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Datasheet

Centaurea debeauxii (meadow knapweed)

Summary

  • Last modified
  • 23 June 2017
  • Datasheet Type(s)
  • Invasive Species
  • Preferred Scientific Name
  • Centaurea debeauxii
  • Preferred Common Name
  • meadow knapweed
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • Centaurea debeauxii, a fertile hybrid of C. nigra and C. jacea, is an invasive perennial of pasture and natural grasslands with an increasing non-native distribution in wet temperate areas of...
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Identity

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Preferred Scientific Name

  • Centaurea debeauxii Godr. & Gren.

Preferred Common Name

  • meadow knapweed

Other Scientific Names

  • Centaurea decipiens subsp. debeauxii (Godr. & Gren.) B.Bock
  • Centaurea jacea nothosubsp. pratensis (W.D.J. Koch) Celak.
  • Centaurea jacea subsp. debeauxii (Godr. & Gren.) Douin
  • Centaurea nemoralis Jord.
  • Centaurea nigra subsp. debeauxii (Godr. & Gren.) Malag.
  • Centaurea nigra subsp. nemoralis (Jord.) Gremli
  • Centaurea pratensis Thuill.
  • Centaurea sessiliflora L.
  • Centaurea sessilifolia Hill
  • Centaurea x moncktonii C.E Britton
  • Jacea debeauxii (Godr. & Gren.) Holub

International Common Names

  • English: chalk knapweed; protean knapweed; slender knapweed
  • French: centaurée de debeaux

Summary of Invasiveness

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Centaurea debeauxii, a fertile hybrid of C. nigra and C. jacea, is an invasive perennial of pasture and natural grasslands with an increasing non-native distribution in wet temperate areas of continental North America, now also recorded in South America and Australia. Favouring mesic and moist situations, it can form dense stands, outcompeting native grasses and displacing broadleaved species. It is listed as a noxious weed in several western US states as well as one Canadian province. In natural and semi-natural plant communities, it threatens rarer endemics such as the rough popcorn flower (Plagiobothrys hirtus). 

Taxonomic Tree

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  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Asterales
  •                         Family: Asteraceae
  •                             Genus: Centaurea
  •                                 Species: Centaurea debeauxii

Notes on Taxonomy and Nomenclature

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Taxonomy of Centaurea is acknowledged to be difficult. There are proposals to split this large genus (Greuter, 2003; Hellwig, 2004).

Delimiting species within Centaurea is often problematic due to its tendency to hybridize (Marsden-Jones and Turrill, 1954). Centaurea debeauxii Godr. & Gren. is a nothospecies which is a fertile hybrid of Centaurea jacea L. and Centaurea nigra L., but all three taxa might best be treated as part of a C. jacea complex (Gardou, 1972).

Several subspecies of Centaurea have been described (Dostál, 1976). A discussion about the delimitation of Centaurea species can be found in Keil and Ochsmann (2006).

Following Stace (2010), C. debeauxii is addressed as a species in this datasheet; as Stace notes, the common intermediates between C. debeauxii and C. nigra are nonetheless troublesome, and may be hybrids or just “an indication that the two taxa are not separate species”.

While the characters differentiating C. debeauxii from C. jacea and C. nigra might be challenging to use, and while populations of C. debeauxii show a wide range of morphological variation, evidence suggests that this hybrid occupies a somewhat different niche to either of the parents - in particular, wet meadows and riparian habitats (Roché and Roché, 1991). In terms of understanding its ecology and potential invasiveness, there is merit in discussing C. debeauxii separately, whether or not taxonomic opinion ultimately upholds it as a species in its own right.

Within the C. jacea aggregate a further difficulty concerns Centaurea nigrescens Willd. This taxon is closely related and often recorded as an introduced species in the northern American flora; like some other taxa in this complex, it is debatable whether it should be recognized at the rank of species. Koutecký (2009) provides context for the taxonomic status of central European species and subspecies.

In terms of higher-level relationships in the genus, phylogenetic reconstruction supports the view that C. jacea and C. nigra (and presumably C. debeauxii, although it is not directly represented in the study) are closely related to western Mediterranean species of section Phrygia (syn. Lepteranthus) (Garcia-Jacas, 2006). The authors suggest this clade (“Jacea-Lepteranthus”) should be treated as a single section. Lopez-Alvarado et al. (2014) discuss limitations of phylogenetic studies in Centaurea.

