To control crabgrass, you have to take action early. Once the weeds sprout, it’s too late. So, before the crabgrass seeds germinate and sprout, apply a preemergence herbicide. This actually applies to any grassy weed. Set up an early warning system for yourself to ensure that you won’t miss your application window for controlling grassy weeds. When you see the forsythia bushes in your neighborhood in bloom – bright yellow leaves – it's time to spread a preemergence herbicide.