When a landscape professional looks at that lawn, however, he sees the pounds of fertilizer that were spread, the irrigation that was carefully scheduled, the mowing that was done to just the right height, the aerating and the dethatching that allowed sun and air to get to each blade of glass, the insecticide that killed the grubs that nibble grass roots, and finally, the herbicides that were used to prevent or remove the weeds.
Weeds, like party crashers, may show up uninvited, and never alone, but with a gaggle of their annoying friends in tow. When they appear, they’re like the girl whose hair, dress and makeup are perfect, but when prom night comes, finds a big, red zit on the tip of her nose. Then, it’s a last-minute scramble to cover up the eyesore.
It would have been better to prevent the pimple from forming in the first place, but as every teenager knows, that’s hardly an exact science.
We’re luckier when it comes to weeds; they’re fairly predictable. We know when they’re going to show up, more or less. Then we can hit them with herbicides, in granular or liquid form.
There are two ways to attack weeds: before they appear (pre-emergently); and after they appear (post-emergently). Which plan of attack works best depends on which species you’re dealing with, your weather and other factors. But in general terms, pre- tends to be better than post-.
“Annual weeds that grow each year from seed are easier to prevent with pre-emergents than perennial weeds that grow back from roots,” says Ashton Ritchie, a lawn and garden expert at the Scotts Miracle-Gro Co., Marysville, Ohio.
Commonly occuring spring-germinating annual summer weeds include crabgrass, foxtail, barnyard grass, spurge and oxalis. They mature during warm weather, produce their seeds and then die with the first frost.
Poa annua, smooth chickweed and henbit are common winter annual weeds that germinate in early fall and mature during cool weather, producing seed in the spring, and then dying out during the hot summer weather.
Both grassy and broadleaf weeds are a problem for Kevin Herrmann, general manager of Fairway Green Inc., in Raleigh, North Carolina. “We have a big issue with a broadleaf called chamberbitter. It’s a weed that came out of Florida about 15 years ago, probably from pine straw. It’s just running rampant. We have a relatively new one that’s been around for the past eight to 12 years, called doveweed. And of course, there’s always Poa annua and crabgrass to deal with, too.”
In Tumwater, Washington, it’s a different weed that struggles for dominance over landscapes. “By far, the worst one we deal with is horsetail,” says Chris McCallum, landscape maintenance director at Controlled Rain LLC.
“It’s one of the oldest plants known to man, and it grows like asparagus, by stolons. The roots are so long that you can’t get a good contact herbicide to translocate (travel through the plant’s phloem) long enough to kill it. You may burn the root back, but you won’t get total kill.”
The tough-to-control weed has been known to grow right through asphalt. And because of the way it propagates, disturbing a site through excavation or rototilling causes it to spread like wildfire; every cut root segment becomes a new plant. McCallum says that, compared with horsetail, controlling crabgrass is “child’s play.”
The only pre-emergent herbicide he’s found to be effective on horsetail is dichlobenil. But it can be rough on the other plants in the landscape, especially in beds. “You have to apply this stuff sparingly,” says McCallum.
When should you start?
What’s the earliest you should start laying down pre-emergent controls? That depends on where you live. In the northern climates, such as the Midwest and Northeast, many companies start in early winter, just before the snow covers the ground.
Mike Friederichs, president and co-owner of Pro Landscape Maintenance LLC, Waconia, Minnesota, prefers to wait until early spring. “We always target April 1 as our start date,” he says. “That’s weather-dependent, of course. There have been years where we’re still plowing snow in April. But generally, we’re looking at a six- to eight-week application period starting on that date, wrapping it up by mid- to late May.”
Mostly, Friederichs battles crabgrass, and his chemical of choice is a granular formulation of dithiopyr. It’s also effective on goosegrass, Poa annua, bittercress, oxalis, chickweed, henbit and spurge.
“We use it because it’s a pre-emergent with post-emergent qualities as well,” says Friederichs. “It works up to the second or third tiller (a shoot that springs from the root or bottom of the original stalk) stage.
“Being that it’s a granular product, we’re able to apply it early, and have a long residual. If things start running late, we’re still able to get control. That’s important because it’s very hard to fight crabgrass once it reaches middle- to late-stage development.”
“The type of pre-emergent we use breaks down microbially,” says Mark Wise of Kiefer Landscaping Inc., Durham, North Carolina.
“Once the temperature gets around 57 degrees, it actually prunes the roots and kills the plants as they start to grow. A lot of people think it keeps weeds from germinating, but that’s not the case.”
Herrmann does his first application in mid-January. “We do splitouts (split applications). Because people tend to mow fescue too short, not giving it enough competition against the crabgrass, we’ve gone to three applications of pre-emergent, with the second around March 15, applying the same amount as the first one. The third will be at a much lower rate, around the first of August.”
“The best efficacy we’ve seen is in split application,” confirms Willie Pennington, a sales representative for BASF, the multinational chemical company hose U.S. headquarters is in Florham Park, New Jersey. “It works much better than one single application.”
Pendimethalin is supposed to be applied at a rate of 3 pounds per acre, but a weed scientist will tell you it is preferential to apply 1.5 pounds initially, followed by another 3.5 pounds four to six weeks later. “You’ll see much better control of Poa annua and crabgrass than you’ll get with a single application,” he adds.
