Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Medicago polymorpha
(bur clover)



Medicago polymorpha (bur clover)


  • Last modified
  • 06 November 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Host Plant
  • Preferred Scientific Name
  • Medicago polymorpha
  • Preferred Common Name
  • bur clover
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • Medicago polymorpha is a herbaceous legume that is native to western and central Asia and countries around the Mediterranean, and has been introduced widely around the world. It is found in particular in region...

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Medicago polymorpha (bur clover); whole plant, on gravel substrate, with many burrs present.
CaptionMedicago polymorpha (bur clover); whole plant, on gravel substrate, with many burrs present.
Copyright©Trevor James/Hamilton, New Zealand-2005
Medicago polymorpha (bur clover); whole plant, on gravel substrate, with many burrs present.
HabitMedicago polymorpha (bur clover); whole plant, on gravel substrate, with many burrs present.©Trevor James/Hamilton, New Zealand-2005
Medicago polymorpha (bur clover); plant growing on gravel substrate.
CaptionMedicago polymorpha (bur clover); plant growing on gravel substrate.
Copyright©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); plant growing on gravel substrate.
HabitMedicago polymorpha (bur clover); plant growing on gravel substrate.©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); leaves and flowers.
TitleLeaves and flowers
CaptionMedicago polymorpha (bur clover); leaves and flowers.
Copyright©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); leaves and flowers.
Leaves and flowersMedicago polymorpha (bur clover); leaves and flowers.©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); cluster of flowers.
TitleCluster of flowers
CaptionMedicago polymorpha (bur clover); cluster of flowers.
Copyright©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); cluster of flowers.
Cluster of flowersMedicago polymorpha (bur clover); cluster of flowers.©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); stipule at base of leaf.
CaptionMedicago polymorpha (bur clover); stipule at base of leaf.
Copyright©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); stipule at base of leaf.
StipuleMedicago polymorpha (bur clover); stipule at base of leaf.©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); bur, seed pod.
TitleBur, seed pod
CaptionMedicago polymorpha (bur clover); bur, seed pod.
Copyright©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); bur, seed pod.
Bur, seed podMedicago polymorpha (bur clover); bur, seed pod.©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); bur, seed pod, lateral view.
TitleBur, seed pod
CaptionMedicago polymorpha (bur clover); bur, seed pod, lateral view.
Copyright©Trevor James/Hamilton, New Zealand-2014
Medicago polymorpha (bur clover); bur, seed pod, lateral view.
Bur, seed podMedicago polymorpha (bur clover); bur, seed pod, lateral view.©Trevor James/Hamilton, New Zealand-2014


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Preferred Scientific Name

  • Medicago polymorpha L.

Preferred Common Name

  • bur clover

Other Scientific Names

  • Medicago apiculata Willd.
  • Medicago denticulata Willd.
  • Medicago hispida Gaertn.
  • Medicago lappacea Desr.
  • Medicago nigra (L.) Krock.
  • Medicago polycarpa Willd.
  • Medicago reticulata Benth.

International Common Names

  • English: bur medic; bur trefoil; burclover; burr medic; burr medick; hairy medic; rough medic; rough medick; toothed bur-clover; toothed medic; trefoil-clover
  • Spanish: carreton de amores; trébol de carretilla; trefol de carretilla
  • French: luzerne hérissée; luzerne hispide
  • Arabic: Nafal
  • Chinese: nan mu xu
  • Portuguese: carrapico

Local Common Names

  • Australia: bur medick
  • Bolivia: alfalfilla
  • Colombia: rodajilla
  • Germany: Borsten-Schneckenklee; Gezähnter Schneckenklee; Rauhe Luzerne; Rauher Schneckenklee; Steifhaariger Schneckenklee
  • Honduras: caretilla
  • Italy: medica ispida
  • Japan: umagoyashi; uma-goyashi
  • Korea, Republic of: gaejari
  • Netherlands: ruiger rupsklaver
  • Nicaragua: cadillo de vaca
  • Sweden: tagglusern
  • UK: hairy medick; toothed medick
  • USA: California burclover; California bur-clover

EPPO code

  • MEDPO (Medicago polymorpha)

Summary of Invasiveness

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Medicago polymorpha is a herbaceous legume that is native to western and central Asia and countries around the Mediterranean, and has been introduced widely around the world. It is found in particular in regions with a Mediterranean climate, but is by no means confined to them. Introduction has been a result of accidental transport of the spiny seed pods and probably also of deliberate introduction as a fodder plant; the relative importance of these two means of spread cannot be determined. The species can be a useful pasture plant, in particular because of its nitrogen-fixing ability, but in other places it is considered to be an invasive weed; it can sometimes be toxic to livestock, and the seed pods can be a serious contaminant of wool.

