When do you put weed killer on a lawn?

Exactly when to apply the weed killer depends upon the herbicide used. Pre-emergent herbicides should be used in the early spring before weeds begin to grow to prevent seed germination. Post-emergent herbicides work on weeds that have already developed and can be applied when the weed is actively growing.

Weed-and-Feed type weed killers should be applied to the lawn in the late spring when the weeds are actively growing. These should be applied no more than twice per year. The product is applied using a drop spreader and should be applied to a damp lawn. Most are developed to kill weeds such as dandelions and other nuisance weeds.