When are weed killers normally applied on home lawns?

Answer 1

Weed killer has to be applied when the plant is actively growing. However there are "pre-emergents" that prevent weed seeds from germinating in the first place. I think those are applied to the lawn in the fall.

Answer 2

Usually we recommend applying a weed killer in the spring and early fall. Weeds are more actively growing and the weed seeds in the soil will have germinated.