Lepidium latifolium
(perennial pepperweed)

Pictures

Picture	Title	Caption	Copyright
Title Flowers Caption Lepidium latifolium (perennial pepperweed); flowering habit. USA. Copyright ©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US	Flowers	Lepidium latifolium (perennial pepperweed); flowering habit. USA.	©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US
Title Flowers Caption Lepidium latifolium (perennial pepperweed); close view of flowers. USA. Copyright ©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US	Flowers	Lepidium latifolium (perennial pepperweed); close view of flowers. USA.	©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US
Title Infestation Caption Lepidium latifolium (perennial pepperweed); roadside infestation. USA. June 2010. Copyright ©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US	Infestation	Lepidium latifolium (perennial pepperweed); roadside infestation. USA. June 2010.	©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US
Title Infestation Caption Lepidium latifolium (perennial pepperweed); infestation. USA. July 2002. Copyright ©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US	Infestation	Lepidium latifolium (perennial pepperweed); infestation. USA. July 2002.	©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US
Title Habit Caption Lepidium latifolium (perennial pepperweed); flowering habit. USA. Copyright ©Pedro Tenorio-Lezama/Bugwood.org - CC BY-NC 3.0 US	Habit	Lepidium latifolium (perennial pepperweed); flowering habit. USA.	©Pedro Tenorio-Lezama/Bugwood.org - CC BY-NC 3.0 US
Title Habit Caption Lepidium latifolium (perennial pepperweed); habit. USA. June 2007. Copyright ©Bonnie Million/National Park Service/Bugwood.org - CC BY-NC 3.0 US	Habit	Lepidium latifolium (perennial pepperweed); habit. USA. June 2007.	©Bonnie Million/National Park Service/Bugwood.org - CC BY-NC 3.0 US
Title Habit Caption Lepidium latifolium (perennial pepperweed); habit and close view of leaves. Copyright ©Bonnie Million/National Park Service/Bugwood.org - CC BY-NC 3.0 US	Habit	Lepidium latifolium (perennial pepperweed); habit and close view of leaves.	©Bonnie Million/National Park Service/Bugwood.org - CC BY-NC 3.0 US
Title Leaves and stem Caption Lepidium latifolium (perennial pepperweed); leaves and stem. USA. Copyright ©Pedro Tenorio-Lezama/Bugwood.org - CC BY-NC 3.0 US	Leaves and stem	Lepidium latifolium (perennial pepperweed); leaves and stem. USA.	©Pedro Tenorio-Lezama/Bugwood.org - CC BY-NC 3.0 US
Title Flowers Caption Lepidium latifolium (perennial pepperweed); flowers and partially formed fruits. USA. Copyright ©Pedro Tenorio-Lezama/Bugwood.org - CC BY-NC 3.0 US	Flowers	Lepidium latifolium (perennial pepperweed); flowers and partially formed fruits. USA.	©Pedro Tenorio-Lezama/Bugwood.org - CC BY-NC 3.0 US
Title Fruits Caption Lepidium latifolium (perennial pepperweed); mature fruits. USA. October 2010. Copyright ©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US	Fruits	Lepidium latifolium (perennial pepperweed); mature fruits. USA. October 2010.	©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US
Title Fruit Caption Lepidium latifolium (perennial pepperweed); intact fruit starting to separate into two halves. Note scale. Copyright ©D. Walters & C. Southwick/Table Grape Weed Disseminule ID/USDA APHIS ITP/Bugwood.org - CC BY-NC 3.0 US	Fruit	Lepidium latifolium (perennial pepperweed); intact fruit starting to separate into two halves. Note scale.	©D. Walters & C. Southwick/Table Grape Weed Disseminule ID/USDA APHIS ITP/Bugwood.org - CC BY-NC 3.0 US
Title Seedling Caption Lepidium latifolium (perennial pepperweed); seedling. USA. October 2005. Copyright ©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US	Seedling	Lepidium latifolium (perennial pepperweed); seedling. USA. October 2005.	©Leslie J. Mehrhoff/University of Connecticut/Bugwood.org - CC BY 3.0 US

Identity

Preferred Scientific Name

• Lepidium latifolium L.