Nomenclature is perhaps more straightforward than taxonomy for C. debeauxii. Published in Grenier and Godron’s Flore de France (1850) it is differentiated from C. nigra, drawing attention to bract characters particularly. C. debeauxii has priority over C. moncktonii C.E. Britton, although the latter synonym is often found in literature and is used (following Stace, 1991) by Keil and Ochsmann (2006) and is an accepted name in The Plant List (2013).

Description

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C. debeauxii is an erect perennial herbaceous plant growing to 80 cm tall. Young plants have a strong tap root which may develop into a tough crown of several roots as plants mature. The basal rosette leaves are entire to deeply lobed and slightly hairy; stem leaves are small and without a stalk (sessile). Growth is openly branching, stems are slender and somewhat rough to the touch.

Each capitulum (flowerhead) is covered on the outside by scale-like rows of involucral bracts (phyllaries). The bracts of C. debeauxii are intermediate between its hybrid parents. The ornamentation on the edges of its bracts may be brown or tan in colour, and do not overlap the neighbouring bracts so as to obscure them fully (Stace, 2010). The central portion of the bracts in C. debeauxii is lanceolate (more or less straight-sided and narrowly triangular) (Stace, 2010). Bract ornamentations in C. debeauxii are somewhat papery (Roché et al., 2003). 

The uppermost portion of flowering stems, just below the capitulum, is slightly swollen in C. debeauxii (Stace, 2010). Capitula may be rayed or rayless.

Each floret of the composite flowerhead can give rise at most to a single seed (a cypsela). Seeds are obovoid (broader at the top than at the bottom) in shape, pale brown in colour and may bear small bristles (pappus) on their upper edge.

Plant Type

Top of page Broadleaved
Herbaceous
Perennial
Seed propagated

Distribution

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In its native range, C. debeauxii occurs from western to central Mediterranean Europe. As this hybrid can arise spontaneously wherever C. nigra and C. jacea occur together, and as it is morphologically quite similar to C. nigra and C. nigrescens, it may be under-recorded.

With regard particularly to its US distribution, C. nigrescens and C. debeauxii seem to have been treated ambiguously in USDA-NRCS (2015) distribution maps: C.debeauxii (referred to as Centaurea × moncktonii [jacea × nigra] in this resource) is only pinpointed in a few states (many fewer than the range indicated in other sources) while C. nigrescens is shown occurring in 27 states. Given the overall similarity of these taxa and long history of introduction, it seems warranted to amalgamate USDA distribution data for C. nigrescens and C. × moncktonii. Accordingly, in the Distribution Table, C. debeauxii is shown for all the states where either of these taxa is recorded.

C. debeauxii is considered a noxious weed in Alberta and British Columbia, Canada (Meidinger et al., 2004) and Washington, Idaho, Oregon and Colorado, USA (Keil and Ochsmann, 2006, diTomaso, 2013) and is expanding rapidly in the Pacific Northwest (Roché and Roché, 1991).

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 ReportedInvasiveReferenceNotes

North America

Canada
-British ColumbiaPresentIntroduced Invasive Meidinger et al., 2004; USDA-NRCS, 2015
-Nova ScotiaPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea × moncktonii
-OntarioPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea × moncktonii
-QuebecPresentIntroducedUSDA-NRCS, 2015Recorded as synonym Centaurea × moncktonii
USA
-CaliforniaPresentIntroduced Invasive DiTomaso and Kyser, 2013; USDA-NRCS, 2015
-ColoradoPresentIntroduced Invasive DiTomaso and Kyser, 2013
-ConnecticutPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea × moncktonii
-DelawarePresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-IdahoPresentIntroduced Invasive DiTomaso and Kyser, 2013; USDA-NRCS, 2015
-IllinoisPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-IndianaPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-MainePresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-MarylandPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-MassachusettsPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea × moncktonii
-MichiganPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-MissouriPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-MontanaPresentIntroducedDiTomaso and Kyser, 2013; USDA-NRCS, 2015
-NebraskaPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-New JerseyPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-New YorkPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea × moncktonii
-North CarolinaPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea × moncktonii
-OhioPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-OregonPresentIntroduced Invasive DiTomaso and Kyser, 2013; USDA-NRCS, 2015
-PennsylvaniaPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-Rhode IslandPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-VermontPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-VirginiaPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-WashingtonPresentIntroduced Invasive DiTomaso and Kyser, 2013; USDA-NRCS, 2015
-West VirginiaPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-WisconsinPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens
-WyomingPresentIntroducedUSDA-NRCS, 2015Recorded as Centaurea nigrescens

