After you've done your weed prevention work and we come into the summer season, you hope that a weed-free green vista awaits you and your clients. Good luck with that.

Mother Nature might have some other ideas. Those inconvenient and invasive plants we call weeds have a way of getting around all of our preventive measures. Then it’s time for post-emergent weed control.

Unlike pre-emergents that prevent weeds from appearing, post-emergent herbicides control only existing plants, and provide no residual weed control. Their primary function is to control young annual species, but they’re also used to control certain perennials.

There are two types of post-emergent controls, selective and nonselective. Nonselectives are the machine guns; they’ll take out anything in their swath. Selectives are the snipers. They target only specific weeds for assassination.

“Selectives are often tied to a specific turf type, for instance, a Kentucky bluegrass,” said Jeff Michel, field development manager at Environmental Science, a division of Bayer Crop Science in Research Triangle Park, North Carolina. “The chemical won’t enter the grass, but will control the weeds. You want to be able to spray something on your grass to kill the weeds, but not end up with giant brown spots in the middle of a lawn.”

Glyphosate is the perfect example of a nonselective. It’ll injure any plant it touches, so you’ve got to be careful where you use it. It’s great for certain applications, though, such as taking out weeds growing up through the cracks in a driveway or patio.

Dandelion, clover and chickweed are the foes the workers at Weed Man of Columbus, Ohio, face come spring. How much work they’ll have to do will depend on how much prevention was done the previous year.

“We try to do really good work in the fall with our post-emergent controls, to make sure that the weeds aren’t as big of an issue in the springtime,” said owner/manager Corbin Schlatter.

In the fall, they apply broadleaf weed controls, usually three-way post-emergent products. They’re called that because they contain three different active ingredients. But in the main, they try to get the lawn as thick as possible to help out-compete any weeds that might want to show their not-so-attractive little heads in the spring.

Rob Nelson is lawn maintenance manager at Katerberg Verhage, Inc., a company that services residential and commercial landscapes in Grand Rapids, Michigan.

He finds plenty of broadleaf and other post-emergent weeds to keep him busy. “It’s a broad spectrum. We get everything from dandelions, to clover, to chickweed—the whole gamut. Then there are the hard-to-control weeds, like wild violet and spurge. Veronica is another really hard one.”

In the spring, David Woehler, owner of Woehler Landscaping, LLC, in Pittsburgh, Pennsylvania, sees a lot of dandelions, clover, black medic, oxalis and thistle. “For the most part, annual weeds are very easy to control, as long as you’re applying the chemicals at the recommended rates at the recommended times of the year, with the proper soil temperatures.”

When he maintains his clients’ turf, he regularly applies herbicide combined with fertilizer. He’ll usually do five rounds of fertilization throughout the year. The first round combines a straight pre-emergent herbicide for crabgrass control with a 30-0-0 fertilizer.

The second fertilizer application contains a post-emergent weed chemical for broadleaf weed control. He’ll mix a liquid 2, 4-D with another liquid product whose active ingredient is dithiopyr.

“It’s when you start getting into the perennial weeds, like violet or wild violet, then things become very difficult to control. 2, 4-D alone will not wipe violet out; you have to use different products. And even then the products that you do use will only kill 60 to 70 percent of it. It usually requires multiple sprays.”

Woehler says perennials are difficult because they have a much hardier structure to them. If you did absolutely nothing, the clover, oxalis and dandelions would all die when the first frost comes, because they’re annuals.

“It’s kind of like you’re starting fresh the following year. But the perennial weeds are not going to die. They’ll go dormant, but liven up again in fall and spring.”

When attacking weeds, Nelson’s found that timing is critical. “You need to get at them before the seed heads form. Once the weeds go to seed, controlling them becomes a lot more difficult.”

Temperature

What the thermometer reads outside has a lot to do with how successful your post-emergent weed control program will be. Again, timing is critical.

Before you apply any herbicide, take a glance at the mercury. Michel says that a good number of these products have warnings against putting them down when it’s too hot outside, as they can injure turf. If it says, “Do not apply if the temperature is over 85 degrees,” take it as gospel.

“The plants are more actively growing at that time,” he explains. “They’re in the midst of running a marathon. If you put down a product that’s trying to affect the weeds, it can also have a secondary effect on the grass. It’s not true of all herbicides, but it’s an important thing to be aware of.”

Almost as bad as spraying when it’s too hot is spraying when it’s too cold. “Every year I go through this with customers,” says Woehler. “They’ll say, ‘My neighbor’s yard has already been treated. Why aren’t you out here?’ I have to explain to them that it’s much too early.”

If the soil temperature is below 55 degrees, the herbicide will simply decompose, and never get down to the thatch level because the grass isn’t actively growing yet.

The tough guys

There are some weeds that are tougher opponents than others. Every region has its local gang. “One that’s extremely difficult, Dallas grass, grows in the South, Texas, the Carolinas and Georgia,” says Michel. “Trying to control a grass within a grass, without injuring the good turf that you want to keep, is very hard.”