Preferred Common Name

• perennial pepperweed

Other Scientific Names

• Cardaria latifolia (L.) Spach
• Crucifera latifolia (L.) E.H.L.Krause
• Lepidium affine Ledeb
• Lepidium dioscorides Bubani
• Lepidium sativum var. latifolium DC DC
• Lepidium sibiricum Pall
• Lepidium sibiricum Schweigg
• Nasturtiastrum latifolium (L.) Gillet & Magne
• Nasturtium latifolium (L.) Kuntze

International Common Names

• English: broadleaf peppergrass; broadleaf pepperwort; dittander; giant whiteweed; ironweed; peppergrass; peppergrass mustard; perennial peppercress; perennial peppergrass; perennial pepperwort; tall whitetop; Virginia pepperweed
• Spanish: lepidio; mastuerzo montesino; piperisa
• French: grande passerage

Local Common Names

• Austria: Breitblatt-Kresse
• China: kuan ye du xing cai
• Germany: Breitblättrige Kresse
• Latvia: placialape pipirne
• Lithuania: platlapu cietkersa
• Norway: strandkarse
• Portugal: erva-pimenteira
• Sweden: bitterkrassing

Summary of Invasiveness

L. latifolium is an erect, branching perennial native to southern Europe and western Asia. It was accidentally introduced into countries outside of its native range as a contaminant of seeds such as Beta vulgaris. L. latifolium exhibits a wide ecological adaptation to different environmental factors, tolerating a range of soil moisture and salinity conditions, which has allowed it to spread explosively in recent years in wetlands and riparian areas especially in the western USA. L. latifolium thrives in many lowland ecosystems and is extremely competitive, forming monospecific stands that can crowd out desirable native species and a number of threatened and endangered species.L. latifolium alters the ecosystem in which it grows, acting as a ‘salt pump’ which takes salt ions from deep in the soil profile and deposits them near the surface, thereby shifting plant composition and altering diversity.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Capparidales
•                         Family: Brassicaceae
•                             Genus: Lepidium
•                                 Species: Lepidium latifolium

Notes on Taxonomy and Nomenclature

L. latifolium was named by Linnaeus and although a number of other names have been applied since, including Cardaria latifolia, none of these are now commonly used. 

Five subspecies have been described: subsp. affine,amplexicaule, latifolium, obtusum and sibiricum. These however, are not recognised in the most recent taxonomic treatments of the species (Francis and Warwick, 2007). This is supported by a study by Gaskin et al. (2013) who found no correlation between morphological characteristics used in taxonomic keys to distinguish the subspecies latifolium, affine and obtusum and genotypic lineages or clusters.

Common names such as pepperweed and peppercress derive from the pungent, peppery taste of the foliage.

Description

L. latifolium is a perennial herb 1-2 m high, with a creeping root system emanating from a semi-woody crown. Francis and Warwick (2007) describe the underground structures as both rhizomes and roots. Other authors quoted by Zouhar (2004) conclude otherwise, that they are all true roots. It seems likely that both types of structure can occur – short rhizomes (horizontal stems from which buds develop at the nodes) and much longer horizontal roots 10-20 cm deep, on which adventitious buds can develop at any point, especially when fragmented. Other roots can occur much more deeply, even down to 3 m (Zouhar, 2004). A number of erect stems arise from the crown, and are much branched above. Lower leaves are up to 30 cm long by 5-8 cm wide on petioles up to 10 cm long, elliptic-ovate or oblong, finely serrate on the margins and with a whitish mid-rib. Upper leaves are smaller up to 10 cm long, sessile, with entire margins, cuneate base and acute apex. Leaf surfaces may have some hairs, but are generally glabrous, leathery and glaucous.

Francis and Warwick (2007) describe the inflorescence as ‘paniculate, terminating in numerous, many-flowered, often compounded racemes; sparsely pubescent or glabrous; pedicels slender, 2–5 mm long. Sepals deciduous, oblong, suborbicular, 1–1.4 mm long by 0.8–0.9 mm wide, glabrous or pubescent, white at margin and apex. Petals milky white, obovate, 1.8–2.5 mm long by (0.8)1–1.3 mm wide, apex rounded. Stamens 6, with 4 long and 2 short filaments 0.9–1.4 mm long; anthers ovate, 0.4–0.5 mm long. Pistil 2 mm long, style nearly obsolete (scarcely visible), stigma prominent, sessile, 2–3 times broader than sepals, persistent on fruit. Fruits (silicules) 2- chambered, slightly flattened, oblong ellipsoid to oval-ellipsoid, or suborbicular, (1.6)1.8–2.4(2.7) mm long by about 1.3 mm wide, sparsely hairy with soft, crinkly hairs or glabrous, not emarginate or very minutely so. Seeds 1 per chamber, light reddish brown, flattened, wingless, finely papillate (with small swellings) with long, simple hairs; oblong-ovoid, (0.8)1–1.3 mm long by 0.7–0.9 mm wide.’ 

