When is the best time to apply crabgrass preventer?
The quick and dirty answer is "before the seed germinates...duh!" But really, to understand how to best control crabgrass is to understand it's lifecycle. Crabgrass is an annual weed, which means that each crabgrass plant is "born" and dies within each growing season. It regenerates by developing seed heads that become the next years crop. Crabgrass begins to germinate when soil temperatures reach 60 degrees. This is why we tend to see more crabgrass along driveways and sidewalks. The radiant heat from the sidewalk or driveway warms soils quicker, and earlier, than the middle of a yard and typically crabgrass preventive applications are "late to the table". Crabgrass preventatives contain chemicals that are absorbed by crabgrass seedlings and halt the growth of the plant and subsequently die. These chemicals must be present in the soil where crabgrass seeds exist. Any mechanical actions like aeration or dethatching can inhibit the effectiveness of the chemical if these operations are performed after application. The best approach to controlling crabgrass is to combine a pre AND a post emergent application. Any crabgrass that makes it though the first round is likely to be wiped out in the post emergent application. Typically, liquid solutions provide a better "blanket of coverage" than granular controls. Other factors that can make or break your control program are climate, soil types and rainfall.