Medicago hispida
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