Pollen morphology has been described by Tang et al. (2005).

Distribution

L. latifolium has a wide native range across Europe, Asia and northern Africa but has been introduced to Australasia and to the Americas.

Distribution Table

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.

History of Introduction and Spread

Introduction of L. latifolium into North America apparently dates back to the early twentieth century. Francis and Warwick (2007) indicate the 1920s as the period of introduction, corresponding with numerous records for herbarium specimens from across Massachusetts and Connecticut in the 1920s and 1930s (GBIF, 2012). However, there are earlier herbarium specimens recorded by GBIF (2012) from ‘Shaw’s Garden’ in St Louis, Missouri in 1902 and from New York Botanical Garden in 1908. It is possible that some introductions occurred via botanic gardens or nurseries, but there are suggestions that several separate introductions may have been involved, some involving contaminated Beta vulgaris (sugar beet) seed (Zouhar, 2004).

In the western USA L. latifolium was first sighted in California in sugar beet seed in 1936 (Bellue, 1936 in Howald, 2000) and was recognised as invasive in the 1980s, first in California and then in adjacent western states. The first herbarium record in Arizona dates from 1987 (Brown, 2005). Although established in eastern USA long before the west, it has until recently, been apparently less invasive there, but Orth et al. (2006) now report increasing concern that it is spreading in Massachusetts and Connecticut. 

In Canada, L. latifolium was first identified as invasive in the late 1990s in British Columbia (Francis and Warwick, 2007). 

In Mexico the earliest herbarium specimen recorded by GBIF (2012) is 1944. Vibrans (2003) reports that infestations in the Valley of Mexico are continuing to expand. 

In Australia, it is thought to have been introduced via contaminated sugar beet seed (Kloot, 1973). The earliest herbarium specimen recorded by GBIF (2012) is from 1972. 

In Norway the earliest herbarium specimen recorded by GBIF (2012) is from 1921. It is now of increasing concern as it is spreading, perhaps via soil introduced as ballast (Halvorsen and Grøstad, 1998) or in seaweed or seawater after storms (Størmer, 2011).

Introductions

Risk of Introduction

L. latifolium is not traded commercially in horticulture, nor does it occur commonly in crops. Hence risks of long-distance introduction are relatively low. However, it can reproduce by seed which is often accidentally a contaminant of other seeds such as Beta vulgaris (sugar beet). This species can also reproduce via root fragments thereby increasing the risk of introduction into new areas locally.

Habitat

L. latifolium is a plant of wet places, especially coastal saline wetlands, but also non-saline stream-sides, marshes, roadsides, railways, waste ground, ditches and irrigated cropland; also non-irrigated cereal, lucerne, hay and pasture crops (Francis and Warwick, 2007). 

Habitat List

Biology and Ecology

Genetics

Most authors indicate that L. latifolium has a chromosome number of 2n=24 (Missouri Botanical Garden, 2012), but Pogan (1980, in Poland) and Queiros (1979, in Portugal) record tetraploids with 2n=48. 

Reproductive Biology

L. latifolium may spread by seed and also vegetatively via its spreading root system. Seed production is potentially very high but Zouhar (2004) notes that seeds may fail to mature in dry years and under wet conditions they may be damaged by the oomycete, Albugo sp. Leininger and Foin (2009) found that while inflorescence size was not affected by salinity, seed production was much higher in dry, non-saline conditions. Seed production at a high salinity site was reduced by 29% from a freshwater site and seed production at the wettest site in this San Francisco Bay study had an 87% reduction from the driest site.