South America

UruguayPresentIntroducedOrfeo and Bancheva, 2006Naturalized

Europe

BelgiumPresentNative Not invasive Tutin et al., 1976
Czechoslovakia (former)PresentNative Not invasive Tutin et al., 1976
FrancePresentNative Not invasive Tutin et al., 1976
-CorsicaPresentNative Not invasive Tutin et al., 1976
GermanyPresentNative Not invasive Tutin et al., 1976; Otto et al., 2015
ItalyPresentNative Not invasive Tutin et al., 1976
NetherlandsPresentNative Not invasive Tutin et al., 1976
SpainPresentNative Not invasive Tutin et al., 1976
UKPresentNative Not invasive Stace, 2010Southern England, not Scotland

Oceania

Australia
-South AustraliaPresentIntroducedKellermann et al., 2005
-TasmaniaPresentIntroducedKellermann et al., 2005
-VictoriaPresentIntroducedKellermann et al., 2005

History of Introduction and Spread

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C. debeauxii has probably been introduced to North America multiple times (Keil and Ochsmann, 2006), perhaps in shipping ballast and perhaps bringing the full genetic payload of a hybrid swarm (Susanna and Roché, 2011). C. jacea was introduced originally as a forage crop, while C. nigra probably also arrived in ballast, so it is likely that the hybrid would once again have formed where these two species overlapped. C. debeauxii is recorded by the early nineteenth century in Pacific Northwest America (Roché and Talbott, 1986).

While C. jacea and C. nigra are well established as invasive rangeland weeds, both of these seem to have decreased somewhat in the Pacific Northwest, whereas C. debeauxii has been increasing rapidly (Roché and Roché, 1991).

Introductions

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Introduced toIntroduced fromYearReasonIntroduced byEstablished in wild throughReferencesNotes
Natural reproductionContinuous restocking
USA Europe 1800-1900 Forage (pathway cause) Yes Keil and Ochsmann, 2006 Multiple introductions

Risk of Introduction

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There is some risk of long-distance accidental introduction where seed contaminates other agricultural crops (included in seed crops such as lucerne).

Local accidental introduction may occur where C. debeauxii seed, or ripening capitula, which are capable of releasing viable seed, are transported in hay.

C. debeauxii is not known to have been deliberately introduced as an ornamental plant (unlike C. nigra).

Habitat

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Highly favourable sites for this perennial species are those where a mowing or grazing regime allows plant to flower and set seed but prevents establishment of trees and species which would later shade out C. debeauxii.

Habitat List

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CategoryHabitatPresenceStatus
Terrestrial-managed
Cultivated / agricultural land Present, no further details Harmful (pest or invasive)
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) Principal habitat Harmful (pest or invasive)
Rail / roadsides Present, no further details Harmful (pest or invasive)
Terrestrial-natural/semi-natural
Natural forests Present, no further details Harmful (pest or invasive)
Natural grasslands Principal habitat Harmful (pest or invasive)
Riverbanks Present, no further details Harmful (pest or invasive)
Wetlands Present, no further details Harmful (pest or invasive)

Host Plants and Other Plants Affected

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Plant nameFamilyContext
Plagiobothrys hirtus (rough popcorn flower)Unknown

Growth Stages

Top of page Flowering stage, Fruiting stage, Seedling stage, Vegetative growing stage

Biology and Ecology

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Genetics

C. debeauxii is tetraploid, 2n=44 (Bancheva and Greilhuber, 2006); though it is noted that diploid counts have been published for some North American material. It easily backcrosses with C. nigra and C. jacea, which may be either diploid or tetraploid (Keil and Ochsmann, 2006).

