 1973).

Uses List

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Environmental

  • Erosion control or dune stabilization
  • Soil conservation

General

  • Ornamental

Materials

  • Fibre
  • Poisonous to mammals

Medicinal, pharmaceutical

  • Source of medicine/pharmaceutical
  • Traditional/folklore

Wood Products

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Textiles

Woodware

  • Brushes
  • Industrial and domestic woodware

Similarities to Other Species/Conditions

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Many other exotic weed species occur in the same tribe as C. scoparius. These include three other species of Cytisus and members of the genera Genista (six species), Retama (two species), Calicotome (one species), Spartium (one species) and Ulex (one species) (Holm et al., 1979; Jepson, 1979). Species in Cytisus all have upright, relatively leafless, robust stems and large, showy flowers. Of these, all except C. scoparius have rounded young shoots. Cytisus multiflorus has smaller white flowers and small, few-seeded pods; Cytisus striatus has grey-green shoots with fine, white, wax-filled grooves along the shoot and downy pods; and the largest species is the pale grey-green Cytisus proliferus, which has large, persistent, trifoliate leaves, pendulous branches and clusters of large, cream flowers early in the season (Jepson, 1979). This latter species can attain small tree proportions and is also known as tagasaste or tree lucerne as it is grown as a fodder crop in warmer winter areas (Hosking et al., 1998).

Prevention and Control

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Detection

Mapping of C. scoparius using Landsat imagery has been tested by Hill et al. (2016) in British Columbia, Canada. Areas of low density and small patches of C. scoparius were often missed, while some areas were erroneously classified as C. scoparius, but the authors suggest that despite these limitations, a satellite-based remote sensing approach may be useful for some aspects of C. scoparius management.

Cultural Control

Manual pulling of C. scoparius at flowering or hand sawing individuals at the base in hotter, drier months is commonly used for the removal of outlying C. scoparius individuals (Peterson and Prasad, 1998; Ussery and Krannitz, 1998). Sheep (not all breeds) and goats (particularly meat goats) are effective at suppressing C. scoparius (Rousseau and Loiseau, 1982) even when used strategically on existing stands (Allan et al., 1997), but cannot be used in conservation areas where indiscriminate grazing is undesirable. Large browsing or grazing animals reduce C. scoparius biomass, but exert little control (Bossard and Rejmanek, 1994). This can be seen in the frequency and ease with which C. scoparius infests cattle-grazed paddocks (Hosking et al., 1998). Rabbits can prevent C. scoparius regeneration at high density (Paynter et al., 2000) but do not provide a practical management option. Fire is often employed to control C. scoparius especially over large and/or remote areas (Robertson et al., 1999); however, it also promotes conditions for subsequent re-invasion. Fire is rarely effectively applied because plants have to be very dry for an effective burn (Downey, 2000). The use of fire also poses significant environmental risks. While hot fires can reduce C. scoparius seed banks to <10%, the heat stimulates the remaining seed sufficiently for stand replacement as seedling survival can be higher in burnt than unburned areas due to low competition in the grass layer (Downey, 2000). Burnt C. scoparius dies but scorched plants will resprout, so fire is useful for C. scoparius control only as part of integrated control.

Kerr et al. (2012) found that prescribed fire and hand pulling were the most cost-efficient methods of control in prairie fields and city parks in Washington, USA.  

Mechanical Control

The mechanical removal of plants before they seed can be used to control isolated plants but is not practical over large areas (Parsons and Cuthbertson, 1992). Small areas can be slashed and cultivated, but this produces a seedbed ideal for C. scoparius seedling establishment (Parsons and Cuthbertson, 1992). Farmers frequently employ bulldozers to remove stands, but this may prove counter-productive, burying seed and spreading regeneration probability over a longer time frame (Hosking et al., 1998). Mulching or rolling C. scoparius with large machinery can prove effective as it sets up a layer of mulch that limits C. scoparius regeneration in the short term (Talbot, 2000) but, like all options, follow up is required.