Plants can self- and cross-pollinate (Brown, 2005; Gaskin et al., 2012). Pollination is believed to occur by insects (Zouhar, 2004). Most authors however comment that germination is rarely observed in the field and most local spread apparently occurs vegetatively.New shoots can arise from anywhere on the undisturbed superficial root system, effectively establishing new plants. After fragmentation by cultivation or by wave or current action, root regeneration can occur from root fragments as small as 2-3 cm long (Wotring et al., 1997). 

Physiology and Phenology

Seed germination requires light and is inhibited by high salinity, though some germination still occurs at 16 dS/m (Larson and Kiemnec, 2005). Germination is generally low at constant temperatures and requires alternating temperatures, anywhere between 0 and 40°C. Ahmed and Khan (2010) found the optimum temperature regime to be alternating between 20/30°C.

Newly-established plants can flower in their first season. Regrowth from existing crowns begins in early spring, with the development of a basal rosette of leaves, followed by flowering shoots. In Europe and North America flowering may occur from May in low-lying coastal areas but later inland, e.g. from August in New England. In the absence of frost, some basal leaves may persist through the winter, but above-ground growth normally dies down in the winter and forms a layer of litter.

Once established, a young colony of L. latifolium may expand by 1-3 m per year as new shoots emerge from the peripheral root system (Zouhar, 2004). However, longer-established patches were found to expand by only 0.85 m per annum (Renz et al., 2012). 

The seasonal flux of photosynthate between roots and shoots has been described in some detail by Renz (2000). L. latifolium has the ability to make available and take up more nitrogen than the vegetation it is replacing (Blank, 2002). 

Longevity

Seeds have no innate dormancy and are not known to persist beyond two years in the soil (Zouhar, 2004). Stands of established L. latifolium have been observed to persist for at least 15 years (Blank et al., 2002).

Environmental Requirements

L. latifolium is a plant of temperate regions, perhaps requiring a cool winter for normal development, with frost-tolerant underground parts. It grows under a wide range of environmental conditions from saline to brackish to fresh and from very wet to quite dry; also in inland alkaline soils. Although it may occur under fully saline conditions it is generally most vigorous in less saline, brackish soils at -0.02 MPa soil matric water potential (Blank et al., 2002). Although thriving in wet conditions and surviving under temporary flooding, L. latifolium is not fully adapted to anaerobic soil conditions and growth is reduced. The plant survives continuous flooding for at least 50 days but photosynthesis is reduced by 60-70% (Chen et al., 2005) and it is eventually killed.

Roots of L. latifolium have metabolically adaptive strategies to anoxia, but there is evidence of oxidative stress under anoxia and of post-anoxic injury from free radicals upon re-exposure to air (Chen et al., 2002; Chen and Qualls, 2003). 

Although mainly a lowland/wetland species it does also occur at high altitudes, up to 2000 m in the USA and in India an ecotype apparently adapted to cold conditions has been the subject of molecular studies (Mohammad Aslam et al., 2010; Mohammad Aslam et al., 2011).

Climate

Latitude/Altitude Ranges

Natural enemies

Notes on Natural Enemies

In its native range, around 100 herbivores were identified from L. latifolium (Gerber et al., 2015). Ceutorhynchus marginellus and Metaculus lepidifolii are currently being studied for their potential as biological control agents.

Means of Movement and Dispersal

Natural Dispersal

Seeds of L latifolium are shed gradually from the pods over a period of months. Dispersal is then largely by water, whether tidal, via river flows or along the coast in seawater. The seeds sink initially in water but then develop a mucilaginous coat which causes them to re-float (Young, 1999). Dispersal of plants can also occur via root fragments. The root system is relatively sparse and does not prevent the erosion of mud in tidal situations and the resultant break-up of root systems (Young et al., 1997). In Norway, local spread is attributed to movement with seaweed or seawater after storms (Størmer, 2011). 

Vector Transmission

Carpinelli et al. (2005) show that the viability of seeds of L. latifolium may be enhanced after 96 hours in the gut of cattle and grazing animals could therefore be a means of dispersal. 

Accidental Introduction

Short-distance dispersal can result from the use of the flowers and seedheads in dry flower arrangement (Washington State Noxious Weed Control Board, 1999). Hay, feed stock, and straw used in stabilisation projects can be contaminated with seed and/or rhizomes and moving dirt or machinery that are contaminated with root fragments can initiate an invasion. Recent localised infestations of L. latifolium in the USA may have been initiated from seed or plant fragments that were contaminants in rice straw bales (ISSG, 2012). In Norway, spread is believed to have occurred via soil introduced as ballast (Halvorsen and Grøstad, 1998).

