A Game of Knowledge and Adaptation

Weeds! They are unavoidable. We hate having to deal with them, yet they’re part of what keeps us in business; clients pay us to get rid of the darned things. You could say it’s a love/hate relationship.

The best way to tame these nasty beasts is to nip them in the bud. fortunately, we have some pretty potent weapons in our arsenal that can do just that; they’re called pre-emergent herbicides. When used properly, pre-emergent herbicides will zap weeds before they can poke their ugly heads up out of the ground.

We’re calling this a game, because in a way, it is. Like an opponent in a game, the other team is always out there strategizing, figuring out ways to get around the barriers we’ve set up for them. We just have to learn how to play the game better than they do.

Healthy turf as a deterrent

When it comes to weed abatement, particularly in turf, prevention is worth several hundred pounds of pre-emergent herbicides. The first move in this grudge match is to make it harder for the weeds to play. And for that, you’ve got to get your players into condition.

“We’re in the business of growing a crop,” said Steve Martinko, president of Waterford, Michigan-based Contenders Tree and Lawn. “Grass is a crop that has to be maintained; it’s not a one- or two-shot per year thing. You have to keep your crop healthy, so it can prevent unsightly weeds from coming in. The better you manage the crop, the easier it is, and the less herbicide is required.”

Matt Topper, lawn division manager at dixie Green Lawn, Tree and Shrub Care in Marietta, Georgia, agrees. “The best thing that you can do is to establish a healthy stand of desirable turfgrasses. The number one pre-emergent control is simply to out-compete the weeds. And then there’s a whole host of cultural practices, such as fertilizing, and keeping turf at the proper height, that help, too.” by keeping the turf healthy and strong, it becomes more difficult for weeds to get established.

“I always say, if you have a good start on weeds, then you’re going to have a good lawn,” says Kassim Al- Khatim, Phd, professor of weed science at the university of California, davis, and director of the state’s Integrated Pest Management program.

“You start with a good, clean lawn. If you maintain its health, you minimize weed problems down the road.

But if you start off with a bad lawn with a lot of weeds in it, and don’t do much about them in the beginning, then you’re going to have a problem for several years before you can really clean it up.”

Bedding plants Of course, weeds aren’t just a problem in turf. “When you’re talking about landscape beds, anywhere from little flower beds to annual color beds that get changed out several times a year, weed control in those types of situations is difficult,” says Laurence Mudge, manager of the Green Solutions Team for bayer Environmental Science, a division of bayer CropScience, Pittsburgh, Pennsylvania. “It depends on whether they’re annual-type plants, annual flowers or woody-type ornamentals. In annual flowerbeds, it can be much more difficult because you’re dealing with tender, succulent plants with shallow root systems. A lot of our herbicides can beat them up pretty good.”

“Weed control in landscape beds that have more permanent-type plants in them, such as perennials, hostas, vines, or small ornamental plants, on up to large landscape beds with trees and shrubs, is a lot easier,” says Mudge. “On those, you can use granular or sprayable pre-emergents.”

It’s all in the “when”

Pre-emergents need to be applied at just the right time, before seeds germinate, usually sometime in mid- March. figuring out exactly what day is the right one, though, is not so easy. “This game is all about the timing,” said Topper. “That’s crucial, but you can’t go by the date onthe calendar. You have to go by soil temperature. Weed seeds don’t germinate until the soil temperature reaches about 50 degrees. That isn’t always easy for us, because we have a wide service area (greater Atlanta) where temperatures can vary a lot.”

“When we apply pre-emergents is based on growing-degree days, and we use soil temperature to determine that,” says Martinko. This is trickier still in the northern states. “In Michigan, March could be frozen over, or it could be 70 degrees, as it was two years ago,” he explains. “We even had a 90-degree day in March of 2011. So by March, you can either be way too early, or way too late. but you definitely don’t want to treat ground that’s frozen.” Treating frozen ground is useless; you’d just be wasting product.

Pre-emergent controls work by knocking out the seed bank. The herbicide forms a kind of carpet under the soil, a layer of protection beyond which the emerging weeds can’t penetrate once the seed bank starts to germinate.

