The warm, wet weather in recent weeks has been ideal for crabgrass. Crabgrass is a warm-season annual grass. Crabgrass seeds germinate from spring to mid-summer. Germination begins when soil temperatures reach 55 to 60Å¡F in the spring. The plants grow rapidly during the summer months and often form dense patches in poorly maintained lawns. There are two species of crabgrass found in Iowa lawns: smooth crabgrass (Digitaria ischaemum) and large crabgrass (Digitaria sanguinalis). Both species have wide leaf blades, a light green color, and prostrate growth habit. Crabgrass plants begin to flower in mid-summer. Seedheads appear as finger-like projections at the top of upright stems. Each plant can produce thousands of seeds until it is destroyed by the first frost in the fall.

The best way to prevent crabgrass infestations is to maintain a dense, healthy lawn. One important cultural practice is proper mowing. In spring and fall, mow bluegrass lawns at a height of 2 1/2 to 3 inches. Bluegrass should be mowed at a height of 3 to 3 1/2 inches during the summer months. Crabgrass seeds require light for germination. A high mowing height produces healthy turfgrass plants but provides little light for crabgrass germination. A low mowing height encourages crabgrass germination and growth.

Another way to control crabgrass is to apply a preemergent herbicide in the spring. To be effective, preemergent herbicides must be applied before the crabgrass seeds germinate. Apply a preemergent herbicide in early April in southern Iowa, mid-April to May 1 in central Iowa, and early May in northern portions of the state. Effective preemergent herbicides include a number of chemical herbicides and corn gluten meal.

If preventative strategies fail, crabgrass can be controlled with postemergent herbicides. Postemergent herbicides are effective when applied to small, actively growing plants within the 3 to 5 leaf stage of development. Postemergent herbicides are not effective against large, well established crabgrass plants. Home gardeners with a lawn badly infested with crabgrass should consider a total renovation of the lawn in the fall.

This article originally appeared in the July 23, 1999 issue, p. 102.