Crabgrass is an annual with branching, spreading stems. Its coarse, blue-green to purplish leaf blades can be smooth or hairy, depending on the species. Flower heads with several fingerlike spikes rise from narrow stems. Crabgrass thrives in lawns mowed shorter than 2 inches, underfed lawns, and those watered frequently and lightly. Thick, deeply irrigated turf is the best control. Dig crabgrass before it seeds. Preemergence crabgrass herbicides are available; apply in spring before soil temperature reaches a steady 60 degrees F.
Yellow woodsorrel is a broadleaf perennial, although it might act as an annual in some regions. Also known as oxalis, it has cloverlike leaves and yellow flowers, each with five petals. Plants spread by roots and seed. This weed is difficult to control, and does best in thin turf watered frequently and lightly. Water thoroughly and fertilize properly. Dig out small plants or spot-treat isolated ones with a postemergence weed killer. Prevent new weeds with a preemergence herbicide with oxalis on the label.
Quackgrass is a perennial grass with flat light-green to blue-green leaves. It spreads by seeds and aggressive underground stems, called rhizomes. Thoroughly dig out roots and pointed rhizomes—remaining pieces regenerate new plants. Spot-treat with a nonselective weed killer.
Yellow nutsedge is a grasslike perennial sedge with triangular stems and 1/4-inch-wide leaves. Also called yellow nutgrass, it reproduces by seed and tubers that grow at the root tips. Tubers often persist in the soil, making established plants difficult to control. Mow high in early to midsummer and water infrequently though thoroughly. Spot-treat with postemergence herbicides labeled for nutsedge. As with most weeds, control is easiest when plants are small.
Spotted spurge is a broadleaf annual that grows close to the ground in a fast-spreading mat. Its small leaves are green with a brown-red spot on top. Cut stems exude a milky liquid. Spotted spurge reseeds heavily. A high-mowed, well-fertilized and vigorous lawn provides tough competition. Pull isolated plants before they seed. Spot-treat with a postemergence weed killer and use appropriate preemergence herbicides to prevent new weeds.