(News article for December 31, 2020; updated)
You may have noticed that cool-season weeds have begun to appear in the lawn. Seeds of these weeds generally begin germinating around October in southern Louisiana.
Good cultural practices are the foundation of good weed management. If turfgrass isn’t healthy, bare or thin spots provide openings for weed growth. Healthy turfgrass is better able to compete with weeds.
For lawns, “good cultural practices” include applying fertilizer at the recommended times and rates, mowing at an appropriate height, and having the soil pH (acidity or alkalinity) within the optimum range for your particular type of turfgrass. It also involves avoiding or reducing soil compaction by, for example, core aerating from time to time.
(Learn more about lawn care in the turfgrass section of our website.)
People sometimes wait until weeds are really obvious to do something about them, but if you’re going to use an herbicide, it’s important to do it while weeds are small. Post-emergence herbicides are more effective at this stage. (Post-emergence herbicides are what we use once weeds are present. Pre-emergence herbicides are used to prevent seed germination.)
Not only are weeds easier to kill when they’re small, but it’s important to kill them before they flower and produce seed. Annual weeds and many perennial weeds come back from seed. So, if a weed is allowed to go to seed, there is more potential for problems in the future.
The winter weed I probably get the most questions about is lawn burweed, or “stickers.” Lawn burweed produces spines as it produces seeds. Once the spine is there, even if you manage to kill the plant with an herbicide, this spine will still be there to stick in feet that run barefoot around the yard.
Herbicides containing the combination of 2,4-D, dicamba, and mecoprop are commonly available and effective on a wide range of broadleaf weeds. Combinations of penoxulam, sulfentrazone, 2,4-D, and dicamba have become available in home lawn herbicides, as well. Metsulfuron is likewise effective on a wide range of broadleaf weeds. It’s found in MSM Turf, Martin’s TopShot, and Manor.
There are herbicides with the above-mentioned ingredients can be used on most of the warm season turfgrasses that we grow, including centipedegrass, zoysiagrass, bermudagrass, and St. Augustinegrass. (Some products cannot be used on the St. Augustinegrass variety Floratam.) Check the label of a product to make sure it's labeled for the type of turfgrass you have before using it.
Besides broadleaf weeds, we have some weeds that are grasses. Annual bluegrass is a common cool-season annual weed that tends to be a problem in areas with compacted soils.
It can be hard to kill a grassy weed that is growing in turfgrass, and there are more pre-emergence than post-emergence herbicide options for managing annual bluegrass in centipedegrass and St. Augustinegrass. However, atrazine is an option in some cases. It has both pre-emergence and post-emergence activity on annual bluegrass and is effective on many broadleaf weeds, as well.
Home lawn atrazine product are generally labeled only for use on centipedegrass and St. Augustinegrass, while some commercial atrazine-containing herbicides are labeled for use on zoysiagrass and bermudagrass, also. Do not use atrazine over the root zones of trees or shrubs.
When using post-emergence herbicides, it’s important to use them on weeds that are actively growing. Warm days (temperatures above 60 F) during January and February are generally good times to use a post-emergence herbicide for winter weeds, if needed.
Be sure to read and follow label instructions when using any herbicide or other pesticide.
Contact Mary Helen Ferguson.
Young lawn burweed (“stickers”) grows in an area with thin turf. (Photo by M.H. Ferguson)
Foliage of young lawn burweed (“stickers”). (Photo by M.H. Ferguson)