PRODUCT TRENDS: Preventing Weeds with Preemergents

Getting your money’s worth from a preemergent herbicide is tricky business. No two formulations are the same and there is no set standard for when to apply the product. Most of what you need to know should be on the product label, but there are some things a label won’t tell you, like when to make your applications.

Some lawn care operators (LCOs) make applications in the spring but a trend toward fall preemergent applications is beginning to, well, emerge. Is it a trend that deserves LCOs’ attention? That depends. Considerations like location, soil temperature, turf type all play an important role in an LCO’s decision. What’s more, different preemergent herbicides target different weeds so getting familiar with the product is essential. Square one, however, is knowing what preemergent herbicides are and what they do.

FALL FORWARD. SPRING BACK. Preemergent herbicides are formulated to prevent weed seeds from germinating. This, in turn, prevents those weeds from establishing in the spring. And while most preemergents are used to control annual grasses, they also can be used to help control annual broadleaf weeds to minimize the need for control of those weeds later in the season. This means contractors shopping for a herbicide should have a good idea of what types of weeds they are up against. Whether the weed is an annual, biennial or perennial will make a difference in a preemergent’s effectiveness and, in some cases, dictate the best time of year to make the application.

"Preemergents prevent top weeds," explains Kyle Miller, senior technical specialist, BASF, Research Triangle Park, N.C. "Annual bluegrass, poa annua and chick weed are commonly targeted, as well as broadleaf weeds like cudweed and speedwell. Fall preemergent applications generally target broadleaf weeds that are trying to germinate from seed, so deciding when to make the application hinges on the weeds being targeted."

Since crabgrass is usually at the top of a weed-hunting contractor’s most wanted list, preemergents have traditionally been applied in the spring to cool-season turf, Miller says. "But because they control crabgrass so effectively, it’s sometimes overlooked that many preemergents also control several weeds that develop during the winter," he explains.

Meanwhile, in areas of the country where turf goes dormant, weeds are more likely to germinate, resulting in a weedy lawn in the winter. "In southern turf or warm-season turfgrasses, it’s a very different situation," Miller explains. "Normally, you don’t have anything competing with weeds because the turf is either dormant or near dormancy. Because turf is less likely to compete, more weeds end up emerging in the fall, which is why fall is a very good time to use a preemergent herbicide in warm-season turfgrass. You won’t have anything competing with those weeds."

David Ross, technical manager, turf and ornamentals, Syngenta Professional Products, Greensboro, N.C., agrees that deciding between a fall or spring application largely depends on that old cliché: location, location, location. "There are some advantages to fall application," he says. "One is that you can take care of applications early so that you don’t have to do it in the spring when you’re busy doing other types of work. Often, it’s a time management issue."

Fall applications increase efficiency because many preemergents are good for controlling broadleaf weeds in addition to the basic grassy weeds, says George Raymond, business manager for herbicides and plant growth regulators, Bayer Environmental Science, Montvale, N.J. "If you do a good job of preventing broadleaf weeds, your late summer application of postemergent materials would be significantly decreased," he says. "The fact that you’re able to put down a preemergent before late fall, winter or early spring when the majority of weeds germinate gives you the opportunity for better control," Raymond shares.

"In a region where you have a lot of winter weeds it makes good sense to use a preemergent product to try and get that under control before it becomes a problem," Miller adds. "No one wants to wait and see how many weeds they’ll get before they’re controlled. Most want them controlled before they’re even seen."

Scott Eicher, senior marketing manager, DowAgro Sciences, Indianapolis, Ind., points out that customer expectations are also a consideration when making fall applications. "Putting out a preemergent herbicide next spring doesn’t really do anything to aid the customers early perception of quality," he explains. "But if they can do it in the fall when labor flexibility is a little greater, that frees up their time next spring to do the visible services early that will influence the perception of quality."

GROUND RULES. Deciding whether to apply preemergents in spring or fall is, above all, a matter of personal preference with location and efficiency being the primary considerations. A contractor who may be thinking of switching to fall applications may wonder then how it differs from a preemergent application made in the spring – if it does at all.

To arrive at this answer, an LCO must consider the primary goal of a preemergent application. "The main objective with a preemergent application, whether it’s granular or liquid, is getting it down into the soil profile," Raymond says. "A number of preemergent herbicides are broken down by heat and microbial degradation in the soil, but in the winter time this doesn’t happen." Depending on the product, the herbicide will break down in the soil by various mechanisms whether it be moisture or sunlight. Most often though microbes in the soil break down the product into non-useful constituents until there is nothing left. If a product is applied to turf in the fall, there will be fewer elements working against the herbicide, and when the winter comes, there will be less sunlight. Though you may have moisture, the microbes aren’t as active as they would be when the soil is warm, so the residual of the herbicide is extended and the active ingredients breakdown at a slower rate. But you need to keep in mind that the product is going to be out there longer so to get it’s maximum effect you may have to bump up the rate a little bit. Typically a product’s label will give some indication of the rate to use per increment of time."

