Bur clover (Medicago polymorpha) is a common broadleaf weed found infesting turfgrass across various climates. This annual plant is troublesome because its low-growing, mat-forming habit allows it to evade standard mowing practices. The plant is difficult to manage due to its prolific seeding capacity and the distinctive, sharp burs it produces. These spiky seed pods are the primary mechanism for dispersal, making the weed a nuisance in lawns and gardens. Successful management requires understanding its lifecycle and employing targeted control strategies.
Key Features for Identifying Bur Clover
Accurate identification of bur clover relies on distinct physical markers, beginning with its foliage. Like true clover, the leaves are trifoliate, composed of three leaflets that are typically heart-shaped or wedge-shaped with serrated edges. The plant maintains a prostrate, sprawling growth habit, forming dense patches that suppress surrounding turfgrass. This low profile allows the weed to thrive even in frequently mowed areas.
Small, bright yellow flowers appear in clusters of two to five during the spring. Following pollination, these develop into the namesake structure: small, coiled seed pods, or burs, covered in sharp, hooked spines. Bur clover typically behaves as a winter annual, germinating in the cool, moist conditions of late fall and winter. The plants mature, flower, and set seed in the spring before dying back in the heat of summer, leaving behind the hardened burs to germinate the following season.
Physical and Cultural Removal Techniques
For homeowners seeking non-chemical solutions, mechanical removal is effective for small, isolated infestations of bur clover. Hand-pulling the weed is most successful when the soil is moist, allowing the removal of the entire taproot, which is necessary for preventing regrowth. This manual task must be performed before the spiny burs have formed, as mature burs will detach and remain in the soil, ensuring a new crop of weeds the next season.
Adjusting mowing practices can also significantly reduce the weed’s spread and vigor. Experts suggest temporarily lowering the mower blade to “scalp” large, dense patches of bur clover to remove the flowering heads and reduce seed production. Immediately after this, the blade height should be raised to the highest setting appropriate for the turfgrass species to encourage dense, tall growth. A taller, denser lawn canopy shades the soil, blocking the sunlight required for bur clover seeds to germinate and establish.
Larger, localized patches can be managed using smothering techniques, such as solarization or heavy mulching, which deprive the weeds of light and heat. Solarization involves covering the infested area with clear plastic sheeting for several weeks during the hottest part of the summer. This generates lethal temperatures beneath the plastic, effectively killing existing plants and many seeds present in the upper soil profile.
Long-term cultural control involves improving the overall health and competitiveness of the turfgrass stand. Bur clover often establishes itself in areas with poor drainage or compacted soil, which stresses the lawn but not the weed. Aerating compacted soil and amending the area to improve drainage creates an environment where desirable turfgrass thrives and actively outcompetes the low-growing weed for resources like water and nutrients.
Chemical Control and Long-Term Prevention
When cultural methods prove insufficient for large-scale infestations, chemical herbicides offer a targeted means of control, divided into pre-emergent and post-emergent applications. Pre-emergent herbicides prevent seed germination and must be applied before seeds sprout, typically in late summer or early fall before the winter annual cycle begins. These chemicals create a barrier in the upper layer of the soil, disrupting the cellular division necessary for the seedling to emerge.
Timing the pre-emergent application is important, as applying it too late will have no effect on seeds that have already begun to germinate. For actively growing plants, post-emergent herbicides are necessary, and products containing active ingredients like 2,4-D, dicamba, or MCPP are commonly effective against this broadleaf weed. These systemic herbicides are absorbed by the plant’s foliage and translocated throughout the plant, disrupting growth hormones and causing the weed to die.
Post-emergent treatments are most successful when applied to young, actively growing bur clover plants, generally in the fall or early spring before the plant flowers. Once the plant has matured and produced the tough, spiny burs, the efficacy of most chemical treatments decreases. Always read and adhere strictly to the manufacturer’s directions on the herbicide label, paying close attention to application rates and safety precautions.
Long-term prevention focuses on altering the lawn environment to make it inhospitable to bur clover. This weed frequently thrives in soils that are slightly alkaline or have low fertility, giving it a competitive advantage over many common turfgrasses. Conducting a soil test to determine pH and nutrient levels allows for precise application of amendments to favor the lawn species.
Proper fertilization encourages a dense stand of turf, which is the most effective biological defense against weed invasion. Using a fertilizer that slowly releases nitrogen helps maintain consistent, vigorous grass growth throughout the season without causing excessive surges. Adjusting watering practices to deep, infrequent irrigation cycles encourages the deep root growth of the lawn, enabling it to better withstand stress and crowd out shallow-rooted weeds like bur clover.