Crabgrass is a common weed in many lawns. Crabgrass is an annual, warm-season grass. Germination of crabgrass seeds usually begins about mid-April in southern Iowa, early May in northern parts of the state. Crabgrass seeds continue to germinate over several weeks from spring into summer.
The best way to prevent crabgrass infestations in lawns is to maintain a thick, healthy lawn through proper mowing, irrigation, and fertilization. Crabgrass has a difficult time germinating and surviving in thick turf. Gardeners who have had crabgrass problems in recent years will need to apply a preemergence herbicide in spring.
A key to the successful control of crabgrass in lawns is the correct timing of the preemergence herbicide application. Preemergence herbicides must be applied before the crabgrass seeds germinate. 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.
Longtime residents of Iowa realize that this state's weather is rarely "normal." This has been especially true in the last few months. The unseasonably warm weather in February and March has resulted in trees and shrubs leafing or flowering 2 to 4 weeks earlier than normal. Lawns have also greened up much earlier than normal. The warm weather has prompted questions as to when to apply preemergence herbicides. In 2012, we are recommending the following application dates; March 25 to April 7 in southern Iowa, April 1 to April 15 in central Iowa, and April 10 to April 25 in northen portions of the state.
It's important to remember that crabgrass is a warm-season grass. If a few crabgrass seeds germinate early, freezing temperatures in early to mid-April (likely in most of Iowa) will destroy the crabgrass seedlings.