I am self taught. Wait til the forsythias bloom. If you do it too early it just doesn't last as long. Those yellow bushes all around in late April early May, I do crabgrass preventer on the south side in last half of April most years north side in Early to Mid May. You wont get it perfect every year its an art and that's what makes it fun for me. forsythia below.What's the key to timing the application of crabgrass preventer? My lawn care guy (who is very cheap but has varying degrees of quality service directly related to his level of sobriety) didn't put it down until mid-June last year and the weeds got really bad in sections of my lawn. I usually will supplement what my guy puts on with some additional fertilizer or other treatment if necessary so I will probably do crabgrass preventer on my own this year.