And you do want it to germinate. “If the seed doesn’t germinate, or is dormant, the herbicide is not going to work,” says Al-Khatib. “You want the seedling to be exposed to the herbicide so it’ll be killed.” Otherwise, the seed bank will just sit there, waiting for its next opportunity. “The ideal is to put the herbicide down before the seed emerges, before germination,” he continues.

Once a pre-emergent control is laid down, the next step is watering it in. This is crucial; it says so, right on the directions. “You need moisture to activate the herbicide,” says Al-Khatib. “Without the moisture, the control isn’t worth the money. You don’t want too much moisture, because it’ll leach the herbicide, and lose the activity. You don’t want to have too little moisture, either; just enough to activate the herbicide.”

But what if you can’t? “You can’t do that here in Michigan, because our nighttime temperatures in March are too cold for sprinkler systems,” says Martinko. “We’ll get 50- degree days, but then it’ll drop below 30 degrees at night, so no sprinklers are turned on until May. So we really are at the mercy of nature, waiting for a rainstorm.”

Mind the gaps

With any herbicide, the label is the law. The specific weed species that is killed by a pre-emergent chemical will be listed on its label; if you don’t see it there, it won’t kill it. “We try and stay ahead of the gaps that any one herbicide might have,” says Phil Fogarty, owner and president of Weed Man Lawn Care, North East Cleveland, Ohio, and Weed Man master franchiser for Ohio, Pennsylvania and upstate New York. “And every herbicide has gaps.”

“After a few years of using a product, you’ll have more and more of whatever weed wasn’t killed by it,” fogarty adds. “So, for example, if there are a couple of weeds that aren’t on the label, eventually you’ll end up with a monoculture of those weeds. Then you’ll have to go to another herbicide to clear those up.

Meanwhile, you’re creating an opportunity for some of the weeds you’d eradicated before to come back.”

So why not simply combine different products to cover the gap? “That would be dependent upon cost, and also if there’s going to be any negative buildup,” says Fogarty. “You don’t want to get a compounding of herbicides in the plants you’re trying to help grow. There’s not a limitless tolerance to this kind of stuff.”

Resistance and adaptation

Also keep in mind that you shouldn’t apply the same herbicide year after year. Organisms have an uncanny ability to adapt. Over time, bacteria develop resistance to antibiotics. Insects develop resistance to pesticides. So it shouldn’t be a surprise that weeds can develop the same sort of resistance to herbicides.

Mudge explains how resistance works. “Say you’re trying to control poa annua in a big field, with ten million seeds. In that population of 10 million seeds, there may be a couple of thousand that are genetically different, another biotype.”

He explains further. “When you apply herbicides, you’re applying ‘selection pressure.’ You’ll kill most of the poa annua, but a few of those plants will have a genetic mutation that makes them resistant. You keep applying that same chemical for five, six, seven years, and after awhile, you’ve selected out the susceptible biotype, and you’re left with just the resistant biotype.”

“If you’re a guy that likes to buy the same product, year after year, in the end you’re going to get resistance,” says Al-Khatim. “To manage resistance, you need to use the integrated pest management (IPM) approach.”

IPM includes cutting grass to the proper height, changing the timing of herbicide applications, keeping turf healthy through proper fertilization and irrigation and rotating herbicides, not relying on a single mode of action. Putting all of these things together can prevent resistance from developing.

“Mode of action” is another important part of the resistance story.

That’s the biochemical process by which an herbicide works.

There are three main classes of herbicides with three different modes of action. The first class works by disturbing the way a weed processes light (photosynthesis). Another class works by interfering with the metabolism of a weed’s cells. The third class inhibits a weed’s growth by impeding the division of its cells.

When you’re rotating herbicides, you don’t want to simply use a different chemical. Pay attention to how the chemical works, and pick one that has a different mode of action than the last one you used.

