Pre-Emergence
The lawn weed, crabgrass is a warm-season annual weed, which means it reproduces by seed. People often ask me how to kill crabgrass. The short answer: applying pre-emergent herbicides at the right time is the best way to kill crabgrass.
To get rid of crabgrass, it helps to know its life cycle. When spring soil temps (at a depth of 2"-3") reach 55-60 degrees Fahrenheit, the first crabgrass seed will germinate. From mid-summer to fall, crabgrass produces seed. The crabgrass plants (but not the seeds) are killed by frosts in autumn.
Pre-emergent herbicides come in granular form and kill crabgrass seedlings as they germinate. Think of pre-emergent herbicides as forming an invisible shield across the soil surface that stops emerging crabgrass dead in its tracks.