Disturbing the soil (verticutting, aerating, dethatching or raking) exposes soil and any pre-existing weed seeds that lay dormant in the soil. These weed seeds could be from last year or several years ago, suspended under the soil, too deep to germinate waiting for their opportunity to spring into life.
Most of these new weeds will be annual grassy weeds. Grassy weed seeds appreciate the same fertilizer and additional watering from your fall renovation program and flourish. Don't panic annual grassy weeds will be slow to germinate as soil temperatures cool in fall. Mother Nature will take care of these annual weeds at first frost. Using PREVENT, crabgrass preventer in mid-April will eliminate their return.
Dandelion, clover, spurge and numerous other broadleaf weeds are stimulated as well. Fall is an excellent time to control those broadleaf weeds. Perennial broadleaf weeds are busy sucking up much needed nutrients to store for overwintering. An application of Trimec or Speedzone can be applied 28 days after grass seed germination. A fall application of Trimec with Uncle's Stikit, spreader sticker or a granular application of Loveland Weed and Feed will be quickly absorbed by the broadleaf weeds and is sure to kill all the way to the roots.