From Craig Roussel, Horticulture Agent with the Ascension/St. James Parishes LSU AgCenter
Now is the time to start applying liquid herbicides for winter weeds in lawns before they become mature and start producing seeds.
Liquid atrazine has tested out best in LSU AgCenter winter weed trials and can be applied for winter weed control in lawns now through early May. Atrazine controls weeds like annual bluegrass (Poa), white clover, chickweed, bedstraw, and lawn burweed or sticker weed. The herbicide is good but not perfect. Atrazine is not effective on wild onion/false garlic, and blue-eyed grass.
Some common trade names for atrazine include: Image with Atrazine for St. Augustinegrass, Hi-Yield Atrazine, and Spectracide Weed Stop for St. Augustine and Centipede Lawns (hose end sprayer).
Herbicides containing “trimec” - The next best option for winter weed management in lawns after atrazine are herbicides that contain 2,4-D as one or more active ingredients. Most consumer herbicides will also have dicamba, and mecoprop, and possibly carfentrazone as additional active ingredients. We usually call these weed killers “trimec” type herbicides. Ortho Weed B Gon, Weed Free Zone, Weed Out, and Trimec work pretty well when sprayed on clovers, chickweed, and other broadleaves but do not have any activity on annual bluegrass. These herbicides are somewhat effective on wild onion, false garlic, and blue-eyed grass. Weed control with these trimec herbicides can be improved with follow-up applications 2 weeks after the initial application.
Weed and Feed Herbicides
- Some homeowners like to use weed and feed products for their winter weed problems. These products are very easy to apply and seem to save a step since you are actually putting out a weed killer while fertilizing your yard — killing two birds with one stone. The problem with weed & feeds are the products are typically high in nitrogen fertilizer. High nitrogen applied in the late winter encourages vigorous lawn growth and green-up which can lead to frost injury and increased brown patch disease susceptibility. Another problem with weed & feed is in the application. If you are not real careful with granular broadcast applicators, atrazine will end up where it is not supposed to be, like in flowerbeds. Atrazine can be very tough on bedding plants.
Not all weed & feed products contain atrazine. Some products contain trimec impregnated on a fertilizer granule. This type of weed and feed is applied when there is heavy dew on the grass or after the lawn has been watered to wet the foliage of the weeds. The herbicide/fertilizer granules must stick to the weed leaves to achieve success. Of the weed and feeds, atrazine based products have out-performed trimec-types of weed and feed in trials.
Overall, granular weed & feed performance has not been as good in field trials as liquid atrazine or liquid trimec products. Liquid herbicides are very active on emerged winter broadleaf weeds and usually eliminate them with one or two applications. The key is to apply early to small actively growing weeds. Flowering weeds in late March and April are much harder to kill.
If you insist on using weed and feed, wait until it’s time to fertilize the yard in April.