Taxonomic Tree

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

Notes on Taxonomy and Nomenclature

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M. polymorpha has been given several different botanical names over the years, M. denticulata Willd. and M. hispida Gaertn. being two of the more common. In a few countries like New Zealand the preferred name is M. nigra (L.), following the treatment by Lesins and Lesins (1979), quoted in Webb et al. (1998). Many different cultivars have been named: plant breeders show most interest in Medicago polymorpha var. brevispina, distinguished by the lack of hooked spines on the burs. This means it is much less likely to become entangled in and downgrade wool.


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The following description is from eFloras (2013):

‘Annual or biennial herbs, 20-90 cm. Stems prostrate or ascending, branched at base, subquadrangular, glabrescent. Stipules ovate-oblong, 4-7 mm, base auriculate, margin irregularly laciniate or deeply incised, apex acuminate; petioles long and thin, 1-5 cm; leaflets obovate or triangular-obovate, 7-20 × 5-15 mm, papery, sparsely hairy abaxially, glabrous adaxially, base broadly cuneate, margin shallowly serrate in apical 1/3, apex obtuse, truncate, or emarginate, apiculate. Flowers (1 or) 2-10 in axillary racemes; peduncles slender, 3-15 mm, usually longer than leaves; pedicel less than 1 mm. Corolla yellow, 3-4 mm; standard obovate, emarginate. Legume ash-green to greenish brown, discoid, 4-6(-10) mm, tightly coiled in 1.5-2.5(-6) spirals, turning clockwise, radial veins connected near edge on coil face, spines or tubercles 15 in each row. Seed brown, reniform, ca. 2.5 × 1.25 mm, smooth.’

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


AfghanistanPresentNativeUSDA-ARS, 2013
AzerbaijanPresentNativeUSDA-ARS, 2013
ChinaPresentPresent based on regional distribution.
-AnhuiPresentIntroducedeFloras, 2013
-FujianPresentIntroducedeFloras, 2013
-GansuPresentIntroducedeFloras, 2013
-GuangdongPresentIntroducedeFloras, 2013
-GuangxiPresentIntroducedeFloras, 2013
-GuizhouPresentIntroducedeFloras, 2013
-HainanPresentIntroducedeFloras, 2013
-Hong KongPresentIntroducedPIER, 2013
-HubeiPresentIntroducedeFloras, 2013
-HunanPresentIntroducedeFloras, 2013
-JiangsuPresentIntroducedeFloras, 2013
-JiangxiPresentIntroducedeFloras, 2013
-ShaanxiPresentIntroducedeFloras, 2013
-SichuanPresentIntroducedeFloras, 2013
-YunnanPresentIntroducedeFloras, 2013
-ZhejiangPresentIntroducedeFloras, 2013
East TimorPresentIntroducedCouncil of Heads of Australasian Herbaria, 2013
Georgia (Republic of)PresentNativeUSDA-ARS, 2013
IndiaPresentIntroducedUSDA-ARS, 2013
IranPresentNativeUSDA-ARS, 2013
IraqPresentNativeUSDA-ARS, 2013
IsraelPresentNativeUSDA-ARS, 2013
JapanPresentIntroducedPIER, 2013
JordanPresentNativeUSDA-ARS, 2013
KazakhstanPresentNativeUSDA-ARS, 2013
KyrgyzstanPresentNativeUSDA-ARS, 2013
LebanonPresentNativeUSDA-ARS, 2013
PakistanPresentIntroducedUSDA-ARS, 2013
Saudi ArabiaPresentNativeUSDA-ARS, 2013
SyriaPresentNativeUSDA-ARS, 2013
TajikistanPresentNativeUSDA-ARS, 2013
TurkeyPresentNativeCimalová, 2012; USDA-ARS, 2013
TurkmenistanPresentNativeUSDA-ARS, 2013
United Arab EmiratesPresentNativeUSDA-ARS, 2013
UzbekistanPresentNativeUSDA-ARS, 2013
VietnamPresentIntroducedUSDA-ARS, 2013
YemenPresentNativeUSDA-ARS, 2013


