This species has been variously termed bur clover (Finch and Sharp. 1983; McKee, 1910), burr medic (Mackay and Barnard, 1981; Quinlivan et al., 1986), California burclover (Duke, 1981), and toothed burclover (Duke, 1981).

The scientific name is Medicago polymorpha L. (Duke, 1981; Mackay and Barnard, 1981).

Cv 'Serena' is very early maturing, whereas cv 'Circle Valley' is a medium-season type (Miller et al., 1989).

Seed is produced in rounded, coiled burrs surrounded by short, hooked spines which cling to animal coats, assisting seed distribution. Sheep fleece with burrs is discounted in value (Murphy et al., 1976). There are 3 to 11 seeds per burr, separated by partitions. The seeds are 2 to 4 mm long and oval, rather than kidney-shaped as in barrel medic (Quinlivan et al., 1986). Mackay and Barnard (1981) state that cvv 'Circle Valley' and 'Serena' normally have 5 seeds per burr. A high percentage of the seed is hard, ensuring stand regeneration in the face of variable weather conditions (Murphy et al., 1976). According to Murphy et al. (1976), there are 145,000 seeds/lb., whereas Mackay and Barnard (1981) estimate 280,000 seeds per kg.(about 127,000 per lb.) for both cvv 'Circle Valley' and 'Serena.' 'Circle Valley' seeds are oval to kidney shaped and glossy yellow; those of 'Serena' are yellow brown (Mackay and Barnard, 1981).

In dry areas, seedlings may require 1-2 irrigations before winter rains commence (Duke, 1981). Root growth of burr medic seedlings is faster than that of seedlings of subterranean clovers, facilitating establishment with more erratic rainfall (Fred Thomas, pers. comm.).

Burr medic is the best known plant among the medics (Murphy et al., 1976). The plant branches profusely at the base, and the stems are decumbent and attain a height of about 6 to 10 inches (Finch and Sharp, 1983). There are numerous prostrate stems; in a dense stand they may become erect. Prolific seed production, rapid growth following fall rains, and ability to fix nitrogen make it closely resemble a true clover, which it is not (Murphy et al., 1976).

Burr medic leaves are almost hairless. The leaflets are wide and rather flat at the top, and sometimes have purple and white flecks on the upper surface or a small purple inverted V mark at the base of the leaf. The flowers are in groups of two to eight and the wing petals are distinctly longer than the keel petals. Among Western Australian medics, only 'Calvary' medic, an extremely rare species, shares this trait; others have the keel longer, or equal to the wing petals. The length of the flower stalk is not consistent in this species as it may be longer or shorter than the corresponding leaf stalk. The fruits are hairless and have two to six anticlockwise coils. The common name burr medic relates to the pods, which have long, slender spines hooked at the tip which readily adheres to wool. There are forms with short spines and a variety brevispina which is spineless. This variety is rare but found scattered throughout the range of the whole species. The spineless burrs can be distinguished from spineless barrel medic or disc medic burrs by the radiating veins on the coil surface which are strongly curved, fine and close together, whereas in the latter two species the radiating veins are fewer, almost straight, and rather indistinct (Quinlivan et al., 1986).

Cv 'Circle Valley' is similar morphologically to 'Serena.' Stems are ribbed and develop a purple anthocyanin flush where they are exposed to the sun. Leaflets are obovate to wedge shaped, 12-23 mm long, 10-15 mm wide and glabrous. Leaflets have basal purple anthocyanin markings and, during the winter, have further scattered anthocyanin flecking prominent on the leaflet. Stipules are spiny and large, green and glabrous. Peduncle are normally shorter than adjacent petiole and normally with three yellow flowers, the corolla being approximately twice the length of the calyx. Mature fruit is cylindrical 7-9 mm in length and coiling anti-clockwise, with 4-5 coils (Mackay and Barnard., 1981).