Intentional Introduction

L. latifolium is not traded commercially in horticulture and no introductions are known to have been intentional.

Impact Summary

Economic Impact

In northeastern California, invasions of L. latifolium moved beyond the irrigated meadows used as winter forage into intensive agricultural crops such as cereal grains and alfalfa, where infestations led to depreciation in land values for affected farms (Young et al., 1997). Zouhar (2004) also notes economic losses through reduced forage quantity and hay quality. On land used for grazing plus hay harvest, the costs of herbicide use (metsulfuron or 2,4-D) were only recouped after 4-5 years, while on land used only for grazing it would take 15 years (Eiswerth et al., 2005).

Environmental Impact

Among impacts listed by Zouhar (2004) are: altered species diversity, structure and function; displaced native species; decreased food and habitat for several wildlife species; changes in biogeochemical cycles; and increased streamside soil erosion. In summary, it has altered species diversity, structure, function, and succession in many wetland and riparian areas in the western USA.

Impact on Habitats

Blank and Young (1997) have shown that L. latifolium can act as a ‘salt pump’ which brings salt ions from deep in the soil profile and deposits them near the surface. This can favour halophytes and put other species at a disadvantage, thereby shifting plant composition and diversity. Conversely, Reynolds and Boyer (2010) recorded lower salinities under L. latifolium compared with those under Sarcocornia pacifica. L. latifolium was also shown to elevate soil solution levels of Mg⁺² and Ca⁺², thereby reducing sodium adsorption ratios that could lead to sodic soil amelioration (Blank and Young, 2002; Blank and Young, 2004). Leonard et al. (1998a; 1998b) demonstrated increased emission of mercury from contaminated soils into the atmosphere. Invasion by L. latifolium thus has the potential to alter soil properties and processes, thereby altering the trajectory of soil evolution. These effects may be exaggerated under elevated carbon dioxide conditions (Blank and Denner, 2004). 

Impact on Biodiversity

L. latifolium is extremely competitive, forming monospecific stands that can crowd out desirable native species and a number of threatened and endangered species. Significant amounts of litter can build up in dense infestations, forming a layer impenetrable to light. This layer prevents the emergence of annual plants in these areas and may reduce competition from other species decreasing the biodiversity in an area. Reports of the impact of L. latifolium on biodiversity include that from the US Fish and Wildlife Service (2010a) noting threats to the endangered thistle Cirsium hydrophilum var. hydrophilum in California from rapid invasion of brackish tidal marsh by the weed which can readily invade both diked and tidal brackish marshes with low salinity, forming dense stands in the better-drained areas where C. hydrophilum is most likely to occur. L. latifolium is especially invasive on physically disturbed soils and where vegetation cover has been reduced, forming a continuous leaf canopy, eliminating the vegetation gaps that may be essential for seedling establishment of C.hydrophilum.

The same report indicates that Sarcocornia pacifica [Salicornia pacifica] (pickleweed) is also reduced by L. latifolium with indirect consequences for the hemi-parasitic Cordylanthus maritimus for which it is an important host; also the salt marsh harvest mouse (Reithrodontomys raviventri). California black rail (Laterallus jamaicensis coturniculus), a threatened bird of California and in the IUCN Red List of Threatened Species, and California clapper rail (Rallus longirostris obsoletus), a threatened bird of California, are also affected via loss of nesting sites in favoured native vegetation (ISSG, 2012). The US Fish and Wildlife Service (2009b) also record threats from L. latifolium to the endangered Solano grass Orcuttia mucronata [Tuctoria mucronata] in California, resulting from shading and changes in water chemistry. 

An inventory of rare and endangered plants in California indicates that L. latifolium is encroaching on several rare plant populations at Grizzly Island Wildlife Area in Suisun Marsh, including soft bird's-beak (Cordylanthus mollis ssp. mollis), Suisun thistle (Cirsium hydrophilum var. hydrophilum) and Suisun Marsh aster (Symphyotrichumlentum) (Zouhar, 2004). This author also refers to detrimental effects on nesting sites for several species of rare and endangered waterfowl. 