AlgeriaPresentNativeUSDA-ARS, 2013
EgyptPresentNativeUSDA-ARS, 2013
EthiopiaPresentMengistu, 1987Quite frequent above 1700m
LibyaPresentNativeUSDA-ARS, 2013
MoroccoPresentNativeUSDA-ARS, 2013
South AfricaPresentIntroducedUSDA-ARS, 2013
-Canary IslandsPresentNativeUSDA-ARS, 2013
TunisiaPresentNativeUSDA-ARS, 2013

North America

CanadaPresentPresent based on regional distribution.
-British ColumbiaPresentIntroducedUSDA-ARS, 2013
-New BrunswickPresentIntroducedUSDA-ARS, 2013
-OntarioPresentIntroducedUSDA-ARS, 2013
-QuebecPresentIntroducedUSDA-ARS, 2013
-SaskatchewanPresentIntroducedUSDA-ARS, 2013
USAPresentPresent based on regional distribution.
-AlabamaPresentIntroducedUSDA-NRCS, 2013
-AlaskaPresentIntroducedUSDA-NRCS, 2013
-ArizonaPresentIntroducedUSDA-NRCS, 2013
-ArkansasPresentIntroducedUSDA-NRCS, 2013
-CaliforniaPresentIntroducedUSDA-NRCS, 2013
-ConnecticutPresentIntroducedUSDA-NRCS, 2013
-FloridaPresentIntroducedUSDA-NRCS, 2013
-GeorgiaPresentIntroducedUSDA-NRCS, 2013
-HawaiiPresentIntroduced Invasive PIER, 2013; USDA-NRCS, 2013Cultivated and invasive
-IdahoPresentIntroducedUSDA-NRCS, 2013
-LouisianaPresentIntroducedUSDA-NRCS, 2013
-MainePresentIntroducedUSDA-NRCS, 2013
-MassachusettsPresentIntroducedUSDA-NRCS, 2013
-MichiganPresentIntroducedUSDA-NRCS, 2013
-MississippiPresentIntroducedUSDA-NRCS, 2013
-MissouriPresent, few occurrencesIntroducedUSDA-NRCS, 2013
-MontanaPresentIntroducedUSDA-NRCS, 2013
-NevadaPresentIntroducedUSDA-NRCS, 2013
-New JerseyPresentIntroducedUSDA-NRCS, 2013
-New MexicoPresentIntroducedUSDA-NRCS, 2013
-New YorkPresentIntroducedUSDA-NRCS, 2013
-North CarolinaPresentIntroducedUSDA-NRCS, 2013
-OhioPresentIntroducedUSDA-NRCS, 2013
-OklahomaPresentIntroducedUSDA-NRCS, 2013
-OregonPresentIntroducedUSDA-NRCS, 2013
-PennsylvaniaPresentIntroducedUSDA-NRCS, 2013
-Rhode IslandPresentIntroducedUSDA-NRCS, 2013
-South CarolinaPresentIntroducedUSDA-NRCS, 2013
-TennesseePresentIntroducedUSDA-NRCS, 2013
-TexasPresentIntroducedUSDA-NRCS, 2013
-UtahPresentIntroducedUSDA-NRCS, 2013
-VermontPresentIntroducedUSDA-NRCS, 2013
-VirginiaPresentIntroducedUSDA-NRCS, 2013
-WashingtonPresentIntroducedUSDA-NRCS, 2013
-WyomingPresentIntroducedUSDA-NRCS, 2013

Central America and Caribbean

Costa RicaPresentIntroducedUSDA-ARS, 2013
GuatemalaPresentIntroducedUSDA-ARS, 2013

South America

ArgentinaPresentIntroducedUSDA-ARS, 2013
BrazilPresentIntroducedUSDA-ARS, 2013
ChilePresentIntroducedUSDA-ARS, 2013
EcuadorPresentIntroducedUSDA-ARS, 2013
PeruPresentIntroducedUSDA-ARS, 2013
UruguayPresentIntroducedUSDA-ARS, 2013


