Application of Preemergence Herbicides to Lawns

Crabgrass is a annual warm season grass and a common weed in many lawns. Crabgrass seeds begin to germinate when soil temperatures reach 55°F. Germination usually begins about mid-April in southern Iowa, early May in northern parts of the state, and continues to germinate over several weeks from spring into summer.

While crabgrass germination begins in spring, plants usually do not become highly visible in lawns until summer. Crabgrass is a low growing, spreading plant with light blue-green foliage. The leaf blades are approximately 1/4 inch wide and seedheads appear as several finger-like projections atop upright stems. Crabgrass grows rapidly during warm summer weather. Growth slows with the arrival of cooler temperatures in late summer. Plants are destroyed with the first hard frost in the fall. However, before it dies a single crabgrass plant can produce up to 150,000 seeds.

The best way to prevent crabgrass infestations in lawns is to maintain a thick, healthy lawn through proper mowing, irrigation, and fertilization. Crabgrass will have a difficult time germinating and surviving in a thick turf. Gardeners who have had crabgrass problems in recent years will probably need to apply preemergence herbicides in spring. Preemergence herbicides must be applied before the seeds germinate and are not effective on emerged weeds. Several preemergence herbicides will control crabgrass in established lawns. These include benefin, bensulide, pendimethalin, and dithiopyr. Often these herbicides are combined with a turf-type fertilizer. This allows the gardener to apply a preemergence herbicide and fertilizer with one application. If you intend to establish a new lawn from seed in spring, the only preemergence herbicide that can be used is siduron (Tupersan). This herbicide effectively controls crabgrass without affecting the germination of the turfgrass seeds.

Gardeners can also control crabgrass by applying corn gluten meal. A corn milling byproduct, corn gluten meal inhibits the root growth of crabgrass seedlings. Unable to develop roots, the crabgrass seedlings die. In addition to crabgrass, corn gluten meal offers pre-emergent control of dandelion, plantain, lambsquarter, and other weeds. Corn gluten meal is approximately 10 percent nitrogen by weight, thus making it a natural "weed and feed" product. Products containing corn gluten meal are available at garden centers and mail-order companies. (Additional information on corn gluten meal can be found at

The keys to successful control of crabgrass in lawns are correct timing of the preemergence herbicide application and proper application of the material. Preemergence herbicides must be applied before the crabgrass germinates. If the material is applied too early, crabgrass seeds that germinate late in the season will not be controlled. If applied too late, some crabgrass will have already germinated. Preemergence herbicides should normally be applied in early to mid-April in southern Iowa, mid-April to May 1 in central Iowa, and late April to early May in northern areas of the state. Weather often varies considerably from year to year in Iowa. Accordingly, gardeners should make adjustments in the timing of the preemergence herbicide application. If the weather in March and April is consistently warmer than normal, apply the preemergence herbicide early in the normal time period. Apply the herbicide late in the recommended time period if Iowa is experiencing a cold, early spring. If you're still uncertain as to when to apply the preemergence herbicide, Mother Nature does supply some helpful clues. Crabgrass seed germination usually begins with the end of the forsythia bloom season or when redbud trees reach full bloom.

To insure the herbicide is applied properly, carefully read and follow the label directions on the package. Also, make sure the spreader has been correctly calibrated and is working properly.

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