Cv 'Serena' is prostrate or semi-erect, and attains heights up to 20 cm depending on plant density. Stems exposed to the sun may develop a purple anthocyanin flush. Stems, petioles and peduncles glabrous or with sparse hairs on peduncle. Leaflets cuneate or obovate, apex apiculate, margin almost entire or with upper half serrate, upper and lower surfaces glabrous. Leaves have very rare purple flecks and rare development of a basal purple mark at the junction of the petiole and the lamina. Stipules incised and glabrous on both sides. Peduncle usually shorter but sometimes longer than the subtending petiole. Three flowers, 1-2 fruits per peduncle. Flowers pale yellow, standard about 4 mm long, wings longer than the keel. Fruit coiling is anti-clockwise: the young fruit is contracted and protrudes sideways from the calyx: young fruits are sparsely hairy, mature fruits glabrous: spineless with 4-4.5 coils forming a small cylinder or truncated cone. Many (15-20 per cil) radial veins strongly curved, anastomosing into a net and joining a lateral vein which is separated from the wide dorsal suture by a narrow groove (Mackay and Barnard, 1981).

Burr medic will easily tolerate a freeze of 22-24°F even in the seedling stage. At 18°F it will be killed.

This species is cultivated in dry, warm temperate regions of the world, is not frost tolerant, and tolerates mean annual temperatures of from 10.5 to 27.5 degrees C (Duke, 1981). It has been called moderately resistant to cold (Madson, 1951).

The following account is from Duke (1981). Burr medic is assigned to the Eurosiberian and Hindustani Centers of Diversity, and is native in southern Europe north to Britain, east to Russian and Crimea, eastward to Punjab, Kumaon, North Bengal, and Madras, western and Central Asia, China, Japan, North Africa, south to Ethiopia, Eritrea, Kenya, and Tanzania. It is also cultivated in Argentina, Australia, and the Pacific Coast of the U.S.A. It occurs at elevations up to 1,500 m in the Himalayas and to 1,900 m in Tanzania. It is also cultivated in South America where the cv. 'Santiago' was developed.

Duke (1981) wrote that burr medic requires about 80 cm (31.50 inches) of rainfall per year but goes on to say that it tolerates annual precipitation of from 31-191 cm (12.20-75.20 inches). Optimum rainfall is 10-25 inches (Murphy et al., 1976), and it does best where ample moisture is available early in the fall (Finch and Sharp, 1983). It will produce plants in low- rainfall years if winters temperatures are not too cold (Murphy et al., 1976).

Superphosphate is often needed, as are potassium and even nitrogen on poor soils; it grows well in alkali soil containing 12.5 MT/ha carbonate of soda in the top 75 cm of soil (Duke, 1981). Based on pot experiments and literature review, burr medic and barrel medic are not as efficient at absorbing P as is subterranean clover. Yellow serradella is more efficient at absorbing P from soil than either subterranean clover or burr medic (Paynter, 1990).

On rangeland sites, burr medic can be reestablished with the application of phosphorous and sulfur.

Burr medic tolerates soil pH of from 5.3-8.2, with a mean of 30 cases of 6.8; it is deemed tolerant of alkali and high pH (Duke, 1981). Murphy et al. (1976) stated that burr medic tolerates soil pH of from 4.7-8. Burr medic is said to be well adapted to all orchard and vineyard soils that are neutral to slightly alkaline (Finch and Sharp, 1983). It is less tolerant of acid soil than is subclover (Murphy et al., 1976). Medicago murex, M. polymorpha, and M. soleirolii are particularly good at nodulating on acid soils when acid-tolerant rhizobia are used (Rhizobia meliloti strain WSM419). M. tornata, M. truncatula, and M. littoralis did not perform so well (Howieson and Ewing, 1989).

Burr medic does well on many soil types, including poorly-drained adobe soils; it tolerates soil pH of from 5.3-8.2, with a mean of 30 cases of 6.8; it is deemed tolerant of heavy soil, alkali, and high pH (Duke, 1981). Burr medic is said to be well adapted to all orchard and vineyard soils that are neutral to slightly alkaline (Finch and Sharp, 1983). On poor, mildly-acid soils, burr medic grows better than black medic (McLeod, 1982), but it is less tolerant of acid soil than is subclover (Murphy et al., 1976). Madson (1951) recommended loam soils, whereas Murphy et al. (1976) suggested heavy soils. Finch and Sharp (1983) said that burr medic is best adapted to soils with good moisture holding capacity.