An infertile hybrid, C. x kleinii, is reported with the highly invasive C. solstitialis. C. debeauxii may also hybridize with C. diffusa (Susanna and Roché, 2011).

Reproductive Biology

The insect-pollinated, composite flowerheads of C. debeauxii are made up of 40-100 individual flowers (Keil and Ochsmann, 2006). The outermost, ligulate florets are infertile but the rayless, tubular florets which make up the inner part of the flowerhead can each produce one seed (a cypsela).

Dispersal occurs as seed falls near the parent plant or is transported a short distance by wind, or by water in streamside habitats (Roché et al., 2003). Seeds are relatively heavy and although they may possess a pappus (short bristles), they are not adapted to wind dispersal like a dandelion’s. Some animal (including bird) dispersal may occur (diTomaso, 2013).

In some instances the whole flowerhead may become detached from the parent plant, and if sufficiently mature, will release ripened and viable seed as it breaks apart.

Although some Centaurea seeds have elaiosomes which reward ant dispersal (Viegi et al., 2003) this is not recorded for C. debeauxii.

Physiology and Phenology

The natural distribution of C. debeauxii suggests that it has less physiological tolerance of cold than its parents C. jacea and C. nigra, both of which are found as far north as Scandinavia and Finland. An alternative explanation may be that parts of its range are not fully recorded.

In its native range C. debeauxii is often encountered as a plant of pastures and meadows: the species is quite tolerant of grazing (or mowing several times a year) and will easily produce flowers and seed on short stems near ground level where it cannot grow to full height (Miller and Lucero, 2014).

Associations

C. debeauxii is an early successional plant which can colonize disturbed ground, forest clearings and ruderal habitats.

In its native range it would typically be associated with other chalk grassland species. 

Climate

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ClimateStatusDescriptionRemark
Cw - Warm temperate climate with dry winter Preferred Warm temperate climate with dry winter (Warm average temp. > 10°C, Cold average temp. > 0°C, dry winters)

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Agapeta zoegana Herbivore Seeds USA
Bangasternus fausti Herbivore Seeds USA
Larinus minutus Herbivore Seeds USA
Larinus obtusus Herbivore Seeds USA
Metzneria paucipunctella Herbivore Seeds USA
Urophora quadrifasciata Herbivore Seeds USA, Canada

Means of Movement and Dispersal

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Natural Dispersal

Seed may be blown a short distance by wind or fall near the parent plant. Quite frequently, the whole capitulum functions as a dispersal unit when it breaks off and is carried by wind (Roché et al., 1992).

Irrigation canals and watercourses allow waterborne dispersal (diTomaso, 2013).

Vector Transmission

diTomaso (2013) suggests that seeds are dispersed by animals, including birds. Capitula may also adhere to animal fur (Roché et al., 1992) but C. debeauxii is not highly adapted to this.

Accidental Introduction

Seed may be transported on vehicle tyres or tracks, with crop seed, in gravel, and by other accidental means (Roché et al., 1992).

A much less significant form of accidental dispersal may occur where shallow ploughing breaks up and transports fragments of rosettes with enough root to re-establish elsewhere. This is not a major means of dispersal but is to be taken into consideration when preparing ground with agricultural machinery (diTomaso, 2013).

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Land vehicles Yes
Machinery and equipment Yes
Mulch, straw, baskets and sodin hay Yes
Plants or parts of plantsInfrequent – as possible contaminant of commercial agricultural seed Yes
Soil, sand and gravel Yes Yes
Water Yes
Wind Yes

Impact Summary

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CategoryImpact
Economic/livelihood Negative
Environment (generally) Negative

Economic Impact

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Although introduced knapweed species have serious economic impacts on US and Canadian agriculture, affecting millions of acres of rangelands, evaluations have not yet been carried out specifically for C. debeauxii.

On agricultural land, C. debeauxii diminishes fodder value and may form dense monotypic stands, displacing more desirable pasture crops, including some grasses. Although the species does have some limited value as fodder (and in the past was occasionally grown as a crop for this purpose) it becomes less palatable to cattle as plants reach flowering maturity (Roché and Roché, 1991).

