Medicago polymorpha
(bur clover)

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.

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.

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.

Genetics

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.

Longevity

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.

Nutrition

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.

Associations

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’.

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.

Animal feed, fodder, forage

• Forage

Environmental

• Soil improvement

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’.

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.

13/03/13: Original text by:

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