Blank (2002) concludes that L. latifolium is an effective competitor due to its ability to make available and take up more nitrogen than the vegetation it is replacing.

Threatened Species

Social Impact

Zouhar (2004) refers to increased difficulty in the control of mosquitoes in Utah, where L. latifolium has become dominant, due to its height.

Risk and Impact Factors

• Proved invasive outside its native range
• Has a broad native range
• Highly adaptable to different environments
• Pioneering in disturbed areas
• Long lived
• Has high reproductive potential
• Gregarious
• Has propagules that can remain viable for more than one year
• Reproduces asexually

• Altered trophic level
• Ecosystem change/ habitat alteration
• Modification of nutrient regime
• Modification of successional patterns
• Monoculture formation
• Negatively impacts agriculture
• Reduced native biodiversity
• Soil accretion
• Threat to/ loss of endangered species
• Threat to/ loss of native species

• Allelopathic
• Competition - monopolizing resources
• Competition - shading
• Competition - smothering

• Difficult to identify/detect as a commodity contaminant
• Difficult to identify/detect in the field
• Difficult/costly to control

Uses

L. latifolium may be grazed by cattle, sheep and goats but is not considered a useful forage (Zouhar, 2004). Carpinelli et al. (2005) caution that the viability of seeds is, if anything, enhanced after 96 hours in the gut of cattle, so grazing animals could be a source of further spread. 

L. latifolium has been widely used medicinally, especially as a diuretic. Wright et al. (2007) concluded that it was among the more effective herbal diuretic preparations. It has been found to have a hypotensive effect due to its diuretic action in rats. The aqueous leaf extract given in doses of 50 and 100 mg/kg through intraperitoneal and oral routes, respectively, produced significant and dose-dependent diuretic and hypotensive activities. Attempts to extrapolate the diuretic action of L. latifolium extracts from rats to man led to the recommended daily dose of 3-5 g L. latifolium extract per man per day, administered as tea, which is equivalent to 43 to 71 mg/kg body weight in a 70 kg subject (Navarro et al., 1994). L. latifolium has also been used as a folk medicine in the Canary Islands for renal lithiasis and six months of oral treatment with a suspension of L. latifolium significantly reduced prostate size and volume in castrated rats where the hyperplasia was induced by steroid treatment (Martínez Caballero et al., 2004). L. latifolium is also used in India both medicinally and as a food (Rana et al., 2012) and in the Ladakh Himalayas there is interest in it as forage (Anju Verma et al., 2008).

Similarities to Other Species/Conditions

Zouhar (2004) notes that none of the Lepidium species native to North America are similar in size or growth habit to L. latifolium. There is a possibility of confusion with L. draba if only because of the overlap of common names, the latter also being known as whitetop and perennial peppergrass. However they are quite distinct in a number of characters and L. draba is a much shorter plant, usually about 50 cm and never exceeding 1 m high.

Prevention and Control

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

SPS measures

L. latifolium is listed as noxious in the US states of Alaska, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Utah, Washington and Wyoming. It is ‘regulated’ in South Dakota, ‘designated’ in Oregon, ‘banned’ in Connecticut and ‘prohibited’ in Massachusetts (USDA-ARS, 2012).

Public awareness

In respect of L. latifolium in Nevada, USA, Donaldson et al. (2002) have suggested a combination of a public awareness campaign (increasing the capacity of the public to identify weeds and encouraging public participation in weed control programmes), mapping and inventory (locating and mapping infested sites using geographic information systems), and demonstration and weed control projects (implementing workshops/projects where control methods are developed and demonstrated).

Control

Cultural Control and Sanitary Measures

L. latifolium is susceptible to continuous flooding and is apparently controlled after two full seasons of flooding (Renz, 2000). 

Physical/Mechanical Control

Mechanical control is generally ineffective because of the ability of the weed to recover from crowns and underground structures, and cultivation may even encourage its spread. Renz et al. (2012) record three times the rate of expansion of L. latifolium patches after disking, compared with undisturbed patches. Only in a pasture situation repeated mowing for hay may achieve some reduction. Disking after an effective herbicide application may help to desiccate surviving roots.

