Black medic (Medicago lupulina) is a thrifty yellow-flowered clover that usually grows as an annual. Common in lawns and gardens around the world, black medic often colonizes dry, infertile spots where little else will grow. Plants stay close to the ground until they are ready to bloom. By the time flowers appear, the stems may be 6 to 26 inches long. Providing lawn grass with adequate nitrogen and phosphorous discourages this weed. It is also important to pull young weeds during spring and early summer. In addition, a corn gluten herbicide applied in spring will help prevent germination of black medic seeds.
Pulling. Most young weeds can be pulled from the soil. They will slide out most easily if you pull them when the soil is wet. Getting the root up is crucial, so think of the main stem as the root's handle, and grasp it as close to the soil line as you can. If you find that the weeds are breaking off at the crown as you pull, slip a kitchen fork, dandelion weeder, or similar tool under the weed, and pry and twist as you pull it up. Weeds that have taproots, such as dandelion and plantain, usually must be pried out. A flexible pair of waterproof gloves will keep your hands comfortable as you weed, and it's good to have a nice sitting pad, too. Let pulled weeds bake in the sun for a day or so before composting them. If pulled weeds are holding mature seeds, compost them separately in a hot, moist pile before using this compost in the garden.
Corn gluten herbicides. Powdered herbicides made from corn gluten keep crabgrass and other weed seeds from germinating and growing. They are typically spread on established lawns, but they also can be used in gardens where no seeds will be planted, such as in perennial beds. As the corn gluten degrades, it provides a small amount of nitrogen to the soil. Crabgrass begins to germinate at about the time that azaleas, dogwoods, and forsythias bloom, so spread corn gluten at that time for best results. Application procedures vary with the particular product; be sure to read and follow the directions on the label. Do not use corn gluten in newly seeded lawns, or in garden beds where you plan to sow seeds.Image courtesy of Plant Stock Photos