Rather than a date on a calendar, you want to time your crabgrass preventer by using two early flowering shrubs; the forsythia and lilac. Crabgrass preventer works by killing crabgrass seeds as they sprout, This coincides with the time when the yellow flowers of the forsythia are falling to the ground which is also when the very first lilac flowers are opening. Most people can’t wait and put the crabgrass preventer on WAY too early. Put on too early, the crabgrass preventer barrier breaks down and won’t be there when the crabgrass seeds are sprouting and you’ll end up with crabgrass anyway. The season is running late this year so I estimate we’re 3 weeks or so from when crabgrass preventer should be applied to be effective. The beauty of using forsythias and lilacs to time your application is that they will show the correct time to apply whether the season is running very early or very late.