You want to put crabgrass preventer down before crabgrass has a chance to germinate. Some will say to use a “GDD Tracker” or Growing Degree Day tracker to track the soil temperatures. Others will say to use a soil temperature probe.
It’s actually easier than that. Just look for a forsynthia starting to bloom. It’s those bright yellow bushes that are hard to miss (pictured above). When that happens, it’s actually the perfect time to put down crabgrass pre-emergence.
What if I am too early or late?
If you are early, that’s a good thing. Most preventers last months so it’s better to be early than late. It’s actually not the end of the world if you don’t do it right away. You should still put it down as soon as possible because all the crabgrass doesn’t germinate at the same time. Crabgrass preventer also prevents many other weeds so you want to get it down asap.