9 minute read
Feature
Can We Control Weeds in St. Augustine?
By Jeffrey Derr, Ph.D. and Adam Nichols
At our research station located in southeastern Virginia, we receive questions about weed control in certain warmseason turf species that are not grown in western parts of the state. One common lawn species in our area is St. Augustine. It tolerates the heat and drought of summer and has better shade tolerance than bermudagrass or zoysia. We have evaluated cold tolerance in St. Augustine cultivars through an NTEP project. Although we have seen significant winter damage in certain St. Augustine cultivars, some, such as the Raleigh types, generally tolerate the winter temperatures we see in Hampton Roads.
Weed control, however, is a challenge in this turf species. In this article we will discuss current suggestions for controlling grass, sedge, and broadleaf weeds in St. Augustine, as well as some of the research we are currently conducting. Although you may not be maintaining St. Augustine, some of the treatments we are evaluating are proving useful in other turf species and thus may benefit your program.
Herbicide tolerance in St. Augustine: There are some common turf herbicides that cannot be used in St. Augustine due to severe injury. Drive (quinclorac) and products containing quinclorac cannot be used in this turf species. MSMA, currently registered for golf course, sod, and rights-of-way use, also causes severe injury to St. Augustine. The sulfonylurea herbicides Monument (trifloxysulfuron), Revolver (foramsulfuron), and Katana (flazasulfuron) are not registered for use in this turf species.
Since St. Augustine has overall lower herbicide tolerance than bermudagrass and zoysia, one needs to pay special attention to rates when making applications. Often application rates are lower for St. Augustine than other turf species. Treating small areas first to gain knowledge of turf tolerance when working with a new herbicide or herbicide formulation in St. Augustine is a good and safe practice.
Annual grass control preemergence: In established St. Augustine we can use the preemergence crabgrass preventers used in other turf species. For example, prodiamine (Barricade, others), oxadiazon (Ronstar), benefin (Balan), bensulide (Bensumec), pendimethalin (Pendulum, others), dimethenamid (Tower), dithiopyr (Dimension), napropamide (Devrinol), and indaziflam (Specticle) can all be applied to St. Augustine. Read the label, though, as some of these chemicals, such as oxadiazon, cannot be used on home lawns. Also, sprayable formulations of oxadiazon can only be applied to dormant St. Augustine.
These crabgrass preventers herbicides can also be applied in August for preemergence control of annual bluegrass as well as control of common chickweed and certain other winter annual broadleaf weeds. Use lower rates of indaziflam on St. Augustine compared to bermudagrass due to injury concerns. There is a combination product available that combines prodiamine, simazine, and imazaquin (Coastal Herbicide). This product will provide preemergence control of a range of annual grass and broadleaf weeds as well as early postemergence control of certain weeds, such as annual bluegrass.
St. Augustine will probably be dormant when the application needs to be made for preemergence crabgrass control. If there was any winter kill, these preemergence herbicides will prevent the pegging down of St. Augustine stolons that try to fill in dead areas. St. Augustine does not produce rhizomes so it is dependent on spread through stolons to cover an area. So, if you have observed unusually low temperatures during the winter, or a rapid decrease in temperature as we observed in the winter of 2022–2023, you may want to hold off on application of a crabgrass preventer until St. Augustine has greened up and any winter damage will be apparent. The problem with that approach, though, is that crabgrass probably has started germinating before St. Augustine is fully greened up. There are limited options for postemergence crabgrass control in this turf species.
Annual grass control postemergence: Atrazine (Aatrex, others), sulfosulfuron (Certainty) and amicarbazone (Xonerate) are options for postemergence annual bluegrass control. The Certainty label cautions users, though, to test this product on small areas first as St. Augustine is less tolerant compared to other warm-season turf species such as bermudagrass.
Postemergence crabgrass control: This is a challenge. Dimension and Celsius will control pre-tillered crabgrass and atrazine will suppress this weed. Xonerate will suppress crabgrass but the label warns against summer applications when temperatures are high. Other postemergence crabgrass herbicides used in other turf species, such as quinclorac, will damage St. Augustine.
Perennial grass control: There are not too many options here. Atrazine could be used for suppression of actively growing bermudagrass as well as removal of cool-season grasses like tall fescue. We have also evaluated Prograss for bermudagrass suppression. Based on our work, with some assistance from these chemical treatments, St. Augustine appears to be able to outcompete bermudagrass. We have evaluated the addition of mesotrione (Tenacity) to atrazine, which has a synergistic benefit, but Tenacity is only registered for St. Augustine grown for sod. Certainty suppresses dallisgrass and controls cool-season grasses like ryegrass and tall fescue but check the tolerance on a small area first.