Eicher agrees. "You don’t want to overapply the product, but you’re probably going to have to go with a higher rate," he says. "And when you get south of the Mason Dixon line you’re still going to need a second application next spring, but you’ve got a lot more flexibility on when you do it."

LCOs should also be mindful of a weed’s germination schedule when making their applications to ensure control, Miller suggests. "The idea is to apply it before the winter weeds begin to germinate," he says. "Too often people associate seeing the weed with being the right time to apply the product and that’s not the case. You need to be applying that product when the weeds haven’t germinated yet. This means making applications as early as August in some northern regions. It all depends on the geography and when you can expect the germination of your weeds to occur."

If an LCO has a good idea of when germination will occur, he or she should plan to make a preemergent application about one month in advance, Miller recommends. "The most important thing with preemergents is the timing. If you don’t put it down before the weeds begin to emerge, you’re not doing any good," he explains. "Obviously this coincides with soil temperature. Winter weeds won’t germinate when the soil temperatures are high. That’s why you wait for the weather to become cool and fall-like."

"As long as the temperature is above freezing, you’ll be able to move the product into the soil," Ross adds, explaining that most preemergent products have to be in to the top 1 inch or so of soil so that germinating weeds will pick up the product. He suggests then that the ideal soil temperature for preemergent applications is above freezing but below the temperature at which the targeted weed germinates. "For example, crabgrass will typically germinate at 65 degrees F," he says.

SEEDY OBSTACLES. Obviously, there is more going on in a yard than a battle between herbicides and weeds, and a careless LCO might let innocent turfgrass get caught in the crossfire.

Raymond asserts that it pays to be mindful of a property’s seeding schedule, particularly if the contractor is making a fall application. "If any overseeding has been done, the contractor needs to wait until the overseeded grasses are up and that they’ve been mowed a couple of times so that you don’t inhibit germination," he says. "If it’s a well-established lawn that’s not thin and it’s clear that there won’t be any overseeding done the next spring, a fall preemergent application is appropriate."

However, if overseeding is a factor and there are concerns about winter kill or if an LCO is faced with generally thin turf, Raymond suggests that the residual of the preemergent might be too long and could potentially interfere with the overseeding. "You have to keep those intervals in mind and check the label of the product," he says. "See what the overseeding intervals are with the types of grass receiving the overseeding."

Miller points out other reasons that resident turf should be a consideration. "In the southern United States, you may not have any competition from desirable turf because you have warm-season grass that goes dormant," he says. "In that situation, you could potentially use a couple of applications. In the northern states, however, on cool-season turf you would probably be fine with one application."

TRENDS TO TEND TO. Whether an LCO makes one or many fall or spring applications, the most fundamental decision has yet to made: what product to pick?

It’s probably true that most LCOs come to their "favorite" preemergent herbicide by basic trial and error. Miller has some suggestions for shortening that process.

"You have a spectrum of weeds that are on a particular products label," Miller says. "An LCO will want to make sure they get a product that controls as many weeds as possible, and each product is a little different when it comes to that. Price shopping is particularly wise with preemergents as they tend to be pricier than postemergent products."

Sometimes, the type of application equipment being used affects the preemergent product an LCO chooses. "If they primarily apply product using rotary spreaders and they don’t do very much spraying at all, they’ll want to use a product they can get on a fertilizer carrier," Miller explains.

In face, there is a trend of contractors using combination products that incorporate herbicides with fertilizers. "I think the reason is that it’s a little easier to apply because you don’t need spray equipment and some LCOs are trying to move from liquid to granular products because of customers’ negative perceptions of spraying. However, there are still a number of weeds that are most effectively controlled by spraying and a lot of weeds that you cannot really control by using a herbicide that’s on a fertilizer carrier."

But with regard to industry trends the market for preemergents has been relatively static, according to Raymond and Ross. "Most of the preemergent herbicides that are used have been around awhile," Raymond says. "There haven’t been too many new ones, though there are more combination products for fall application which are better for root growth without pushing shoot development."

Formulators also are beginning to develop blends that are specific to geographic areas, according to Raymond. "In fact, many contractors talk to their formulator representatives and actually get formulations that are practically custom-made," he says. "The formulation would be based on geography and agronomics in that area. If an LCO can partner with a good formulator in their area, they can really maximize their efficiency."