Crabgrass preventer is, as the name suggests, a preventer, not a killer. It is important to understand that it must be put down in the spring before or as the crabgrass is germinating.
The preventer dissolves and forms a barrier on the surface of the soil. ANY seed that is germinating in the treated area is killed as it attempts to penetrate the barrier. Because crabgrass is an annual grass (grows from seed dropped from a mother plant the previous season) a pre-emergent crabgrass preventer is the most effective control. Weeds (dandelions, plantain) come from an already established root system and are not affected by crabgrass preventer.
Three important factors to remember:
1. To be most effective, crabgrass preventer must be applied in early spring. The best device for timing the application is the forsythia bush. The forsythia is the first shrub to flower in the spring (bright yellow flowers open even before the bush has leaves). Ideally, crabgrass preventer should be applied anytime before the forsythia has completely dropped its flowers. If the lilac are fully in flower then it is too late to apply a pre-emergent.
2. Because crabgrass preventer preventer ANY seeds from germinating, lawn seedling projects should be postponed as the preventer will also prevent blue grass, ryegrass, fescue or any other seed from germinating.
3. An area treated with crabgrass preventer should not be raked or roughed up (kids, dogs, etc.) for four to six weeks so that the preventive barrier is not disturbed.

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