Cv 'Circle Valley' has regenerated strongly at Speddingup, Australia, in a transitional soil zone between the inland alkaline malle soils and the acid scrub-plain soils nearer the coast. At the same site various cultivars and varieties of barrel and disc medic failed to establish and persist effectively. Subterranean clover also has proved only partially satisfactory on these extensive soil types (in Australia) which consist of a slightly acid grey-brown surface sand or loamy sand becoming increasingly loamy at depth. They tend towards a neutral reaction at 5-20 cm and normally have a domed clay profile at 30-60 cm (Mackay and Barnard, 1981).

Duke (1981) wrote that burr medic is tolerant of shade. Bugg (pers. comm.) noted that it is a common volunteer in the understories of Californian orchards, including walnut orchards, which are heavily shaded from April through November.

Burr medic does not tolerate high soil salinity (Duke, 1981).

This species is more sensitive to 2,4-D than are members of the genus Trifolium (Duke, 1981). On neutral and alkaline soils, competition by burclover and its rhizobia can interfere with establishing subterranean clover stands. Burmedic can be removed with an application of 0.75 lb/a of 2,4-D (Bade et al., 1988).

Burr medic is a winter annual herb that produces hard seed which will not germinate for 2-3 years. In dry years, 1-2 irrigations may be required to ensure establishment prior to fall rains (Duke, 1981). This early-maturing, reseeding annual legume germinates in the winter but puts on most of its growth during the spring (Finch and Sharp, 1983). 'Circle Valley' shows unusually vigorous early winter growth (Mackay and Barnard, 1981).

From Nedlands, Western Australia, Cotterill (1990) reported that subterranean clover (cv 'Seaton Park') produced less dry matter biomass early (by the equivalent of December 15 here) but had produced more by the equivalent of March 7 here than 'Serena' burr medic. A mixture of the grasses Bromus diandrus, Hordeum leporinum, Lolium rigidum, and Vulpia myuros reduced growth by both legumes.

Seeding rates are variously given as 9 lbs/acre (Finch and Sharp, 1983), 13.3 lbs/acre (Duke, 1981), 15 to 20 lbs/acre (Miller et al., 1989), and 20 to 30 lbs/acre (Madson, 1951).

The seeding rate of 20-30 lb. is excessive, while 12-20 lb. is the optimum for burr medic as a cover crop (Fred Thomas, pers. comm.).

Seed should be covered lightly with soil, which could be obtained by broadcasting seed and harrowing lightly (Duke, 1981). 1/2-inch seeding depth suffices (Bugg, pers. comm.).

Broadcasting followed by light harrowing is a good seeding method; seed should only be covered lightly (Duke, 1981); a firm seedbed was recommended by Duke (1981) and by Finch and Sharp (1983).

Harrowing even when described as light would be too aggressive for the small seeded burr medic. Broadcasting followed by light cultipacking, as with alfalfa, is the best method.

Seed should be sown in late summer (Duke, 1981) or fall (Duke, 1981; Finch and Sharp, 1983), with October-November recommended by Madson (1951).

Rhizobial inoculant type is "Medicago Spec. 1" (Nitragin Co.) (Burton and Martinez, 1980; Duke, 1981).

Inoculation of burr medic seedlings with the bacterium Azospirillum brasilense Strain CD increased the number of lateral roots and root hairs. Similar symptoms can be caused by application of indole acetic acid to growth media. High concentrations of the bacterium reduced root elongation, but the reduction was lessened by addition of rhizobia (Yahalom et al., 1991).

Inoculation of burr medic and berseem clover seedlings with the bacterium Azospirillum brasilense Strain CD accelerated the development of nodules. There was also an increase in root surface area and nutrient uptake (Yahalom et al., 1989).

Medicago murex, M. polymorpha, and M. soleirolii are particularly good at nodulating on acid soils when acid-tolerant rhizobia are used (Rhizobia meliloti strain WSM419). M. tornata, M. truncatula, and M. littoralis did not perform so well (Howieson and Ewing, 1989).