Chemical Control

The main chemicals used to control C. scoparius are picloram, triclopyr, glyphosate, fluroxypyr and metsulfuron-methyl (Parsons and Cuthbertson, 1992). Specific chemicals are appropriate for specific situations, such as proximity to watercourses. The addition of some surfactants to glyphosate and metsulfuron-methyl increases the level of control achieved by these chemicals (Hosking et al., 1998). Herbicide treatment of C. scoparius can result in initial kill rates of 50-100%. Nevertheless, within a few years C. scoparius plants fully regenerate when no future management strategy is applied and, in some cases, spraying recruiting plants may be required for many years. In Australia, Pascoe et al. (2014) found that although herbicides reduced C. scoparius cover, it returned to initial levels in plots which then weren’t treated for two successive years. Chemical control can be applied as a foliar spray, by injecting stem bases (for large isolated individuals) or by painting stumps after cutting mature plants to prevent regeneration (Peterson and Prasad, 1998).

In New Zealand, where C. scoparius is an important weed in pine plantations, Watt and Rolando (2014) found that a mixture of clopyralid, triclopyr and picloram gave good control and increased tree volume during the first year of Pinus radiata establishment. Harrington et al. (2016) also found that mixtures based on clopyralid with triclopyr gave the best selective control, while also preserving grass ground cover. Fluroxypyr was unsuitable for use on its own due to damage to young Pinus radiata.

Biological Control

Biological control activities targeted at C. scoparius started in 1951 in Europe. The twig-mining moth Leucoptera spartifoliella and the weevil Exapion fuscirostre were released in the field in California, USA, in 1960 and 1964, respectively (Andres et al., 1967) and then widely redistributed. The weevil has been recorded destroying between 60 and 90% of seed produced (Syrett et al., 1999) with attack rates of three weevils per pod. L. spartifoliella initially caused severe damage but was found to be already present in Washington State along with its European parasitoid since at least 1941 and soon failed to maintain the high populations reported regularly in New Zealand. L. spartifoliella was introduced to Australia from New Zealand in 1993, where it has established at several release sites and has reached densities that are starting to stunt growth. The seed beetle Bruchus villosus was released in New Zealand in 1987 and in Australia in 1996 (Syrett et al., 1999). The beetle has established in both countries and, in New Zealand, up to 60% seed loss has been recorded at early release sites. B. villosus was shown to have actively moved between eastern North America, where it arrived accidentally, and western North America, where it didn't previously occur, on the basis of the significant damage it caused (Syrett et al., 1999). The psyllid Arytainilla spartiophila was first released in 1993 in New Zealand and in 1995 in Australia and is now established in both countries (Syrett et al., 1999). In California, where this psyllid is an accidental introduction, it had no statistically detectable impacts on C. scoparius growth in field conditions, and high psyllid mortality was found both in the greenhouse and the field (Hogg et al., 2016).

The broom gall mite (Aceria genistae) has been approved for release in Australia and New Zealand after extensive testing between 1999 and 2001 for specificity towards C. scoparius. Sheppard et al. (2013) reported that there was a 32% establishment rate from 106 releases of the mite in Australia, and 50% establishment from 40 releases in New Zealand. Paynter et al. (2012) investigated dispersal after the mite was first released in New Zealand in 2007, and report that the maximum dispersal rate was unlikely to be fast enough to greatly benefit forestry. In Australia, Pascoe et al. (2014) reported good establishment of released Aceria genistae, Bruchus villosus and the self-established rust fungus Uromyces pisi-sativi in the Alpine National Park of Victoria, and found that the organisms were beginning to reduce plant vigour and/or seed production.

As well as classical biological control, mycoherbicides are being considered for use against C. scoparius. Strains of Fusarium tumidum [Gibberella tumida] are being developed as a mycoherbicide in New Zealand (Syrett et al., 1999; Frohlich and Gianotti, 2000). In Canada, several fungi have been tested under greenhouse conditions and one, Chondrostereum purpureum, is being tested under field conditions as a mycoherbicide (Prasad, 2002).

Jarvis et al. (2006) modelled the economic benefits and costs of introducing new biocontrol agents for C. scoparius control in New Zealand and estimated that benefits would outweigh costs by 2.9:1.

