Q: My lawn was covered with chickweed last year and this spring! I put
down a pre-emergent carefully last fall and watered it in. Why did it not work?
A: Since chickweed is an annual plant that sprouts from seed each winter, my
typical advice would be to apply a pre-emergent herbicide in September. But
if you previously had a big infestation, we need to get creative. Don’t mow
low at the end of summer: grass foliage will shade chickweed seeds and help
prevent germination. Look online and on hardware store garden shelves for
the chemical isoxaben (Gallery) pre-emergent. It has the strongest effect on
broadleaf weed seeds. Make your first application in early September. Be sure
the chemical is applied in the corners and on the edge of your lawn. Put down
at least .5″ of water after application. Make another application of isoxaben 60
days after the first. You should get more than 90% control. Any chickweed
that pops up later can be spot sprayed with a broadleaf weed killer product.