Timing is everything, and in the case of post-emergent herbicide application, targeting weeds during the sweet spot – after “emergence,” of course, but before maturity – makes all the difference in control efficacy.
This is easier said than done, and with the persistent winter most regions in the country weathered, late spring starts will push the weed growing cycle out a bit. “In the northeast and mid-Atlantic states, we are about three weeks behind as far as the development of weeds – depending on where you are, your applications may be more delayed, or earlier,” says Kyle Miller, senior market development specialist, BASF Professional Turf & Ornamentals.
Some suspect the polar vortex and moist soil conditions are ripe conditions for weed mania this year. Old broadleaf weed germination experiments using refrigeration and water lead Jerry Corbett, technical service manager/product development for Quali-Pro, to believe that this year’s broadleaf crop will be “enhanced.”
Lawn care operators (LCOs) likely jump-started the season with post-emergent herbicides labeled for use in cooler temperatures, points out David Loecke, herbicide product manager at PBI-Gordon. “You can still get ahead of weeds and control them when temperatures are down in the 50s,” he says.
Timing and temperature.
Summer solstice, some researchers say, is prime time for pinpointing broadleaf weeds that have “escaped” pre-emergent applications. Earlier spring applications, depending on the region and temperatures – higher than 50 degrees is critical – are effective for spot-treatment of winter annuals, such as chickweed. But for weeds that crop up after the growing season starts, a late-June application time, in general, is a good target. “Those are your longest days of the year – ideal for growing,” says Jason Fausey, research and development specialist at Nufarm Americas.
Weeds must be growing to warrant a post-emergent herbicide application.
By late July and August, when the weather in most areas of the country is hot and dry, the window for effectively treating weeds with post-emergent products has passed until fall. Then, LCOs can lay down another application to manage perennials that germinate at the end of the season.
Ken Hutto, technical service manager for FMC, says ideal application conditions are a calm day (not windy), a temperature no higher than 80 degrees Fahrenheit, with adequate soil moisture.
In very hot and humid conditions, Hutto says, “The target weeds grows slower when the turfgrass is under stress, and when that weed is not growing vigorously, it will not take up as much herbicide so you will not get as good of control. Also, when turf is under stress, you can run into injury problems.”
Timing is all about balance – not too hot or dry, but well enough into the growing season so weeds are blooming. “You have to apply the products when weeds are actively growing so systemic products can be taken up by the plant,” says Dean Mosdell, field technical manager, Syngenta Turf and Landscape.
In other words, get weeds while they’re young. “By the time they get to late summer, they are mature and not easy to control because the cuticle layer is thicker,” Mosdell says. Fausey likes to tell LCOs that the key to controlling weeds is to “start clean and stay clean.” This speaks to how post-emergent herbicides fit into an overall lawn care program. Laurence Mudge, who heads up the Bayer Green Solutions team, emphasizes an “earlier rather than later” post-emergent treatment course. This, he adds, is a challenge for LCOs with full schedules – there’s a limited window of time to service properties that require post-emergent treatment. Generally, post-emergent applications are made in spot-treatment form after a pre-emergent – though broadcast application might be warranted depending on the volume and density of weeds. Another application in fall can be made to manage perennial weeds.
“Perennials are sometimes easier to control in the fall because that’s when perennials are in winter survival mode and there is a lot of movement down into the root systems,” Mosdell says. “That is why you apply your systemic products to perennials in the fall.”
Loecke explains further why a fall post-emergent application can control stubborn perennials. “The fall is when they are taking in a lot of energy and nutrition into the roots system to survive the winter,” he says.
As a rule, the smaller the weed, the easier the control, Miller says. Take crabgrass, for example. “If we wait until late in the season, the weed has a deeper root system and more photosynthetic ability because there is more foliage,” he says. The earlier you catch weeds for post-emergent treatment, the cooler the air temperature may be, depending on the region.
“Esters give better (leaf) cuticle penetration and work better in the winter, but they are more volatile, which is why we don’t use them in summer,” Corbett says. Once weather warms, amine formulations and tricloplyr formulations are effective. Jamie Breuninger, technical leader with Dow Agrosciences, says products containing triclopyr ester are effective during late summer, when applications are made to mature weeds or during periods of drought stress.
Right product, right weed, right time, right rate, right place. These are the intricacies of post-emergent herbicide application. And a change to just one of those variables can impact control.
Corbett recommends not mowing the lawn two days prior or following a post-emergent herbicide application. That’s because complete coverage of the weed is critical to product efficacy. “If you don’t have the surface area, you can disrupt the uptake and translocation of the product you’re trying to spray,” he says.
Coverage also is dependent on proper equipment calibration and water volume. “A lot of post-emergent herbicides are contact herbicides and not systemic, meaning they are not going to get translocated from the leaves to the root, and efficacy is dependent on making contact with that leaf surface to get inside of the plant,” Hutto says. Meanwhile, coverage can go beyond the treatment area if great care is not practiced during spot treatment, warns Mudge. “When you're spraying herbicides, you're trying to control one weed out of another turf or landscape,” he says. "On your property, you have the lawn, plants, trees, shrubs – you have to be very careful with these herbicides because you are trying to selectively control certain plants and not affect desirable plants in the environment or in the turfgrass itself.” he says. Some products will require the use of a surfactant. “The addition of an adjuvant improves post-emergence coverage of the foliage and helps penetrate the waxy leaf surfaces of hard-to-control species,” says Anita Alexander, Dow AgroSciences field scientist.
However, these additions can cause potential for turf damage if not used as instructed. Tim R. Murphy, weed scientist and professor emeritus at The University of Georgia says, “Don’t indiscriminately add an adjuvant – surfactants, cop oil concentrates, etc. Use only if it is specifically recommended on the label.” Corbett recommends applying only reputable surfactants labeled for lawn care turf and ornamental use. “Look for the non-ionic surfactant or surfactant blends with less petroleum – not agriculture-grad surfactants,” he says.