AlbaniaPresentNativeUSDA-ARS, 2013
AustriaPresentNativeUSDA-ARS, 2013
BelgiumPresentIntroducedUSDA-ARS, 2013Possibly introduced
BulgariaPresentNativeUSDA-ARS, 2013
CyprusPresentNativeUSDA-ARS, 2013
Czech RepublicPresent, few occurrencesIntroducedCimalová, 2012
Czechoslovakia (former)PresentIntroducedUSDA-ARS, 2013Possibly introduced
DenmarkPresentIntroduced Not invasive NOBANIS, 2013Rare
FrancePresentNativeUSDA-ARS, 2013
-CorsicaPresentNativeUSDA-ARS, 2013
GermanyPresentNativeUSDA-ARS, 2013
GreecePresentNativeUSDA-ARS, 2013
HungaryPresentIntroducedUSDA-ARS, 2013Possibly introduced
ItalyPresentNativeUSDA-ARS, 2013
NetherlandsPresentIntroducedUSDA-ARS, 2013Possibly introduced
PortugalPresentNativeUSDA-ARS, 2013
-AzoresPresentIntroducedUSDA-ARS, 2013Possibly introduced
-MadeiraPresentNativeUSDA-ARS, 2013
RomaniaPresentNativeUSDA-ARS, 2013
Russian FederationPresentNativeUSDA-ARS, 2013
-Southern RussiaPresentNativeAustralian Weeds Committtee, 2013
SpainPresentNativeUSDA-ARS, 2013
SwedenPresentIntroduced Not invasive NOBANIS, 2013Rare
SwitzerlandPresentIntroducedUSDA-ARS, 2013Possibly introduced
UKPresentIntroducedClapham et al., 1962Localised in coastal Eastern and Southern areas
Yugoslavia (former)PresentNativeUSDA-ARS, 2013


AustraliaPresentIntroduced Invasive Australian Weeds Committtee, 2013
-Australian Northern TerritoryPresentIntroduced Invasive Council of Heads of Australasian Herbaria, 2013
-Lord Howe Is.PresentIntroduced Invasive PIER, 2013
-New South WalesWidespreadIntroduced Invasive Australian Weeds Committtee, 2013
-QueenslandLocalisedIntroduced Invasive Australian Weeds Committtee, 2013Southern and Central Queensland
-South AustraliaWidespreadIntroduced Invasive Australian Weeds Committtee, 2013
-TasmaniaWidespreadIntroduced Invasive Australian Weeds Committtee, 2013
-VictoriaWidespreadIntroduced Invasive Australian Weeds Committtee, 2013
-Western AustraliaLocalisedIntroduced Invasive Australian Weeds Committtee, 2013Southern Western Australia
GuamPresentIntroducedPIER, 2013
New CaledoniaPresentIntroduced Invasive PIER, 2013
New ZealandWidespreadIntroduced Invasive Webb et al., 1988Known as M. nigra
Norfolk IslandPresentIntroducedPIER, 2013
TongaPresentIntroducedPIER, 2013
US Minor Outlying IslandsPresentIntroduced Invasive PIER, 2013Midway Atoll

History of Introduction and Spread

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Viable M. polymorpha seeds were extracted from adobe bricks from eighteenth-century buildings in San Vincente, Mexico (Spira and Wagner, 1983), and the species may have been introduced to Chile as much as 450 years ago (Pozo et al., 1989, quoted in Paredes et al., 2002). Introduction to countries like Australia, New Zealand and South Africa probably occurred during the spread of British emigrants in the nineteenth century, when settlers often took with them such possessions as vegetation-stuffed mattresses, hay and straw for livestock, vegetation as packaging material, and grass and other plant seed for cultivation in the new country.


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Introduced toIntroduced fromYearReasonIntroduced byEstablished in wild throughReferencesNotes
Natural reproductionContinuous restocking
Australia Europe 1848 Hitchhiker (pathway cause) Yes CHAH (2014); Council of Heads of Australasian Herbaria (2013) First found near Adelaide; probably introduced from Europe
Chile Europe 1500s Hitchhiker (pathway cause) Yes Pozo et al. (1989) Probably introduced from Europe
Mexico Europe 1790 Hitchhiker (pathway cause) Yes Spira and Wagner (1983) Probably introduced from Europe
New Zealand Europe 1855 Hitchhiker (pathway cause) Yes Hooker (1867) In Auckland province; probably introduced from Europe

Risk of Introduction

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Intentional introduction as a pasture plant is still likely, especially of new and improved cultivars. Accidental introduction is less likely than it used to be, as M. polymorpha seed ought to be detected in phytosanitary searches of samples of crop or pasture seed crossing international boundaries, but it is still possible. The species is already fairly widely distributed.


