Many herbicides are available to manage annual weeds. Preemergence herbicides are absorbed by germinating seeds and seedlings of annual grassy weeds such as crabgrass. Timing of application is very important: The weed killer should be applied before the crabgrass emerges from the soil. Many preemergence crabgrass herbicides are available in combination with lawn fertilizer at garden supply stores, so crabgrass prevention and spring fertilization can be done at the same time.

Crabgrass germinates when soil temperatures are approximately 60°F at the quarter-inch level for five to seven consecutive days. Other annual grasses germinate as soils get above 60°F.

For central Illinois, mid- to late April is the suggested time for applying a preemergence crabgrass herbicide. In the southern portion of Illinois, make the application one to two weeks earlier, and in northern Illinois, one to two weeks later than the period suggested for the central portion of the state. If the spring is very warm, use the earlier dates; in cold, late springs these materials could be applied later. Using forsythia flowering as a guide is not a dependable method for determining application dates. Some herbicides may be reapplied for extended control; refer to the label for timing and rates. Core aerifying or de-thatching should be done according to label instructions.

Preemergence annual grass herbicides, except siduron (Tupersan), will interfere with seed germination of desirable turfgrasses, whether you are seeding a new lawn or overseeding into an existing lawn. Thus, siduron is the only preemergence herbicide that can safely be used for spring seeding or overseeding. Siduron is often combined with high-phosphorus starter fertilizers and is available at many retail garden outlets.