If you're using herbicide, apply a pre-emergent shortly before annual weeds, such as crabgrass, begin to grow in the spring. A good rule is to apply the pre-emergent before the dogwoods begin to bloom. You may decide to use a pre-emergent combined with fertilizer as an early lawn treatment.f you're using herbicide, apply a pre-emergent shortly before annual weeds, such as crabgrass, begin to grow in the spring. A good rule is to apply the pre-emergent before the dogwoods begin to bloom. You may decide to use a pre-emergent combined with fertilizer as an early lawn treatment.