Q: My lawn was covered with chickweed last spring 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)(click for source) 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 in March can be spot sprayed with a broadleaf weed killer product.