I am told that a conservative basis for pre-emergent treatment of crabgrass is when the temperature of the soil at 4" reaches 40 degrees. What should I be looking at when the temperature reaches 40 degrees this early in the season? Will later freezing kill any emergent shoots or should I be putting down dithiopyr now since it should remain effective until spring thaw?
In general, Crabgrass seeds germinate when soil temperatures average 55° F. for seven days to ten days. There are different micro climates in your yard and soil temperatures may be different in areas such as south facing areas, near sidewalks, etc. If you do not have a soil thermometer, during and shortly after, forsythia bloom is a rough, but not consistently reliable, guide for application timing. Here is more information on crabgrass, preemergents, and tips for application