On neutral and alkaline soils, competition by burr medic and its rhizobia can interfere with establishing subterranean clover stands. Burr medic is very sensitive to 2,4-D herbicide and can be removed with an application of 0.75 lb/a (Bade et al., 1988).

In the past, commercial supplies of seed were sometimes limited (Murphy et al., 1976). Most stands of burr medic are volunteer; limited supplies of seed were available (Finch and Sharp, 1983).

For the past 10 years, burr medic seed has been readily available with the importation of cvs. 'Circle Valley' and 'Santiago'.

Burr medic can grow as a perennial in cool areas of California, leading to large plants through summer and flowering in fall.

In California, flowering occurs from March through June (Munz, 1973). Duke (1981) listed flowering as occurring during summer and fall, but in California, this would presumably occur only if the crop is sown in the spring in some coastal areas (Bugg, pers. comm.).

The cultivar 'Circle Valley' will continue to grow vegetatively with irrigation while the cultivars 'Serena' and 'Santiago' are more determinate, and will stop vegetative growth even though they receive rain or irrigation.

Burr medic shows early and continued (indeterminate) flowering coupled with continued vegetative growth (Bugg, pers. comm.). In San Diego County, cv 'Circle Valley' flowers in 96 days (Weitkamp and Graves, 1988). Flowering is 4-5 weeks after 'Cyprus' barrel according to Murphy et al. (1976) and some 3 or 4 weeks later according to Mackay and Barnard (1981). The latter two workers report that testing over 3 years and at 6 sites in Western Australia has given a mean period to flowering of 140 days from a mid-April germination, 122 days from a mid-May germination, and 112 days from mid-June. Cv 'Circle Valley' flowers prolifically and over a relatively long period (Mackay and Barnard, 1981). 'Serena' requires 62 days to flower according to Weitkamp and Graves (1988), 65 according to Miller et al. (1989), and 70 based on the account of Mackay and Barnard (1981).

Seed maturity is in mid April-May according to Finch and Sharp (1983). Cv 'Serena' shows especially early maturity (Mackay and Barnard, 1981).

Cv 'Circle Valley' can form large quantities of seed (in excess of 1 Mg/ha.) provided there is adequate soil moisture and nutrients (Mackay and Barnard, 1981).

The burrs of 'Serena' have a higher proportion of seed (by weight) than do species such as barrel and disc medics which have thick-walled burrs. The seed yield of 'Serena' was higher than that of Cyprus barrel medic or 'Harbinger' strand medic and comparable to that of 'Tornafield' disc medic in sward trials at Perth (Mackay and Barnard, 1981).

Seed of burr medic retains viability for several years if left in the burr, but deteriorates more rapidly if hulled. Under the latter scenario, germination nevertheless remains high for 3 years or more (Duke, 1981).

Burr medic is an winter annual herb, semierect or prostrate (Duke, 1981). Habit is trailing prostrate or erect, depending on plant density (Mackay and Barnard, 1981). Density of growth is light (Madson, 1951).

Finch and Sharp (1983) said that burr medic is about 6 to 10 inches tall, but cv 'Circle Valley' can reach 40 cm (15.75 inches) in dense stands (Mackay and Barnard, 1981). In Hopland, Mendocino County, California, May 2, 1991. 'Circle Valley' attained a height of 34.9+/-5.4 cm (13.7 +/- 2.1 inches, Mean +/- S.E.M.) (Bugg et al., 1996).

Burr medic seedling roots showed twice as much growth as subclover in slant tube experiments (Matches, et al., 1987).

Burr medic seedlings are said to show much more rapid taproot growth than do those of subterranean clovers, and hence burr medic is better adapted to discontinuous rains that characterize much of California. Early fall rains followed by drought will tend to favor burr medic over subterranean clovers (Fred Thomas, pers. comm.).

Inoculation of burr medic seedlings with the bacterium Azospirillum brasilense Strain CD increased the number of lateral roots and root hairs. Similar symptoms can be caused by application of indole acetic acid to growth media. High concentrations of the bacterium reduced root elongation, but the reduction was lessened by addition of rhizobia (Yahalom et al., 1991).