Mowing and cultivation followed by tarping with heavy duty black plastic for two seasons was successful in controlling L. latifolium but was no more effective than mowing followed by glyphosate. Mow-Till-Tarp treatment is extremely time consuming and has the potential to limit native plant community recovery (Hutchinson and Viers, 2011). 

Rood et al. (2010) show that the spread of L. latifolium downstream can be greatly reduced by the building of dams and reservoirs to create ‘reservoir impediments’. 

Biological Control

Extensive field surveys for biological control agents were conducted in the native range of L. latifolium, (China, Kazakhstan, Turkey, southern Russia and more recently in Armenia, Georgia and Iran) identified a total of 67 organisms. Several of these were identified as potential biological control agents for L. latifolium in the USA and were prioritized. These include the shoot-mining flea beetle Phyllotreta reitteri, the root-mining weevil Melanobaris sp. n. pr. semistriata, the gall-forming weevil Ceutorhynchus marginellus, the chloropid stem-mining fly Lasiosinadeviata and the eriophyid mite Metaculus lepidifolii (Hinz et al., 2008). From 2006 onwards, host-specificity tests have been conducted to investigate the host range of these potential biocontrol agents. Tests with Ph. reitteri revealed that this species is not specific enough to be considered further, while work on L. deviata was postponed due the absence of evident impact. Host-specificity tests with M. sp. n. pr semistriata, C. marginellus and M. lepidifolii are ongoing. So far, none of these potential agents have been introduced to the USA.

Chemical Control

The most effective herbicides appear to be chlorsulfuron, metsulfuron methyl and imazapyr applied at the bud stage (Zouhar, 2004) but the persistence of these herbicides may prevent the re-establishment of more desirable species. In one successful example, a low rate of chlorsulfuron applied at 0.01–0.04 kg/ha gave excellent control for a year and desirable grasses established, helping to prevent reestablishment for several years. Renz (2000) also indicates that chlorsulfuron and metsufuron may be selective in favour of some grass species, but use near water may be restricted. Kilbride et al. (1997) achieved near 100% control with combinations of chlorsulfuron or metsulfuron with disking, but encouraged other undesirable weed growth. 

Glyphosate is not fully effective on this species but can provide adequate control when applied to regrowth at flowerbud stage following mowing, disking or hand pulling (Renz, 2002; Boyer and Burdick, 2010) and has the advantage of leaving no soil residues. Renz and DiTomaso (2004; 2006) show excellent results from combinations of mowing, followed by chlorsulfuron or glyphosate. Optimum timing at the flowerbud stage is earlier than for many other perennial weeds, it is thought because at later stages the inflorescences interfere with foliar absorption (Renz, 2000). 

2,4-D has limited effectiveness as a sole treatment, but when combined with winter burning to destroy accumulated litter, summer and autumn mowing, winter grazing, and autumn disking it allowed successful seeding of desirable grass species such as tall wheatgrass (Elytrigia elongata) where other herbicides tended to be too damaging (Wilson et al., 2008). 

On grazing land, the total economic returns from herbicide use (metsulfuron or 2,4-D) did not equal total costs until 15 years after initial treatment. However, on land used for grazing plus hay harvest, cumulative benefits equalled and began to exceed cumulative costs after four to five years (Eiswerth et al., 2005). 

Monitoring and Surveillance

A series of papers have reported on a programme to monitor the spread of L. latifolium in California's Sacramento-San Joaquin River Delta using remote sensing and to develop a model relating spread to a wide range of environmental and other factors, and predict where there is risk of further spread (Andrew and Ustin, 2006; 2008; 2009a, b; 2010). Predictive modelling has also been developed by Vanderhoof et al. (2009) for the San Francisco Bay area (Gillham et al., 2004).

Gaps in Knowledge/Research Needs

Renz (2000) lists a large number of gaps in our knowledge of L. latifolium, including germination behaviour, the regenerative ability of different types of root, understanding the plant’s ability to grow at widely varying soil salinities, the possible existence of biotypes differing in their behaviour, and response to control measures.

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Links to Websites

Contributors

23/03/2016 Updated by:

Sonja Stutz, CABI, Switzerland

Hariet Hinz, CABI Switzerland

10/04/2012 Original text by:

Chris Parker, Consultant, UK

Distribution Maps