The special case of crabgrass
The conventional wisdom is that crabgrass must be tackled pre-emergently, or not at all. Herrmann disagrees with that notion. “There are some new formulations that do an okay job with it on a post-emergent basis, as long as you catch it pretty early. If it gets to a mature, multileaf stage, it becomes very difficult to control.”
That’s true of some of the other broadleaf and other grassy weeds, too, according to Herrmann. When it starts getting really hot, the waxy cuticle layer on the leaf tissue builds up and makes penetrating it with a post-emergent herbicide difficult.
“Summer annuals like crabgrass germinate like waves hitting a beach,” says Ritchie. The first crabgrass will sprout next to a sidewalk, or on a south slope, where the soil first warms to 55 degrees in the early spring. The next wave to germinate will be where the lawn is thinner; the sunlight and warmth helps the seed get started. The last crabgrass plant will sprout in early to midsummer in newly exposed soil.
To do the best job preventing crabgrass, your pre-emergent should have been spread by the time all the yellow blossoms fall off the forsythia bushes, the lilacs begin blooming, or you’re starting to see dandelion puffballs.
Most pre-emergent herbicides will kill crabgrass seedlings a few days after sprouting when they are still at the two-leaf stage. There are post-emergents containing quinclorac that will kill young crabgrass and other broadleaf weeds after they grow.
Resistance and adaptation
It’d be nice if we could just keep spraying weeds with the same herbicides, year after year. Unfortunately, it doesn’t work that way. Most of the weeds will be killed by the chemical, but a few won’t. Their descendants will be resistant to that particular herbicide, and eventually, the dominant culture. So to kill them, you’ve got to change your plan of attack.
That’s where ‘mode of action’ comes into play, the means by which the chemical kills the weed. For example, dithiopyr acts as a root growth and cell division inhibitor, while pendimethalin inhibits both root and shoot growth, preventing plant cell division and elongation.
Dichlobenil is a cell division inhibitor, prodiamine inhibits plant cell growth, and mesotrione interferes with an enzyme that protects chlorophyll from decomposition by sunlight.
There is a way to break up the resistance/adaptation cycle. If you use an herbicide with one particular mode of action this season, use one with a different mode of action next year.
“If the weeds get to the point where they’re truly resistant to anything you’re using, I’d use glyphosate to clean up and start over,” says Pennington.
Are you sure it’s resistance?
Sometimes, you may think you’re encountering resistance when you’re really dealing with another problem. There are some other possibilities to consider, especially when it comes to crabgrass, where resistance isn’t all that common.
Friederichs has used a dithiopyr-based herbicide on crabgrass for 14 seasons in a row and hasn’t noticed any resistance developing. “Any issues we’ve had were environmental. It could have been the soil composition, or the weather; one year, it was 80 degrees on St. Patrick’s Day. At times, we’ve had very wet springs, with lots of rain, where the herbicide simply washed through the effective zone or became diluted.”
If a granular pre-emergent is spread too lightly, or areas are missed, prevention will not last as long as expected. Coverage can be more hit-or-miss with a large-granule formulation than with smaller-granule types.
If the product was put down too late, you may see weeds popping up near a sidewalk or on a south slope where the soil warmed faster, and the seeds started to sprout before the application of the herbicide.
Was the soil disturbed after you applied the chemical? That just spreads the seeds, especially of crabgrass, and then they’re off to the races. Was the pre-emergent properly watered in? It’ll be less effective unless the lawn gets a ¼- to ½-inch of water within two to three days of application. Grass that’s mowed too short, or thinned by drought, insect or fungus attack could let crabgrass germinate if the preventer is no longer active.
Make sure which weed you’re really dealing with, and that it’s not a look-alike. For instance, dallisgrass and nimblewill are often mistaken for crabgrass, but are much more difficult to prevent.
Some premium organic landscape services will hand-pull weeds. That’s labor-intensive and probably not permanently effective. If any part of the root is still in the ground, the weed will come back. That’s especially true of weeds that propagate via rhizomes or stolons. You might pull the head off, but it’ll grow another.
Hand-pulling isn’t economically feasible in most cases, anyway. As Wise puts it, “Paying eight guys for a whole day of pulling weeds costs a lot more than paying one guy with a spreader and a $150 bag of pre-emergent.”
Weed control in landscape beds is a bit different. Much depends on what types of plants are in the beds. If the bed contains permanent, small ornamental plants, vines, hostas, perennials, and even some trees and shrubs, you can use sprayable or granular pre-emergents.
But use caution if it contains annual plants, flowers or woody ornamentals. Tender plants with shallow root systems can be damaged by herbicides. In Friederichs’ opinion, the best weed control in a landscape bed is prevention. He prefers laying down a thick layer of hardwood mulch, such as cypress chips, every year.
When it comes to preventing weeds in turf, a lot of professionals rely on overseeding, which is really nothing more than spreading grass seed over an existing lawn, to fill in sparse patches. It’s a technique that can be worth many pounds of herbicide. The healthy grass simply outcompetes the weeds.
Cool-season grasses such as tall fescue, fine fescue, perennial and annual ryegrass, and sometimes, bluegrass, are the main candidates for overseeding. It’s not generally done over grasses that spread via runners unless they’re damaged or diseased. One exception is Bermuda grass. It’s occasionally overseeded in the fall with a cool-season variety.
The war against weeds will never be entirely won. But, police actions can keep their incursions in check. Pre-emergents, locked and loaded, are some of the best weapons we have against them. Aim them wisely.
The author is senior editor of Irrigation & Green Industry and can be reached at email@example.com.