With regard to its native range, it is worth noting that forage yield and “pastoral value” may underrepresent the nutritional value of broadleaved herbs in forage (Reiné et al., 2014); these measures may tend to overemphasize the economic value of uniform hay crops.

In drier rangelands it is less of an established problem than other knapweed species such as C. jacea.

Environmental Impact

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Impact on Habitats

While the presence of C. debeauxii does not have significant impact on wider ecosystem processes, it can form very large stands, particularly in wet meadows, outcompeting some native flora.

Impact on Biodiversity

Along with another introduced European herbaceous species, the common teasel (Dipsacus fullonum), C. debeauxii has a serious impact, through competition, on the endangered rough popcorn flower (Plagiobothrys hirtus), a small annual and US endemic species restricted to a few wet meadow sites in Oregon. Both C. debeauxii and D. fullonum invade key seasonally wet meadows and vernal pools at the known localities for P. hirtus.

In the Pacific Northwest, its distribution is expanding fast in damp meadows and riparian habitats (Roché and Roché, 1991).

The evidence that C. debeauxii is a more successful invader of damp habitats than either of the parents, C. jacea and C. nigra, is particularly important to note with regard to future biodiversity impacts and likely further range expansion. 

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Plagiobothrys hirtus (rough popcorn flower)NatureServe; USA ESA listing as endangered speciesUSA/OregonCompetition; Competition - smotheringUSFWS, 2010

Risk and Impact Factors

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Impact mechanisms

  • Competition
  • Competition - monopolizing resources
  • Competition - shading
  • Competition - smothering
  • Herbivory/grazing/browsing
  • Rapid growth

Impact outcomes

  • Ecosystem change/ habitat alteration
  • Monoculture formation
  • Negatively impacts agriculture
  • Negatively impacts livelihoods
  • Threat to/ loss of endangered species
  • Threat to/ loss of native species

Invasiveness

  • Benefits from human association (i.e. it is a human commensal)
  • Gregarious
  • Has a broad native range
  • Has high genetic variability
  • Has propagules that can remain viable for more than one year
  • Is a habitat generalist
  • Pioneering in disturbed areas
  • Proved invasive outside its native range
  • Tolerant of shade
  • Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc

Likelihood of entry/control

  • Difficult to identify/detect as a commodity contaminant
  • Difficult to identify/detect in the field
  • Highly likely to be transported internationally accidentally

Similarities to Other Species/Conditions

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Although characters are intermediate between the two hybrid parents, C. jacea and C. nigra, confusion with C. nigra is more likely. Photographs showing flowers and involucral bracts of all three species are available in Roché et al. (2003); diagnostic drawings of bracts can be found in Stace (2010).

C. nigra has black, clearly divided, comb-like ornamentations (pectinations) on the edges of bracts, overlapping and largely covering up the central portion of their neighbours, whereas the ornamentation on the edges of bracts of C. debeauxii is less divided, the bracts may be brown or tan in colour, and do not overlap the neighbouring bracts so as to obscure them fully (Stace, 2010).

The shape of the central portion of bracts also differs between C. nigra and C. debeauxii: the central part of the bracts of C. nigra is ovate, whereas it is lanceolate (more or less straight-sided and narrowly triangular) in C. debeauxii (Stace, 2010). Bract ornamentations in C. debeauxii are somewhat papery, like those of C. jacea (Roché et al., 2003). 

The uppermost portion of flowering stems, just below the capitulum, is quite swollen in C. nigra and only slightly swollen in C. debeauxii (Stace, 2010) and the basal rosette leaves of C. debeauxii are often more deeply lobed than those of C. nigra.

Prevention and Control

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Cultural Control and Sanitary Measures

Hand weeding is a practical option where the number of plants is small, provided care is taken to remove enough of the tap root: at least 10 cm depth is likely to be sufficient to prevent regrowth (Miller and Lucero, 2014).

Repeated cultivation will gradually eliminate C. debeauxii altogether. Cultivation followed by a “cleanup” crop of hay or a green manure (which may also be sprayed to control remaining C. debeauxii), followed by re-seeding with permanent forage species, is a viable option on agricultural land (DiTomaso, 2013).

