Characteristics. This and its close relative black medic (Medicago lupulina) are native to Europe and Asia and are currently very common in lawns, waste areas, gardens and along roadsides in the western US. The burs of burclover are especially problematic as they dry and readily work their way into pet hair and ears and human clothing. These weeds grow as annuals or short lived perennials that reproduce from seeds. Flowers are formed over a long period of time typically from April to February. Burr clover may have an annual or perennial life cycle germinating in fall when soil temperatures are in the 50° to 60°F range. Germination continues throughout the winter and early spring months. Winter rainfall will sustain the annual clovers, but irrigation is required for survival of the perennial species during the dry summer months.
Identifying Characteristics. The stems
typically grow prostrate branching from a base reaching a length from 1 to 2
feet. The leaves are compound, finely toothed
with prominent veins. The central
leaflet is attached by a short stalk. The
central leaflet has a short stem whereas the other two are almost stemless. Flowers
are small, bright yellow, and borne in clusters at the end of a stem. The
burclover seedpod is light brown and curls into a tight bur that is typically
spiny. The burs contain several seeds.
Cotyledons (seed leaves) are oblong. The
first true leaf is rounded. Later leaves are composed of three leaflets and
have a characteristic clover-like shape. Stems grow to 2 feet (60 cm) long and
tend to trail along the ground, but may grow upright. Leaves divide into three
round leaflets, resembling those of clover and usually have reddish-tinged
midveins. Leaflets have serrated edges. Bloom takes place from March to June.
Flowers are small, bright yellow, and cluster into flower heads at the stem
tips. The fruit consists of a pod that usually appears tightly coiled two to
six times. The pods are mostly brown, hairless, and smooth—or have two to three
rows of prickles on the outer face. Prickles often end in tiny
"hooks". Several yellowish or
tan, kidney-shaped seeds are contained in each pod.
Impact. Clover can be a concern in turfgrass or landscaped areas for at least three reasons. First, during the flowering period bees are attracted to the clover blooms and people playing or using the turfgrass may be stung. Second, clovers reduce the uniformity of the turfgrass because its texture, color, and growth rate are different from that of grasses. And third, the mature burs of burr clover are a problem for people walking barefoot and when they become attached to clothing or pets.
Control. In small stands hand-pulling, cultivation, and the application of mulch can result in effective control. In larger where these methods at labor intensive herbicides may required. Because of the hard heat tolerant seed coat composting and solarization are not as effective in reducing viability as they are with other weed species. Clover seeds can germinate over many years, making the control of these plants an ongoing effort. A thick stand of grass can help exclude clovers in turf. Fertilization can also influence clover growth. Programs that include more nitrogen and less phosphorus in turfgrass discourage the growth of clover. In addition mulches can be effective in excluding clovers and other weeds in landscapes.
Landscaped Areas. Burr clover can be easily controlled by hand-pulling, hoeing, or cultivation. Mulching, depending on the size and depth of the mulch, can prevent seedling establishment. Before seeds germinate, a 4-inch thick organic mulch (e.g., compost, wood chips, etc.) can prevent establishment of clovers. Organic mulch can also be applied after the seedlings have germinated but must be applied in a thicker layer (4 to 6 inches) and must cover the plants completely to block out all light. Organic mulches need to be reapplied each year to maintain the 4-inch-thick layer because they decompose and the thickness of the mulch declines over time. Woven black landscape fabric can exclude weeds over a number of years. Larger plants are more difficult to control with mulching, but they can be hand-pulled or hoed.
Preemergent Control. Preemergent herbicides available for landscape use are effective but generally unnecessary in the home landscape where annual clovers are easily controlled by the methods mentioned. For landscape professionals, herbicide formulations that contain isoxaben are effective for controlling annual clovers and can be used around many woody shrubs and trees. Most established annual flowers tolerate this herbicide. Herbicide formulations containing oryzalin, trifluralin, or pendimethalin will control most grass species and some broadleaf weeds but will miss many other broadleaf species (mustard, aster, legume, and cheeseweed families).
Postemergent Control. Postemergent control of clover is difficult. If the seedlings are small, glyphosate can be used in open areas provided desirable plants are not sprayed. Once annual clover plants reach 3 to 4 inches in height, control with herbicides is more difficult. The top may be burned, but the plants often regrow. None of the herbicides used in turfgrass for clover control is safe to use in ornamental plantings because they can damage desirable plants.
Perennial clovers can also be controlled with glyphosate when the plants are seedlings, but once the clover is established, it cannot be controlled except by digging it out. Glyphosate at high rates will suppress some clovers.
Turfgrass Areas. Yellow turf and green clover is a good indication of low nitrogen fertility. The invasion of clover into turfgrass can be reduced by using levels of nitrogen fertilizer that will promote grass growth but not the growth of clover; this can be achieved by applying 1 pound of active nitrogen per 1,000 square feet of turfgrass during each month of active turfgrass growth (not to exceed 4 lb active nitrogen/1,000 sq ft/year). Also, high phosphorus in the soil promotes the invasion of clovers. However, nitrogen applications should be carefully calculated and applied to avoid runoff of excess fertilizer to municipal drainage systems. Clover in established turfgrass cannot be controlled by fertilization or mowing of the grass. Once clover is established, the annual clovers can be controlled by hand-pulling before seeds are formed. Hand-pulling will need to be repeated as new germination occurs and desirable turfgrass is planted in weeded areas.
Herbicide. Both established annual and perennial clovers can be controlled with postemergent herbicides. The best herbicide to use depends upon the species of turfgrass. Warm-season turfgrasses such as bermudagrass, zoysiagrass, and kikuyugrass will tolerate products containing mecoprop and dicamba but not triclopyr. Cool-season turfgrasses will tolerate all of the herbicides that control clover. The herbicide 2,4-D is not effective for clover control; it will injure the plant but does not control it.
Control Methods from UC Online IPM Programs