Critical control

It’s the kickoff to the lawn care season, and the first application in the annual series is a critical step in preventing stubborn grassy weeds. Pre-emergent herbicides go down as early as January in some areas of the country, and late spring after snow melt in northern and Midwest states. Their purpose is to provide a barrier of protection against crabgrass, goosegrass, foxtail, barnyard grass and even some broadleaf weeds that crop up in the summer, such as oxalis or prostrate spurge.

“For lawn care operators, pre-emergents are the most critical weed control they have the whole season,” says Jason Fausey, research and development specialist at NuFarm. “They really only have one opportunity to make that application, and that’s prior to weeds germinating.”

As with any lawn care application, timing is everything – and that’s tricky business when balancing a generous roster of accounts that essentially need this pre-emergent application during the same relatively narrow window of time. Specifically, that’s before weed seed germination and after soil temperatures level at 55 to 58 degrees at a 1-inch depth for four to five consecutive days.

“Timing is going to vary across the country, so the key is to monitor those soil temperatures,” says Bobby Walls, product development manager for herbicides and fungicides at FMC.

Also important are application technique and product selection. “This is where the planning comes in,” says Walls, suggesting that LCOs review their pre-emergent herbicide programs in fall, so they’re prepared to dispatch the crews once the ground defrosts.

Still, it’s not too late to seriously evaluate your pre-emergent herbicide protocol, including a review of best practices for application and understanding how tank mixing and adjuvants play into this, and other lawn care applications. Because, as Kyle Miller, senior technical specialist at BASF, points out, “The easiest and cheapest way to battle crabgrass is with a pre-emergent herbicide.”
 

Application timing.

Pre-emergent herbicides must be applied before weed seed germination occurs, and that can pose a challenge because there are varying environmental conditions even within a single lawn care route.

While application timing is based on an average soil temperature of 55 degrees (or some look to when forsythia bloom in the north, and dogwood in the south,) location gets even more specific than that.

Soil temperatures warm up faster on south-facing slopes. Bare spots in the turf also get warmer because they’re directly exposed to sunlight, whereas lush turf keeps soil cooler for longer.

The strips of lawn abutting sidewalks or driveways pull in heat from pavement surfaces, which ups the soil temperature compared to the rest of the yard.

“Most LCOs take a look at the calendar, but they also need to take look at the application site,” says David Loecke, herbicide product manager at PBI Gordon. “Take a look at the areas in which you need to make your application and make sure you’re timing accordingly.”

That means that properties on a route with south-facing lawns, for example, might get positioned first on the list for receiving a pre-emergent herbicide application. Or, one product might be used to treat properties earlier on in the route, while properties falling toward the end of the service cycle might get another product – perhaps one with a post-emergent mix to catch any weeds that have already started germinating.

As a rule, regions with a longer growing season will receive more than one pre-emergent herbicide application, Miller says.

For example, in the transition zone that includes his part of Virginia, first pre-emergent herbicide applications go down around Feb. 15 (depending on the weather), with a second round of product applied toward the end of March.

“With warm-season grasses further down south, you see LCOs doing up to four pre-emergent applications,” Miller says. Specifically, in Florida, LCOs might plan for three or four applications; they might do two applications in the Atlanta, Ga., area heading north toward the transition zone.

“But one thing that we notice with some of the warm-season grass is they are more competitive to crabgrass because they’re a tropical grass just like crabgrass,” Miller says. “Cool-season grasses start to get weak from summer stress and can’t compete with the crabgrass quite as well.”

Meanwhile, split applications can prolong control on a property and meet that every-eight-weeks visit window that many LCOs promise customers, says Laurence Mudge, manager of the green solutions team at Bayer.

“A lot of companies will apply a half rate, then maybe two months later they’ll come back and make another pre-emergent application,” he says. “There are agronomic and business reasons for doing that.” Some products just won’t last through summer to prevent weed growth, Mudge says. “So you need to do two applications vs. one single one because if you only do one application in February, it may run out of gas by the time you get to July and August.”

Mudge notes that this all depends on turf type and “the situation.” Mainly, lawns that have a healthy turf stand will stand up to weeds.

“You are not putting as much pressure on your pre-emergent herbicide because your turf is providing enough competition that weeds don’t germinate,” he says. Mudge also points out that fall is an important season for making pre-emergent herbicide applications for managing winter weeds, especially poa annua. (This applies to transition zone and down south.)

 

Improving efficacy.

Aside from timing, even application of the product is critical, Fausey says.

Sometimes, complaints from clients that weeds are sprouting in the lawn in spite of a pre-emergent application are because of missed spots during application.

“With pre-emergents, it’s really important to have a perfect barrier across the entire lawn to have perfect weed control,” Fausey says. “If there are any skips or overlaps or breaks in the barrier, you’ll find weeds that will germinate in those areas.”

Also, these products need to be watered in within an average of seven to 10 days of application. “If you don’t receive a rainfall and fail to use irrigation, the herbicide will not be released into the soil,” Walls says.

On the other hand, too much rain can sabotage pre-emergent herbicide applications, says Dean Mosdell, technical manager for the western U.S. at Syngenta. “During warm, wet springs, the active ingredients tend to break down faster in the soil,” he says, noting that “crabgrass breakthroughs” can be treated upon germination with a post-emergent herbicide.

If you’re planning on aerating properties in the spring, the pre-emergent application will still be effective. “Some of our research with pre-emergent herbicides shows that cultural practices like aerification will not affect the level of control,” Mosdell says.

Same goes for seeding, Loecke points out. “If you are planning on seeding, there are certain pre-emergent products out there you can use,” he says, noting that it’s critical to read the label because in a lot of instances, pre-emergent products can actually prevent the grass seed from germinating.

So many times, the success an LCO has with a lawn care product all depends on reading and following the label’s recommendations.

“Some products might have a larger window of application time,” Walls points out, noting that the label dictates application timing, technique and proper tank-mixing partners (if applicable). “It all comes back to the label,” he says.