Preemergence annual broadleaf control: Both atrazine and simazine (Princep) provide control of many broadleaf weeds at germination. Isoxaben (Gallery) will also control a wide range of annual broadleaf weeds, especially winter annuals.
Postemergence broadleaf control: St. Augustine is sensitive to some of the postemergence broadleaf herbicides used in turf, such as 2,4-D. Products for use in use in sensitive southern grasses such as St. Augustine utilize reduced rates of the broadleaf herbicides. For example, Speedzone EW, which is not labeled for use in St. Augustine, contains 1.27 pounds 2,4-D and 0.12 pounds dicamba per gallon while Speedzone Southern EW, which is labeled, contains 0.51 pounds 2,4-D and 0.05 pounds dicamba per gallon. The two products have a similar use rate (pints per acre).
One product we regularly use for broadleaf control in St. Augustine is Celsius, which has three active ingredients: iodosulfuron, thiencarbazone, and dicamba. It controls a wide range of broadleaf weed and certain annual grasses. Celsius XTRA is a similar product that also contains iodosulfuron and thiencarbazone but instead of dicamba it contains halosulfuron. Halosulfuron is the active ingredient in SedgeHammer and ProSedge. So Celsius may be the preferred product if only broadleaf weeds are present. Celsius XTRA would be the better choice if one is trying to control both broadleaf weeds and yellow nutsedge. Another combination product where acceptable tolerance exists in St. Augustine is Avenue South, which contains 2,4-D, dicamba, penoxsulam, and sulfentrazone. St. Augustine does not possess acceptable tolerance to Tribute Total, which contains foramsulfuron, halosulfuron, and thiencarbazone.
We would like to be able to recommend an effective postemergence broadleaf herbicide for all warm season and cool-season turf species, especially in our areas due to the variety of turf species being maintained. Unfortunately, we have not identified a product that fits this need. One of the limiting species is St. Augustine, where products containing higher rates of 2,4-D can be injurious, and products containing lower doses of 2,4-D and related herbicides are less effective for weed control. The other limiting species is tall fescue, where Avenue South can be injurious, and other products such as Celsius and Certainty cannot be used.
Yellow nutsedge control: Celsius XTRA, sulfentrazone (Dismiss), halosulfuron, Certainty, Basagran and Vexis are options for control of emerged yellow nutsedge and other sedges. Tower is a possibility for preemergence yellow nutsedge control, but we have not evaluated it in St. Augustine. We have observed delayed greenup in bermudagrass with this product.
Current research – Recognition: The active ingredient in Monument is trifloxysulfuron but Monument cannot be used in St. Augustine due to injury. Recognition also contains trifloxysulfuron but along with this active ingredient it also contains a safener, metcamifen, that increases the tolerance in St. Augustine and certain other grasses. Besides increasing tolerance to sulfonylurea herbicides like trifloxysulfuron, it increases the tolerance in St. Augustine to the postemergence broadleaf herbicide like triclopyr (Turflon Ester), post grass herbicides like fluazifop (Fusilade), and bleaching herbicides like Tenacity.
We have seen a slight off-color to St. Augustine following a Recognition application, but the turf outgrows this effect fairly quickly. So, Recognition, according to the label, is an option in St. Augustine for control of ryegrass, a range of broadleaf weeds, and yellow nutsedge, as well as dallisgrass suppression. We are currently conducting research to determine if Recognition is as effective as Monument on a variety of weed species as it is possible the safener in Recognition could also increase herbicide tolerance in weed species.
We have evaluated Recognition plus Fusilade for suppression of bermudagrass, as well as Recognition plus Turflon Ester for improved broadleaf control and bermudagrass suppression. Although multiple applications will probably be required to eradicate bermudagrass, the bermudagrass suppression makes St. Augustine more competitive. Recognition plus Tenacity has shown promise for control of doveweed and crabgrass.
We also see potential for Recognition plus Fusilade for improved bermudagrass control in zoysia. We have evaluated adding 24 fluid ounces per acre of Fusilade with Recognition. This rate of Fusilade is much higher than can be applied with Fusilade alone in zoysia, showing the benefit of the safener metcamifen. Use of higher rates due to the safener allows for better bermudagrass control. Metcamifen does not appear to increase herbicide tolerance in bermudagrass. There should be similar benefits of Recognition plus Fusilade for control of dallisgrass. We plan to continue our research with Recognition and Recognition combinations in St. Augustine and zoysia.
Jeffrey Derr and Adam Nichols are located at Virginia Tech’s Hampton Roads Agricultural Research and Extension Center in Virginia Beach.