Physical/Mechanical Control

Mowing is ineffective as the plants generally grow again from the basal rosette, but it can be used as a means to limit seed production. It does not significantly improve response to chemical control (Miller and Lucero, 2014).

Chemical Control

Selective broadleaf herbicides (aminopyralid, clopyralid) with residual activity are the most effective chemical means of control (Miller and Lucero, 2014). Where large areas are involved, spraying is very effective in reducing C. debeauxii without significant damage to monocots such as pasture grasses. Grass crop biomass is increased and weed biomass successfully reduced through treatments; both chemicals can be used pre-emergence as well as on growing plants (DiTomaso et al., 2013). Optimal control is achieved with applications when plants are growing rapidly, from rosette to bolting stage, but treating regrowth in autumn is also effective.

As most Asteraceae and Fabaceae are highly sensitive to pyridines, care should be taken over potential adverse impacts on other species (DiTomaso et al., 2013). Residual activity can also mean that where livestock are fed on treated hay, pyridines persist in farmyard manure and may later damage susceptible crops (House and Pool, 2009).

Treatments using 2,4-D and dicamba are less successful (Miller and Lucero, 2014) but may be used as spot treatments (DiTomaso et al., 2013).

Non-selective systemic glyphosate can be used to control C. debeauxii effectively. Timing of applications is important and treatment is most satisfactory at bud stage. Application with a weed-wiper may be the best method in small areas: where a sprayer is used, patches of dead vegetation have to be re-seeded as these can otherwise be re-colonized from the seedbank (DiTomaso et al., 2013).  

Biological Control

With respect to the complex of knapweed species affecting rangelands in continental North America, there is a fairly long-established body of knowledge on biological control methods, which started in the USA with the importation of two species of gall and seedhead flies, Urophora affinis and U. quadrifasciata, in the 1960s (Wilson and Randall, 2005).

Further introductions have since taken place. Phytophagous insects which will use C. debeauxii as a host plant can be found in the Natural Enemy table (Wilson and Randall, 2005). Other possible insect natural enemies have been trialled/are being evaluated in the USA and Canada (Winston et al., 2014).

Currently, the most effective insect biological control agents for C. debeauxii are Urophora quadrifasciata (fly), Larinus minutus (weevil) and Metzneria paucipunctella (moth) (DiTomaso et al., 2013). L. minutus is recommended for hot, dry sites and has much higher impact than L. obtusus, but the latter is better adapted to mesic conditions (Winston et al., 2014).

Generally, two or more of these biological control agents have to be present to achieve partial control of C. debeauxii populations (Wilson and Randall, 2005); when used in conjunction, they can significantly reduce seed-set and vigour.

A detailed overview of these insects as control agents, their identification in the field, and the stages at which they will attack knapweeds is provided in Wilson and Randall (2005).

IPM

Biological control agents, even when established in C. debeauxii populations, are not currently enough to halt the spread of this species (Wilson and Randall, 2005). As mowing and grazing have little, if any, effect on the reproductive success of this meadow plant, there is likely to be a role for chemical herbicide treatment both in conservation sites and on agricultural land where C. debeauxii has invaded. A full IPM programme for managing C. debeauxii has not yet been elaborated.

Gaps in Knowledge/Research Needs

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Distribution data are likely to be incomplete due to under-recording.

There is increasing knowledge of the effectiveness of biological control agents but further evaluation would be beneficial. As yet, there does not appear to have been any investigation into use of pathogenic fungi to control C. debeauxii, although the rust fungus Puccinia jaceae and P. jaceae var. solstialis have been trialled against Centaurea diffusa and C. solstitialis (Fisher, 2008). 

References

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Bancheva S; Greilhuber J, 2006. Genome size in Bulgarian Centaurea sl (Asteraceae). Plant Systematics and Evolution, 257:95-117.

DiTomaso JM; Kyser GB, 2013. Weed Control in Natural Areas in the Western United States. Weed Research and Information Center, University of California, 544 pp. http://wric.ucdavis.edu/information/natural%20areas/wr_C/Centaurea_debeauxii.pdf

Dostal J, 1976. Centaurea L. In: Flora Europaea, volume 4. Cambridge, UK: Cambridge University Press, 254-301.

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14/06/2015 Original text by:

Rupert Jensen, Consultant, UK

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