If you see light green patches mixed within your fall lawn, there is a good chance you have common chickweed (Stellaria media). It’s a winter annual that’s found in all parts of the country.
Chickweed has small egg-shaped leaves, and petite white flowers. These weeds are found as spreading clumps, 4-12 inches tall. It needs seeds to reproduce, so be sure to keep your lawn mowed, which will reduce the chances of this weed going to seed.
There are two ways to control Chickweed chemically. One is with a post-emergent broadleaf weed killer. They’re marketed under many different product names, including Weed-B-Gone, Bayer Advanced Southern Lawn Weed Killer, and Green Light Spot Weed Killer. Most use active ingredients of 2,4-D, MCPP, and Dicamba. Always read the label to be sure the product is designed to treat chickweed.
It’s best to spot treat your problem areas. You also want to be careful to not treat newly seeded lawns until they’ve had a chance to mature. The best way to gauge this is to wait until you’ve mowed your new lawn at least three times. Otherwise, the weed killer may harm the tender new grass as well.
The other option is to use a pre-emergent. However, if you plan on seeding your lawn in the fall, this is not the best option because the pre-emergent will prevent not only the chickweed from sprouting, but your grass seed as well. If you take this route, you’ll have to sow the grass seed in early September, and then apply a pre-emergent after you’ve mowed three times, or waited at least two months.
To be most effective, pre-emergent must be applied before chickweed germinates, so apply it in September. You can then apply new grass seed next spring. Your new grass won’t be as established when summer comes, but that’s the tradeoff you’ll have to make. Pre-emergent is also marketed under several different brand names. With pre-emergent, you apply it to your entire lawn.
Finally, taking the above precautions doesn’t guarantee a chickweed-free lawn each year. You still have birds, wind, and other factors that will continue to make deposits of this unwanted seed. But you will make a big improvement in the amount of chickweed growing in your lawn.