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Where M. polymorpha has been introduced, as in California, it occurs in a wide variety of ‘disturbed’ sites, including lawns and turf, roadsides, fields, grasslands, pastures, agricultural sites (UC IPM, 2013); Parker and Gilbert (2007) noted that it was invasive and one of the commonest legumes in coastal Californian prairies. In Hawaii it is 'naturalized in open, dry to occasionally mesic, disturbed areas such as pastures, roadsides and vacant lots, 0-1220 m [altitude?]' (Wagner et al., 1999, cited in PIER, 2013). In Australia it is found in a wide variety of vegetation communities ranging from open grasslands to shrublands and woodlands, and grows on a wide range of soils, but does best in alkaline soils, growing most prolifically on heavy clays (Department of Primary Industries, Victoria, 2013). In Chile it is distributed over a remarkable range of bioclimatic and soil conditions throughout the Mediterranean-climate region of the country (Pozo et al., 1989, quoted in Paredes et al., 2002).

Around its native Mediterranean, the website Flowers in Israel (Modzelevich, 2013) describes its habitat as ‘woodlands and shrublands, semi-steppe shrublands, shrub-steppes, deserts and extreme deserts, montane vegetation of Mt. Hermon.’

In Britain, the Online Atlas of the British and Irish Flora (Biological Records Centre, 2013) reports that the species is ‘found in open sandy and gravelly habitats by the coast. It occurs in short, open grassland on summer-parched banks and cliffs with other annuals, particularly in S.W. England. Inland, like other Medicago species, it occurs as a casual, especially with wool shoddy.’ The same source says that the species ‘has declined at its coastal stations through scrub encroachment and lack of grazing, although it may be overlooked or confused with M. arabica. It is much scarcer nowadays as a casual inland’.  The spiny pods stick to wool and other fibres, which helps their dispersal and explains its association in Britain with wool shoddy (recycled wool).

In Syria, Ehrman and Cocks (1990) found that M. polymorpha occurred on almost all soil types, and was found in almost every climatic zone in that country.

Habitat List

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Terrestrial – ManagedCultivated / agricultural land Principal habitat Natural
Managed forests, plantations and orchards Present, no further details Natural
Managed grasslands (grazing systems) Principal habitat Natural
Disturbed areas Principal habitat Natural
Rail / roadsides Present, no further details Natural
Terrestrial ‑ Natural / Semi-naturalNatural forests Present, no further details Natural
Natural grasslands Present, no further details Natural
Scrub / shrublands Present, no further details Natural
Deserts Present, no further details Natural
Arid regions Present, no further details Natural

Biology and Ecology

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2n=14. As its botanical name indicates, M. polymorpha displays a wide range of phenotypic characters, which often relate to ecological factors. Studies of phenotypic and genetic variability have been carried out in Chile (Pozo et al., 2002a, b; Paredes et al., 2002), Tunisia (Hannachi et al., 1998), Sardinia (Bullitta et al., 1994) and elsewhere, often in the search for improved cultivars for agricultural use. Although there seems to be appreciable variation in the genotypes of the species in Tunisia, this is not so in Chile, one reason for which may be, as Paredes et al. (2002) speculate, that only a small number of individuals were originally introduced into Chile.

Reproductive Biology

M. polymorpha is an annual or sometimes a biennial plant, reproducing entirely by seed. Depending on the climatic zone in which plants originate, they can be obligatory or facultative long-day plants (Pozo et al., 2000). Accessions from wetter areas (mean annual rainfall over 1200 mm) did not flower under continuous 8-hour photoperiods for the 111-day (4-month) duration of one experiment, whereas most accessions from arid to sub-humid areas flowered even under 8-hour photoperiods: all accessions flowered earlier (in as little as 24 days in one case) with exposure to longer photoperiods. This means that plants from drier areas of Chile, where frosts are less likely, flower earlier than those from wetter areas, where the chance of frosts is greater (Pozo et al., 2000).

