Crabgrass, knotweed, oxalis, spurge, and purslane (summer annual weeds) have already emerged here.
I did not treat my lawn with any PRE this year. Here is a photo of my lawn today. Not a single weed in it.
Only thing I did was apply N,P, and six micronutrients about a month ago.
During summer weather, I mow at 3 1/4 -- 3 1/2 inches..... This is the problem with homeowners. Unless their lawn is professionally mowed >> they are almost ALWAYS mowing too short!!
Homeowners bring this (crabgrass) problem on themselves. Try to tell them, but forget it. Then they blame their lawn service for the problems their lawn encounters. Weeds, thinning out, diseases, etc, etc.
It's the same game every year. Homeowners swear they do every thing right.