Postemergent chemical and cultural treatments

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May 15, 2015 -  By

By applying the following tips, you can improve the vitality of your clients’ turfgrass this summer.

As summer quickly approaches, the next few weeks are critical, especially for lawn care operators (LCOs). After all, a multitude of annual and perennial weeds—common plantain, prostate spurge, dandelions—have recently emerged throughout the U.S. And, if they’re not treated properly, their growth, maturity and dispersion will soon be problematic.

However, through sound chemical and cultural practices, you can significantly shorten the lifespans of these irritating weeds, and help your clients beautify their lawns all summer long.

Timing is everything



Did you know one square foot of crabgrass produces approximately 10,000 seeds every year? That’s one reason it should be controlled with postemergent herbicides in late spring—to prevent the development and future dispersion of seeds—as is the case with most other summer annual weeds and grasses.

“Since summer annuals are actively growing during this time of year, they’re easier to control with herbicides,” says Patrick McCullough, Ph.D, associate professor, crop and soil sciences, University of Georgia. “Not to mention, the turfgrass surrounding the weeds will better tolerate the herbicides since the air temperatures will usually be cooler.”

Regarding herbicides, amine salt formulations are ideal when LCOs apply the treatments to weeds and grasses in May and June, as they are often less volatile than ester formulations and less likely to damage clients’ trees, gardens, and flowers, according to Aaron Patton, Ph.D, associate professor of horticulture and landscape architecture, Purdue University.

If clients have an assortment of weeds and grasses that need to be controlled, LCOs should consider products that have multiple ingredients or are eligible for tank mixtures, thereby applying a wider range of herbicides to the weeds and grasses.



“But make sure herbicides are not applied to weeds and turfgrasses that are stressed as a result of high temperatures or droughts,” McCullough says. “Turfgrass tolerance to postemergence herbicides decreases at air temperatures greater than 90 degrees. Also, weed control is often reduced when herbicides are applied to stressed weeds, rather than actively growing weeds.”

Patton agrees, adding, “Applying postemergence products at temperatures greater than 85 degrees will increase the risk of turf injury.”

At the same time, LCOs should ensure their herbicides are being applied to weeds and grasses correctly so that they don’t waste products or harm surrounding turf.

“You should check your spray pattern to look for good coverage, either on a driveway or in a parking lot,” says Jamie Breuninger, Ph.D, technical leader, Dow AgroSciences. “Also, by applying herbicides at the correct rate, you can prevent over-application, which can result in leaf burn.”

McCullough emphasizes it’s not necessary to “drown” the weed with a postemergent herbicide.

“Any spray that runs off the weed is usually wasted and does not increase chemical control,” he says.

Proven cultural practices

As important as chemical practices are to ensuring a successful weed control program, McCullough believes the impact of cultural practices can’t be overemphasized.

“Properly maintained turfgrasses are more competitive with weeds than turfgrasses that don’t receive good cultural practices,” he says. “The use of herbicides—without following approved cultural practices—will not result in a high-quality, weed-free lawn.”

Aaron Patton advises LCOs to determine the underlying cultural causes of clients’ unhealthy turf before any weed control program is implemented, focusing specifically on fertilization, irrigation and mowing heights and regularity.

“Proper cultural practices can reduce weed populations by 70 percent or more, so herbicides should not be a substitute for a conscientious cultural program,” he says. “When the right turfgrass species is chosen, and proven cultural practices are followed, weeds will be less competitive in turf.”



While mowing, Breuninger believes clients should be advised on height guidelines and mowing frequency, cultural methods that can limit the development of many weed species.

“For Bermudagrass, mowing height should be 1.5-2 inches,” he says. “If clients have tall fescue, the mowing height should be adjusted to 3-4 inches,” he says. “And if they have St. Augustinegrass or centipedegrass, they should be mowed at 2.5-3 inches and 1-1.5 inches, respectively.”

In addition, Breuninger recommends frequent mowing (once or twice a week), as many weed species, such as woody tree seedlings, don’t tolerate regular trimmings.

“By maintaining the proper mowing height and practicing the rule of never removing [more than] a third of the leaf blade of the grass, you’re favoring the development of a healthy turf and helping to keep the turf weed free,” he says. Also, avoid mowing drought- or heat-stressed lawns, which can cause turf injury.

In the meantime, Bill Leuenberger, soil and turf manager at Chalet Landscape, Nursery and Garden Center in Wilmette, Ill., suggests an additional focus on water control and drainage, as overwatering tends to increase the population of spurge and crabgrass.

“Overwatering can stunt the growth of existing turf, due to a lack of oxygen, and, in some cases, disease, which can decrease the competitive edge that healthy turf would have,” he says. “On the other hand, if the soil is allowed to dry and crack, weed seeds are more likely to germinate. A happy medium is important.”

Communicate with your clients



Finally, Patton can’t stress enough the significance of client communication—both before and after postemergent treatments.

In addition to warning clients about the effects of bleaching herbicides like Tenacity and Pylex prior to any application, Patton recommends communication with clients who have pets, children that frequently play in the yard or sensitive areas like greenhouses—before treatments occur.

After LCOs have finished spraying, they should discuss the application with the client and provide brief details about what was applied, he adds.

“They should then revisit some of the treated properties one or two weeks later to ensure there is no visible phototoxicity from the application and, more importantly, to verify that all targeted weeds are being controlled by the herbicide,” Patton says.

Leuenberger also believes clients should have realistic expectations of chemical treatments.

“Herbicides are considered controls, not eradication,” he says. “To control weeds throughout the summer, clients will need an ongoing program that, in reality, should require less herbicides with each visit.”

Photos: Dow AgroSciences.

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