The flowers are followed by indehiscent coiled pods (burrs) which are often covered in hooked spines which make the pods stick to animal fibres or clothing. The pods never open and as some seeds germinate, others remain dormant within the pod (Wagner and Spira, 1994). Jain (1982) found a high level of dormancy in seeds tested in July after collection in June: by September the mean percentage of germinating seeds from 13 populations had risen from 8.4 % to 16.1 %. This hard-seededness or dormancy is a characteristic of some leguminous species of clovers and medics. Some seeds of M. polymorpha can remain dormant for several years, with release from dormancy brought about by long exposure to temperature and moisture fluctuations (Wagner and Spira, 1994).

In California (and presumably in similar Mediterranean climates) germination usually occurs following the first significant autumn rains, between September and December in the Northern Hemisphere (Wagner and Spira, 1994). In early November, following mid-October rains, the same authors analysed sod samples and found that 40.3% of all seeds recovered were dormant, 6.3 % germinated and died, and 53.4 % became established seedlings. Although seedling mortality was high in both wet and dry winters, in an unusually dry year only about 10 % of seedlings survived to fruiting, and plants were small and produced few fruits. In an unusually wet year, by contrast, about 50% of seedlings survived to produce fruits (Wagner and Spira, 1994).

Physiology and Phenology

Frost tolerance in different ecotypes of M. polymorpha is related to the geographical origin of those ecotypes in both Syria (Cocks and Ehrman, 1987) and Chile (Pozo et al., 2002a). Pozo et al. (2002b) found greater winter vigour in plants originally collected from drier, warmer, less frosty northern regions of Chile than in those from humid southern regions, when they were grown together in the intermediate ‘Mediterranean’ climate zone.


Although the plants of M. polymorpha themselves are short lived, 200 year-old viable seeds have been found on the surface of adobe bricks in Northern Mexico (Spira and Wagner, 1983). As these authors point out, the longevity of these seeds may have been favoured by the dry stable environment of adobe bricks, and reduced temperature fluctuation and microbial activity may also have played a part.


M. polymorpha forms a symbiotic relationship with the bacteria Ensifer medicae (= Sinorhizobium medicae) or E. meliloti (= S. meliloti), which invade plant roots and form root nodules. In these the bacteria gain nutrition from the plant and fix atmospheric nitrogen to help sustain the plant. The amount of nitrogen fixed can be substantial (about 25 kg per tonne of legume dry matter) according to Peoples and Baldock (2001) in a study of Australian pasture legumes in general.


M. polymorpha occupies a very wide range of habitats and climatic zones and therefore associates with many other species of plants, some of which often have a similar Mediterranean origin.

Environmental rrequirements

M. polymorpha is widely distributed around the Mediterranean and tolerates a wide variety of habitats. It is well adapted to neutral and slightly acid soils, to altitudes of 10 to 900 m or more, and to annual rainfall from 100 to 800 mm. (Hannachi et al., 1998). The species is also widespread where it has been introduced to other countries with similar climates – Australia, Chile, South Africa and the United States.  As Graziano et al. (2010) say ‘it is now found over a remarkable range of bioclimatic and soil conditions’.


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Cs - Warm temperate climate with dry summer Preferred Warm average temp. > 10°C, Cold average temp. > 0°C, dry summers

Air Temperature

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Parameter Lower limit Upper limit
Mean annual temperature (ºC) 10.5 27.5


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ParameterLower limitUpper limitDescription
Mean annual rainfall1001000mm; lower/upper limits

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Acyrthosiphon kondoi Herbivore
Aphis craccivora Herbivore
Erysiphe trifolii Pathogen
Halotydeus destructor Herbivore
Phoma medicaginis Pathogen
Pratylenchus neglectus Pathogen
Sitona discoideus Herbivore
Sminthurus viridis Herbivore
Thanatephorus cucumeris Pathogen
Therioaphis trifolii Herbivore
Uromyces anthyllidis Pathogen
Uromyces ciceris-arietini Pathogen
Uromyces pisi-sativi Pathogen

Notes on Natural Enemies

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Major pests of M. polymorpha in Australia listed by Howie et al. (2007) are: redlegged earth mite (Halotydeusdestructor), lucerne flea (Sminthurus viridis), bluegreen aphid (Acyrthosiphon kondoi), cowpea aphid (Aphis craccivora), spotted alfalfa aphid (Therioaphis trifolii), sitona weevil (Sitonadiscoideus) and the root lesion nematode Pratylenchus neglectus. Major diseases are phoma black-stem (Phoma medicaginis), rhizoctonia bare-patch (Rhizoctonia solani[Thanatephorus cucumeris]) and powdery mildew (Erysiphe trifolii). Uromyces ciceris-arietini (chickpea rust) (Stuteville et al., 2013), U. pisi-sativi (Storey, 2013), and U. anthyllidis (Landcare Research, 2013) have also been recorded as infesting M. polymorpha.