Integrated Control

The conventional control measures presently carried out, mainly involving herbicides, manual pulling and local burning, have all proved to be ineffective on their own; however, in combination the management of C. scoparius may be more easily achieved (Downey and Smith, 2000). A key to the management of any stand-forming invasive plant species is to make containment and eradication of small outlying infestations the first priority before tackling the main stand (Smith, 2000). Fire can play an important role in reducing the seed bank and stimulating most of what seed remains; however, it is not easy to generate a hot fire in most of the climates in which C. scoparius grows (Downey, 2000). Cutting or spraying C. scoparius with foliar herbicide and letting it cure prior to the burn may generate a hotter fire (CC Bossard, St Mary's College of California, USA, personal communication, 1990). After the fire, regular follow-up management visits will be required to prevent regeneration (Downey, 2000). The costs of this can be minimized by spot spraying regenerated C. scoparius at flowering in its first year of flowering (e.g. approximately 2-3 years after the fire). Encouraging native vegetation cover after the fire may also help. With time, the costs and frequency of follow-up visits are likely to decline as C. scoparius seedling density drops and native vegetation cover increases, though this may take upwards of 20 years surveillance (Hosking et al., 1998). Where fire is impractical, the management of C. scoparius will be harder and mulching might be an alternative (Talbot, 2000) but the depletion of the seed bank will remain the key problem.

Herrera-Reddy et al. (2012) compared biological control alone (with the Scotch broom seed weevil, Exapion fuscirostre) or combined with fire or mowing. Both integrated strategies outperformed biological control alone in reducing seed production and the seed bank. It is suggested that short-rotation prescribed fire could prove to be the more effective strategy for long-term management of Scotch broom due to its potential for slightly greater depletion of the seed bank.

Preventing any kind of disturbance and letting the C. scoparius infestation drop back to lower densities as it appears to do naturally is another, if perhaps risky, option (Downey and Smith, 2000). Grazing with goats is only really practical where goats are to be maintained on site for many years, as stunted C. scoparius will remain alive in goat enclosures and resprout quickly when the goats are removed (Allan et al., 1997). Biological control provides the only solution for C. scoparius in inaccessible areas or areas of low land value; however, results so far suggest that biological control of C. scoparius is still not effective on its own (Syrett et al., 1999).

References

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Allan C, Holst P, Campbell M, 1993. Weed control using goats. Agdex 640, New South Wales Agriculture, Orange, Australia.

Andres LA, Hawkes RB, Rizza A, 1967. Apion seed weevil introduced for biological control of Scotch broom California Agriculture, 21:13.

Arnst A, 1942. Vegetal stabilization of Oregon coastal dune areas. Northwest Science 16:59-67.

Atkinson I, Sheppard A, 2000. Brooms as part of the Australian nursery industry. Plant Protection Quarterly, 15(4):176-178; 8 ref.

Barnes CD, Holz GK, 2000. Broom (Cytisus scoparius (L.) Link) competition and management in eucalyptus tree farms. Plant Protection Quarterly, 15:163-164.

Belov M, 2013. ChileFlora. http://www.chileflora.com/index.html

Boissier PE, 1838. Elenchus Plantarum novarum minusque cognitarum quas in Itinere hispanico legit. Genevae.

Bolos Ode, Vigo J, 1984. Flora dels Paisos Catalans, Vol.1. ORCA.

Bossard CC, 1991. The role of habitat disturbance, seed predation and ant dispersal on establishment of the exotic shrub Cytisus scoparius in California. American Midland Naturalist, 126(1):1-13

Bossard CC, 1993. Seed germination in the exotic shrub Cytisus scoparius (Scotch broom) in California. Madrono, 40(1):47-61

Bossard CC, Rejmanek M, 1994. Herbivory, growth, seed production, and resprouting of an exotic invasive shrub Cytisus scoparius. Biological Conservation, 67(3):193-200

Burkart A, 1952. Las Leguminosas Argentinas. Acme Agency, Buenos Aires.

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