Burr clover (Medicago polymorpha) is a persistent and widespread weed that quickly colonizes lawns and gardens. This plant is a winter annual, meaning it germinates in the cooler temperatures of the fall, grows through the winter, and produces seed pods in the spring before dying off in the summer heat. Successfully managing burr clover requires a multi-step approach that combines timely identification, immediate removal techniques, and long-term cultural practices to prevent its return. This article details the steps necessary to eliminate burr clover and foster a healthy environment that resists reinfestation.
Identifying Burr Clover
Accurate identification is the first step in effective management, as the plant’s life cycle dictates the best time for control. Burr clover is characterized by its trifoliate leaves, which are composed of three leaflets similar in shape to those of a true clover, with each leaflet having serrated edges near the tip. The plant typically has a prostrate, sprawling growth habit, forming dense mats that lie close to the ground.
Small, bright yellow, pea-like flowers appear in clusters at the stem tips, usually blooming from early spring into the summer. The most distinctive feature is the fruit, which is a small, tightly coiled pod that quickly turns brown and hard. These coiled pods, or burrs, are covered in hooked prickles that easily cling to clothing, pet fur, and machinery, which is the primary method of seed dispersal.
Manual and Non-Chemical Control Methods
For small infestations, physically removing the plant is a direct non-chemical approach. Hand-pulling is best done when the soil is moist, which helps ensure the entire root system is removed. Burr clover has a taproot, and leaving any part of it behind can allow the plant to regrow.
It is important to remove the plants before the spiny burrs mature and drop their seeds, significantly increasing the seed bank in the soil. Mowing is generally not an effective control method because the plant’s low, sprawling growth habit allows it to tolerate close cutting. However, raising your mower height can encourage a denser lawn canopy, which helps shade out new burr clover seedlings and limits their establishment.
In larger, non-turf areas, applying a thick layer of organic mulch, approximately four inches deep, can prevent burr clover seeds from germinating. If seedlings have already emerged, a thicker layer of six inches of mulch is required to completely block all light and smother the plants. Solarization, which involves covering the area with clear plastic sheeting during hot months to kill seeds, is often ineffective against burr clover because its seeds have a hard, heat-tolerant coat.
Chemical Treatment Options
Chemical control is often necessary for widespread infestations, and success depends on selecting the correct herbicide and applying it at the optimal time.
Pre-Emergent Herbicides
Pre-emergent herbicides are designed to stop seeds from germinating. They should be applied in the late summer or early fall, just before burr clover seeds begin to sprout in cooler temperatures. Active ingredients like isoxaben or prodiamine are used to create a chemical barrier in the soil that prevents the weed from establishing.
Post-Emergent Herbicides
Post-emergent herbicides are applied directly to actively growing plants. They are most effective when the burr clover is young and not yet flowering or setting seed. Look for broadleaf weed killers containing a combination of active ingredients such as dicamba, MCPP (mecoprop), or triclopyr. While 2,4-D is a common ingredient, it is generally less effective on clovers when used alone.
Application timing is crucial. Apply when temperatures are moderately warm, ideally between 60 and 80 degrees Fahrenheit, and when no rain is expected for at least 24 hours. The weed should be actively growing and not stressed by drought, as the plant needs to absorb the chemical through its leaves. Specialized products, such as those based on chelated iron, offer a selective option that targets broadleaf weeds like burr clover while being safe for most turfgrasses.
Long-Term Prevention Strategies
Preventing the return of burr clover involves addressing the underlying conditions that favor its growth over desirable turfgrass. Burr clover is a legume, meaning it can fix atmospheric nitrogen into the soil through a symbiotic relationship with bacteria in its root nodules. This ability gives it a competitive advantage in soils that are low in nitrogen, which is a common cause of weak, thin lawns.
Conducting a soil test to determine nutrient deficiencies and pH levels allows for a targeted fertilization program. Maintaining a consistent seasonal fertilization regimen, especially one that ensures adequate nitrogen, encourages dense turf growth that physically crowds out the burr clover. Dense, healthy turfgrass prevents bare patches, which are the primary invasion points for weed seeds.
Cultural practices, such as deep and infrequent watering, promote deeper root growth in turfgrass, making it more robust and competitive against shallow-rooted weeds. Burr clover thrives in well-drained soils, but proper aeration in compacted areas can further strengthen the lawn’s ability to resist weed establishment. Creating a thriving, dense turf canopy significantly reduces the light and space available for new burr clover seeds to germinate and establish.