Crabgrass is a warm-season annual grass that plagues almost every homeowner. Crabgrass thrives in under watered, poorly drained and under-fertilized lawns. One crabgrass plant is capable of producing up to 150,000 seeds per season. These seeds lay dormant on the lawn through winter and germinate in the spring when conditions become favorable. Crabgrass seeds are easily killed by applying pre-emergent herbicide in the spring before germination takes place.
Apply pre-emergent herbicide to your lawn when the soil temperature reaches 50 degrees F. Apply it with your broadcast spreader at the bag's indicated application rate. A clue is to watch for local forsythia. When they bloom, it is time to apply the herbicide.
Apply a post-emergent herbicide in the early summer to kill any crabgrass that successfully germinates. Mix the concentrate with the indicated amount of water and dispense using a hose-end sprayer. Spray the entire lawn to ensure any young plants that are not yet visible are killed. MSMA or MAMA are great crabgrass post-emergent herbicides.
Practice a good lawn-maintenance program to prevent crabgrass seeds from forming and germinating. Mow your grass high and often. Crabgrass can't form seeds if the top of the plant is continually cut off.
Water your lawn deeply, but infrequently. Seeds require constant moisture to germinate. If seeds are allowed to dry out between waterings, they will die.