White clover (Trifolium repens) is a cool-season, perennial legume that may not be a weed at all to some. Not too long ago, clover was a common addition to turf seed mixtures. Although a source of nitrogen for lawns, it can be an undesirable species if the turf serves as a play area for children—it attracts bees, which can be a hazard and can also cause stubborn grass stains. Low-growing, it spreads by stolons, creeping stems that root at the nodes and forms patches. Its roots are fibrous. The trifoliate leaves have three unstalked, oval leaflets, each marked with a faint white crescent. Petioles are longer than the leaflets. The pea-shaped flowers are white, sometimes tinged with pink, and are combined into rounded (globose) heads (inflorescences). White clover likes moist, low-fertility soils and cool, moist springtimes.

Black medic (Medicago lupulina), unlike white clover, has a stalked central leaflet; petioles shorter than the leaflets; yellow, pea-shaped flowers; and a shallow taproot. Also, it’s usually a summer annual (sometimes a winter annual or biennial) and likes droughty, low-fertility sites. This spring in central Illinois, black medic appears to be more abundant than in recent years.

Yellow woodsorrel (Oxalis stricta) also has three leaflets, but each leaflet is heart-shaped rather than oval. Like black medic, the flowers are yellow but instead are comprised of five petals. Like white clover, the central leaflet is not stalked. Petioles are much longer than the leaflets. Yellow woodsorrel is a perennial that spreads by rhizomes, but it can act as a summer annual. It often grows more erect than the previously mentioned species and seems to tolerate a wide variety of soil types and site conditions.

After proper identification, controls may be administered. Refer to “Controlling Broadleaf Weeds in Turf,” issue no. 5 of this newsletter (http://www.ag.uiuc.edu/cespubs/hyg/html/200305j.html) for further assistance.