Inoculation of burr medic and berseem clover seedlings with the bacterium Azospirillum brasilense Strain CD accelerated the development of nodules. There was also an increase in root surface area and nutrient uptake (Yahalom et al., 1989).

A firm seedbed is important; in dry parts of California, 1-2 irrigations may be needed before winter rains commence (Duke, 1981). Low seeding rates may suffice, because self-reseeding in subsequent years fills in the stand (Duke, 1981).

Burr medic is a reliable reseeder, and volunteers in many orchards (Finch and Sharp, 1983). Dense volunteer stands of burr medic can be found in commercial orchards despite the lack of conscious encouragement by growers. This suggests that burr medic would be easily managed as a reseeding cover crop for orchards (Bugg, pers. comm.).

Unless the plants are mowed frequently, regrowth points of burr medic will be damaged by close mowing, stunting the plant. If the plant is 6 inches when first mowed, the mowing height should be 3 inches; if the height is 8-9 inches, when first mowed, the mowing height should be 5-6 inches. If mowed several times starting early in January, the plant can be made to grow very prostrate.

Burr medic is best adapted where mowing heights of 3 to 5 inches restricts growth of other plants that would crowd it out (Finch and Sharp, 1983).

Burr medic can be incorporated with disk, rototiller, or power spader (Bugg, pers. comm.). When burr medic is relay intercropped with corn or cotton (inter-row spacing 1.3-1.4 m), remnant strips of the legume can provide sufficient seed to reseed the entire area (Duke, 1981).

Imported burr medic is commercially produced with a vacuum harvester. Some California burr medic is harvested in barley or wheat and is cleaned out at the milling process and sold as burr medic seed.

Seed may be harvested by sweeping by hand with stiff brooms Duke, 1981).

Normal tillage and mowing equipment are appropriate (Bugg, pers. comm.), however, pure stands of burr medic may lie close to the ground and be difficult to mow (Duke, 1981); therefore, if grown for hay, it is often sown in mixture with oats or wheat, at 12.5 bu/ha of burr medic to 5 bu/ha of oat or 4.5 bu/ha of wheat (Duke, 1981). Seed may be harvested by sweeping by hand with stiff brooms (Duke, 1981).

Imported burr medic is commercially produced with a vacuum harvester. Some California burr medic is harvested in barley or wheat and is cleaned out at the milling process and sold as burr medic seed.

Burr medic can be used for green forage, rarely as hay, soil renovation, green manure, cover, and erosion prevention (Duke, 1981). It is not often seeded on range because in most parts of California, where it is adapted, the seed is present and can be encouraged with cultivation, fertilization, and proper grazing. Green plants, dry plants and burrs provide good forage (Murphy et al., 1976). The species is useful in unimproved pastures (Quinlivan et al., 1986). Dense volunteer stands of burr medic can be found in commercial orchards and vineyards despite the lack of conscious encouragement by growers; this suggests that burr medic would be easily managed as a reseeding cover crop for orchards and vineyards (Bugg, pers. comm.).

Solid stands of burr medic have been observed in many nontillage orchards and vineyards in California and can be encouraged with lime, calcium, phosphorous and sulfur.

Burr medic is compatible in mixes with rose clover and subterranean clovers (Bugg, pers. comm.). On neutral and alkaline soils, competition by burr medic and its rhizobia can interfere with establishing subterranean clover stands. Bur clover is very sensitive to 2,4-D herbicide and can be removed with an application of 0.75 lb/a (Bade et al., 1988).

In Hopland, Mendocino County, California, dry above-ground biomass during May for October-seeded cv 'Circle Valley' was 8.3+/-1.8 Mg/ha, Mean +/- S.E.M. (Bugg et al., 1996), whereas cover crop + weeds totalled 9.248+/-1.403 Mg/ha (Bugg et al., 1996). Duke (1981) reported hay yields of burr medic of 5-7.5 Mg/ha.