Means of Movement and Dispersal

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Natural Dispersal (Non-Biotic)

The non-dehiscent fruits are relatively large and are unlikely to move far from the parent plant, except perhaps in flood waters.

Vector Transmission (Biotic)

Green et al. (2008) found an apparently intact seed of M. polymorpha in the faeces of a black swan (Cygnus atratus). Seeds have also been found in cattle dung in Argentina (Vignolio and Fernández, 2010); they easily survive passage through horses (St John-Sweeting and Morris, 1990), and some pass unharmed through the guts of sheep, both in South Africa (Kotzéac et al., 1995) and in Australia (Edward et al., 1998).

The hooked spines commonly, but not always, found on the fruits allow the entire burrs to adhere firmly to the wool of sheep or the hair of horses, American bison (Bison bison L.) (Constible et al., 2005) and other animals, and to the hair and clothes of humans. Their occurrence in sheep’s wool in Australia is described by the Australian Wool Testing Authority (2002) as follows: ‘Burr Medic is very common in Australian wools and is one of the most troublesome types to processors. Not only do its protruding spines catch in the wool, making them difficult to remove, but also, if broken up during carding, its coils tend to unwind into thin ‘eyebrow’ shaped pieces which are even more difficult to remove and can persist into the finished product’.

Accidental Introduction

With modern seed cleaning and inspection services, further accidental introduction of M. polymorpha is less likely than it used to be (although still possible), but the species is already widely distributed throughout much of the world, having probably been spread in hay and straw transported for livestock, in vegetation used as packaging and in plant seed transported for cultivation.

Intentional Introduction

Intentional introduction to countries where grazed grasslands are important is likely to continue, since many countries are searching for improved cultivars to improve the productivity of their pastures.

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
True seeds (inc. grain) seeds Yes Pest or symptoms usually visible to the naked eye

Impact Summary

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Economic/livelihood Positive and negative
Environment (generally) Negative

Economic Impact

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As mentioned in the 'Means of Movement and Dispersal' section, the burrs of M. polymorpha cause serious degradation of the Australian wool clip and reduce its value – they are difficult to remove, and if they are broken up during carding, their coils tend to unwind into thin ‘eyebrow’ shaped pieces which are even more difficult to remove and can persist into the finished product (Australian Wool Testing Authority, 2002).

At the same time in Australia and elsewhere – Chile (Pozo et al., 2002a,b), Sicily (Graziano et al., 2010), Tunisia (Hannachi et al., 1998), Syria (Ehrman and Cocks, 1990) – the species is seen as a valuable self-sustaining, nitrogen-fixing addition to pastures for grazing animals, and the search for better cultivars is continuing. In Australia at least three cultivars (Circle Valley, Serena and Santiago) have been commercially produced (Hannachi et al., 1998). However these cultivars have shown poor adaptability to real Mediterranean environments (Graziano et al., 2010). Chile has also produced its own cultivar of M. polymorpha suitable for the arid, semiarid and subhumid Mediterranean zones of Chile (Pozo et al., 2001).

In an article on the Russian Steppe, Boonman and Mikhalev (2005) say: ‘Although it is well eaten, M. polymorpha can be a most harmful plant because the pods spoil the wool. It can be suppressed by hard and prolonged grazing. Herbicides are also effective.’

In Egypt, M. polymorpha apparently ‘causes great damage to flax production’ (Hozayn et al., 2010).

According to Howie et al. (2007)M. polymorpha can cause photosensitisation in horses, occasionally red gut in sheep, and bloat in cattle. Phytoestrogens/coumestrols can potentially have negative effects on the reproduction of grazing livestock but this is rarely reported (levels tend to be higher under conditions of phosphorus deficiency and Phoma infection).

Environmental Impact

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Many countries with Mediterranean climates, like Chile, Australia and those around the Mediterranean, see M. polymorpha as a valuable grassland component, both providing fodder for livestock and enhancing the supply of nitrogen for grass growth. Parts of the USA (California, Arizona, Nevada), however, regard the species as an invasive species or weed, although in other parts of the USA (the Upper Midwest) Australian cultivars have been evaluated as a possible ‘valuable addition to agricultural systems’ (Haan et al., 2002). 

