Too Late for Crabgrass? When is the best time to treat?

The best way to control crabgrass is to kill it before it grows with a pre-emergent herbicide, but when is it too late to treat? Unfortunately, there’s no single definitive answer to this question. The short answer is to apply it before the crabgrass has a chance to germinate. The longer answer is that it depends on the weather and which pre-emergent is being used. Pre-emergent herbicides work by creating a chemical barrier that kills the crabgrass when it germinates. Some pre-emergent products provide some post-emergent control as well. Determining when a seed germinates depends on many variables in the environment. For crabgrass, the biggest variable is temperature. Crabgrass only germinates at sustained soil temperatures above 57°F at a one-inch depth. The best way to track the soil temperature is with growing degree days (GDD). Because it is tough to track soil temperatures without specialized equipment, we can use air temperature. Air temperatures will usually need to be at around 50°F for 200 degree days for germination. (more…)