'Circle Valley': In an unreplicated trial in southern Georgia, bur medic cv 'Circle Valley' yielded approximately 140 kg/ha of nitrogen in above-ground portions (Bugg, pers. comm.). Bur medic has a mean protein content of 19% by weight (Duke, 1981) and 6.25% of the protein is presumably N. Using these values and Duke's (1981) biomass figures, one can project N yields of 61.75 to 92.63 kg N/ha. Biomass data of Bugg et al. (unpublished data) from Mendocino County translate to 102.92 kg/ha.

Burr medic can cause bloat, which seldom occurs with rose clover or subclovers (Murphy et al., 1976). The coumesterol level of 'Serena' is satisfactory, being intermediate between 'Harbinger' and 'Cyprus' (Mackay and Barnard, 1981). Burr medic causes burr infestation to wool (Quinlivan et al., 1986), for which fleece is discounted in value.

Some people complain about the burrs adhering to clothes or to animals (Bugg, pers. comm.)

Pea aphid (Acyrthosiphon pisum) attacks burr medic.

Alfalfa weevil damage on burr medic can be so widespread that plants are almost completely skeletonized (Murphy et al., 1976).

Burr medic is almost always very susceptible to alfalfa weevil while M. rugosa and M. scutellata have shown tolerance in Glenn Co. (Bell et al., n.d.)

Cvv 'Circle Valley' and 'Serena' both show fair tolerance to Egyptian alfalfa weevil (Weitkamp and Graves, 1988).

In an unreplicated observation in southern Georgia, Lygus lineolaris (Palisot de Beauvois) (tarnished plant bug) attained extremely-high densities on Medicago polymorpha ssp subspina cv 'Circle Valley', placing it well above the 95% confidence interval for densities observed on 11 varieties of subterranean clover (Bugg et al., 1990).

In southern Oregonian pear orchards, certain understory weeds can harbor high densities of twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae). This mite is mainly a secondary pest and a creature of pesticide-disrupted or stressed agroecosystems. Among the plant species suitable for use as cover crops, burr medic appeared particularly prone to outbreaks of the mite. Use of herbicides led to increased movement by mites into trees (Flexner et al., 1990).

Small loopers have also frequently been observed attacking burr medic.

Nematodes associated with burr medic include Ditylenchus dipsaci, Heterodera glycines, H. trifolii, Meloidogyne incognita, and Pratylenchus brachyurus (Duke, 1981).

Whitehead (1992) stated that in a pot study, burr medic and several other annual Medicago spp. (e.g. barrel medic) showed good resistance to the lucerne (alfalfa) race of stem nematode (Ditylenchus dipsaci). Subterranean clover was deemed very resistant, rose clover resistant, and berseem susceptible.

Duke (1981) wrote that burr medic can be afflicted by bean yellow mosaic virus, pea mosaic virus, the parasitic flowering plant Orobanche ramosa, and 13 species of fungi.

Burr medic is said to be tolerant of weeds (Duke, 1981). Mowing heights of 3 to 5 inches favor burr medic by restricting growth of other potentially-competing plants (Finch and Sharp, 1983).

During May in Hopland, Mendocino County, California, for plots seeded in October to cv 'Circle Valley' weed dry above-ground biomass was 0.9+/-0.6 Mg/ha, Mean +/- S.E.M., which was 18.2% of the value obtained in control plots. Dominant winter annual weeds were chickweed, shepherds purse, rattail fescue, and annual ryegrass. Vegetational cover by burr medic in early May was 73.0+/-18.7 % (Bugg et al., 1996).

Cotterill (1990) reported that subterranean clover ('Seaton Park') produced less dry matter early (equiv. of 15 Dec. here) more later (equiv. of Mar. 7 here) than 'Serena' burr medic.

From Nedlands, Western Australia, Cotterill (1990) reported that subterranean clover (cv 'Seaton Park') produced less dry matter biomass early (by the equivalent of December 15 here) but had produced more by the equivalent of March 7 here than 'Serena' burr medic. A mixture of the grasses Bromus diandrus, Hordeum leporinum, Lolium rigidum, and Vulpia myuros reduced growth by both legumes.

Burr medic, with its rapid winter and spring growth, has crowded out many weeds completely even in unmowed understories.

Burr medic in solid stands has caused bloat in cattle numerous times. (Fred Thomas, pers. comm.)