“Another one we hear a lot about, particularly from Florida, is doveweed. Japanese stiltgrass is an up-and-comer in the Northeast that’s very difficult to control. Some things work well on it, but it’s easily mistaken for other plants and other weeds. People get frustrated, because standard herbicides won’t work.”

And then there’s crabgrass. Almost every turf lawn will probably be afflicted with crabgrass at one point or another, and everybody hates it.

The best way to tackle crabgrass is to control it pre-emergence. If applied early enough, the herbicide will establish a chemical barrier, a kind of blanket underneath the surface of the soil. But when weeds pop up despite your best efforts, you have no choice but to treat it at that point.

In that case, Woehler will turn to “the nuclear option,” a powerful post-emergent combo containing quinclorac. It’s a four-way punch, combining the quinclorac with sulfentrazone, 2, 4-D and dicamba. The product is pricey, so rather than apply it liberally with a tank sprayer, he’ll walk a property with a backpack sprayer.

The only other way to deal with crabgrass is to hand-pull it. However, because this plant stashes its seeds underground, pulling it virtually guarantees more plants in the future. It thrives in full sunlight and high temperatures, and easily out-competes cool-season grasses.

Crabgrass is a summer annual. In cold climates, once a frost hits, it’ll die out. Don’t be fooled by that; it’s assured its resurrection many times over. In mid-to-late summer, each plant produces a tremendous amount of seed underground. It can lay dormant for years, waiting for the right conditions.

That’s the reason this particular weed is so hard to eradicate. It’s been said of crabgrass that “One year’s seeding equals seven years’ weeding.”

Landscape beds

Ornamental and shrub beds that are densely planted have a natural protection against weeds. The shade they provide keeps the ground cool and reduces germination.

Weed control can be difficult in a bed. Annual plants and flowers can have tender, shallow root systems. Herbicides, even when selective, could damage them. If the beds aren’t so large that it’d be too labor-intensive to do it, hand-weeding would probably be best here.

If there are permanent plants in the beds, such as trees, shrubs vines, hostas, perennials, or small woody ornamental plants, spot application of granular or spray herbicide could be used.

Don’t help them

It might surprise you, but the best herbicide you have is your lawn mower. It’s been estimated that regular mowing eliminates some 80 percent of weedy species. But your mower can also make things worse; mowing grass too short, for instance. There are other cultural practices that can exacerbate weed growth, such as overwatering or excessive fertilization. Rototilling is another.

“When you open up the soil by rototilling, there are weed seeds down there, and they’ll germinate when the light hits them,” Nelson says. “Weed seeds can stay viable in soil for years, waiting for just the right conditions.”

Eco-green weed control

The greenest way to deal with weeds is to stop them before they happen. Instead of always trying to put out fires, try to get plants and turf as healthy as you can. It’s a proactive rather than a reactive approach.

“That’s what we’re looking for more and more as an industry,” said a spokesman at the green division of Elmsford, New York-based Central Turf and Irrigation Supply. “But the first step to making a lawn weed-free is making it healthy.”

“To focus on trying to eliminate the use of post-emergent herbicides in order to reduce your carbon footprint is looking at the situation backwards. It’s focusing on a problem that’s there because you haven’t managed one of the other steps quite as well.”

There are times when the weed pressure in a landscape is so overwhelming there’s no other choice but to use chemicals. Even super-green types who work preserving fragile wetlands have to turn to herbicides when invasive species threaten engulfment.

In the end, it’s a trade-off. Which one has a smaller environmental footprint, burning weeds out of a lawn with herbicides, or having to mow them down every few days with a gasoline-powered mower?

Schlatter says the biggest challenge the weed-control industry faces is the proper education of customers. “Too many of them believe that by putting down a pre-emergent crabgrass product, we should also be able to prevent dandelions and other broadleaf weeds. That’s a misconception.”

There may be a middle path between what we know works, and the organic, chemical-free one that isn’t as sure-fire. “Commercial applicators know that there’s a growing market segment of people who want to use fewer chemicals, but still want that pristine lawn,” said Reinie Drygala, global director of business development for Intelligro, a division of Suncor Energy, Inc., Mississauga, Ontario, Canada.

The company wanted to create a product with the same performance you get from traditional herbicides, but in a way that reduced the amount of active ingredients used. The result, Civitas WEEDfree, was officially launched last October. “We still use those proven killing actives, 2,4-D, dicamba and mecoprop, combined with some other inert products,” said Drygala. “We put them through a patented process, creating a micro emulsion. The particle size is so small that the product gets absorbed very quickly by the stomata (pores) of the plant. This is what leads to the root kill, not just a ‘top burn’ of the weeds’ leaves.”

Drygala calls his product a “hybrid.” Like one of those vehicles, it goes the same distance, but with much less chemical ‘gas.’ It’s so ecogreen that the EPA doesn’t require the usual warning labels.

Weeds will probably always be with us. So, probably, will herbicides. It’s our job to learn to use them wisely, and as sparingly as we can.