For the best results, gardeners should time the weed killer application to take place before weed seeds start to germinate, generally in the early spring. Gardeners often reapply the weed killer approximately eight weeks after the first spraying to help with exceptionally invasive weeds, such as crabgrass.

When should I put down weed killer?

The best seasons for applying a post-emergent weed killer to control perennial broad leaf weeds are early fall and spring. Apply the weed killer on a still day when the air temperature is between 60 and 80 degrees Fahrenheit, and no rain is forecast for 48 hours.

Do you put weed killer on before or after rain?

For many herbicides, any amount of rainfall soon after spraying has the potential to reduce absorption, translocation, and subsequent weed control. If you apply herbicide and it rains before it's rainfast, herbicide performance will be reduced.