Illustration: Weeds of the North Central States
Black medic usually is found where some soil compaction has occurred, such as along curbs and sidewalks, particularly where turfgrass may already be thin. Relieving that soil compaction is the most important cultural control. For small areas, this might be done by using a spading fork or a pitchfork. The ground should be in a good workable condition, neither dry and hard nor mushy and soggy. Push the fork straight in about every 6" to 12", pull back slightly, then pull it out of the ground.
For larger areas, rent a machine called a core aerator to take plugs of soil out of the ground. Some lawn services also can do the work for you. The best time to aerate is early autumn, when there are still several weeks of good growing conditions for grass. Follow aeration with overseeding to help replace thin, weed-infested turf with good grass.
Other cultural practices that promote lawn growth include high mowing(2½" to 3½") in hot weather, thorough deep watering during hot dry periods, and fertilizing, primarily in the fall. A thick, healthy turf helps to crowd out and prevent weeds such as black medic.
To control black medic chemically, apply a broadleaf lawn herbicide containing a combination of 2,4-D and MCPP or triclopyr as its active ingredients late May or early June. More than one application may be necessary. To be most effective, the herbicide should be applied when temperatures are between 60° and 80°, no rain is forecast for 24-48 hours, and no wind is present to blow the herbicide onto desireable broadleaf plants. Please read and follow label directions carefully.
For spot treatment of small, unwanted patches of black medic, you can avoid the use of a tank sprayer by hand pulling the weeds, or by mixing the herbicide according to label directions and then applying it with a disposable paint brush or a sponge tied to a stick. Glyphosate (Roundup) works well for this since the area may be reseeded seven days after application. Be aware, though, that glyphosate is non-selective--it can kill or damage the grass or other green, living plants that it touches. Be sure to read and follow the product's label instructions and precautions.