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Pre-emergent herbicides act by forming a chemical layer at the surface of the soil. As crabgrass seeds germinate, they take in the herbicide, which prevents them from sprouting and ultimately kills them. This should coincide with the blooming of the forsythia. Buy an inexpensive soil thermometer if you don't know when to time your herbicide application. Always apply the herbicide along with a starter fertilizer. Fertilizer helps thicken the turf, which in turn snuffs out crabgrass seeds that weren't killed by the fertilizer. Apply a starter fertilizer along with your herbicide and you're getting more bang for your buck. As small as the print is, you will find valuable information on time of application and safety precautions. Look for herbicides that use the following active ingredients: dithiopyr, prodiamine or pendimethalin. The herbicide that you use to snuff out unwanted crabgrass is the same stuff that will lay waste to the soft, pretty grass you actually want to cultivate. That means you have to seed during one season and use herbicide during another: Seed during the early fall and use herbicide during the spring, making sure to keep a buffer of at least 50 days in between. These will kill your grass as well so only use if you have more crabgrass than lawn or thick patches of it.
Prevent crabgrass from growing in the first place with pre-emergent herbicides. Apply pre-emergent herbicides in the late winter/early spring when soil temperatures stabilize at 55°F at a depth of 4". When using herbicides, be sure to read the label carefully. Reseed or use the herbicide, but never both. Apply post-emergent herbicides in the summer, if you see crabgrass.