And then there are some weeds that are just plain ornery. “The toughest ones are thistle and horsetail, things that aren’t on any label,” says Fogarty. “Thistle has a very hairy and waxy cuticle that keeps herbicides from entering into the plant. And horsetail has a very small, very waxy leaf surface. It looks like a little mini-Christmas tree growing up out of the ground. It’s hard to get an herbicide to stay on it long enough to seep in.”

“They don’t propagate via seed banks, and they have natural defenses against herbicides. Anything that doesn’t generate from a seed bank, you’re not going to get any control of with a pre-emergent.”

What about a no-herbicide approach?

Organic landscape professionals strive for a chemical-free approach; no manufactured fertilizers, bug or weed killers. but can you really manage tough weeds like crabgrass without using herbicides? Fogarty doesn’t think so. “You’ll have job security if you try and fight weeds without herbicides,” he says.

Of course, you can pull weeds, especially if they’re sparse, as in a flowerbed. “But, if you have a really persistent, high-population invasive weed, you really need the help of an herbicide. I’ve worked with many landscape stewards over the years who try to manage weeds without using pre-emergent controls. They think they’re being environmentally conscious, but all they’re doing is letting problems get out of hand by not nipping them in the bud with the right dose of herbicide.”

“Most of these weeds regenerate quicker when you’re hand-pruning them, pulling them or cutting them down,” says Fogarty. “It encourages more of them to grow.”

Al-Khatib disagrees, to an extent. “Pulling is not a bad thing. but you need to have it as a part of a whole program. It is true that sometimes, with certain perennial weeds that grow via rhizomes, that if you cut those rhizomes, it’ll activate more rhizomes to grow.”

If you’re going to pull a weed, by all means, do it before it’s formed a seed head. Otherwise, all you’ll be doing is helping it propagate. Crabgrass is a good example. “There are 10,000 seeds for every crabgrass plant,” says fogarty. “That’s why it’s so prolific.”

Another weed, oxalis, sounds like something out of an old ‘Star Trek’ episode. “Oxalis shoots its seeds up to eight feet away,” he says. “Touch the stem of the plant at the right time, and you’ll get pelted with them. That’s what makes it such a difficult weed to handle, because it can spread very rapidly.”

Crabgrass is a classic example of why pre-emergent controls were developed in the first place. It has absolutely got to be attacked before it emerges. “If it’s not prevented, the odds of you being successful at eradicating it are slim,” says fogarty.

He describes the way crabgrass propagates as “a perfect system.” As the weed spreads out, it chokes out the other vegetation, creating a “dead mat” wherever it grows. This area of dead and dying grass is exactly where it thrives. You could say that crabgrass is a genius at creating its own ideal circumstances, where it can then drop its seeds and keep spreading. “You need to have control of it over a number of years, and have thick, dense ground cover to prevent it from having the opportunity to spread,” he says.

Bad for the environment?

Nobody wants to hurt the planet. Thankfully, herbicides are not as bad as some other chemicals we use, according to Al-Khatib. “Herbicides are not like insecticides and fungicides,” he says. “They’re less toxic to humans and less toxic to the targets. However, we do need to take precautions against off-target movement and runoff, because herbicides can end up in the water stream, and in areas that we didn’t intend to be exposed. We need to be careful.”

They’ll keep working on it

One thing you can count on is that chemical companies will keep trying to develop new solutions. Fogarty’s happy about that, observing that “chemistry is the thing that’s going to keep enabling us to maintain landscapes affordably.”

Of course, the ‘hit parade’ of weed species differs by region. “Herbicide manufacturers are constantly trying to find the ‘silver bullet’ that’ll kill every weed,” says fogarty. “But the fact that there’s no one-size-fits-all keeps these guys in business.”

“There are so many weed seeds out there in the seed bank,” he says. “We’re always hearing about a new weed coming into town that hasn’t been here before. Like tumbleweed, something you picture rolling down the streets of old Western ghost towns. Well, now it’s here in Ohio.” So is kudzu, the invasive, fast-growing Asian vine notorious for suffocating trees and plants in the South, according to fogarty.

If we’re going to play this game successfully, we have to stay ahead of the other team. We have to learn all we can about our opponent, get comfortable with the equipment, and work on our timing. Then, we just might win.