Cal-IPC (2013) has evaluated M. polymorpha for its ‘level of threat to the ecological health of wildlands through evaluation of its ecological impact, ability to invade natural vegetation communities, and current extent of its invasion.’ These criteria were developed for use in California, Arizona, and Nevada and the outcome of the evaluation describes the ‘ecological amplitude’ of the species as ‘Limited’, meaning that it invades only one major ecological type and two to four minor types.

The points that this evaluation raises are:

  • Greater fire intensity may result from increased grass biomass caused by its  nitrogen fixation,
  • Herbivore populations may be increased by its better nutritional value,
  • It has probably spread to most locations where it can survive,
  • It does not thrive well outside grazed grasslands,
  • It can recover from grazing if this is not too severe,
  • Human-caused dispersal is not expected to be frequent, although seeds can be dispersed as commercial seed contaminants.
  • When livestock are moved any attached seed pods can be moved with them,
  • High levels of infestation (in grasslands) are not generally found unless the site is fertilized and moderately grazed.

Social Impact

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M. polymorpha probably has very little direct social impact.

Risk and Impact Factors

Top of page Invasiveness
  • Proved invasive outside its native range
  • Has a broad native range
  • Abundant in its native range
  • Pioneering in disturbed areas
  • Fast growing
  • Has high reproductive potential
  • Has propagules that can remain viable for more than one year
Impact outcomes
  • Ecosystem change/ habitat alteration
  • Modification of fire regime
  • Modification of nutrient regime
  • Negatively impacts animal health
  • Damages animal/plant products
Impact mechanisms
  • Poisoning
  • Produces spines, thorns or burrs
Likelihood of entry/control
  • Highly likely to be transported internationally deliberately


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Economic Value

The development of improved cultivars of M. polymorpha for oversowing in grazed pasture in Australia and their testing in other countries including the USA and Sicily, as well as the search for improved local cultivars in Syria, Tunisia, Chile and elsewhere (Pozo et al., 2002a, b; Graziano et al., 2010; Hannachi et al., 1998; Ehrman and Cocks, 1990), indicate that in many places it is sought after as a useful addition to grazed pastures, particularly because of its nitrogen-fixing ability.

Social Benefit

Indirect social benefits could arise from the use of improved grazing land in several countries.

Environmental Services

M. polymorpha has the potential to increase the availability of nitrogen to pastoral systems, an effect which can be considered positively by graziers or negatively by environmentalists.

Similarities to Other Species/Conditions

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M. polymorpha is very similar to and often grows with M. arabica (sometimes known as M. polymorpha var. arabica), but differs in NOT having dark spots or markings on the upper surface of each leaflet – instead it has reticulately veined pods and more or less erect spines.

Small and Jomphe (1989) point out that ‘M. polymorpha is the most common ruderal Medicago (except possibly for escaped alfalfa, M. sativa), the most likely to be collected, and the most likely to be confused with other species’.

Prevention and Control

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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.

Physical/Mechanical Control

According to Florabase (Western Australian Herbarium, 2013), M. polymorpha in the Swan River area of Western Australia is relatively tolerant to glyphosate, grazing and mowing. They suggest hand pulling isolated plants in winter before flowering, or the use of herbicides (see below).

Biological Control

Biological control of M. polymorpha has never been considered because the species is seen as a useful component of pastures in several parts of the world. However, a number of insects, nematodes and fungi are known to attack it (see the ‘Notes on Natural Enemies’ section for more details).

Chemical Control

It is suggested in Florabase (Western Australian Herbarium, 2013) that as M. polymorpha in the Swan River area of Western Australia is relatively tolerant to glyphosate, instead clopyralid, metsulfuron or triasulfuron can be used as herbicides to give reasonably selective control, but their use must be repeated annually for several years. In California, isoxaben is suggested for the control of clovers in general (Smith et al., 2013). Boonman and Mikhalev (2005) indicate that herbicides are effective in the Russian steppes.

Control by Grazing

Heavy grazing will certainly reduce the incidence of M. polymorpha in pastures, but not for long, as recovery from buried seed will occur at the next rains; Boonman and Mikhalev (2005) state that ‘hard and prolonged’ grazing can suppress it.


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13/03/13: Original text by:

Ian Popay, consultant, New Zealand, with the support of Landcare Research.

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