July 27 2020 06:00 AM

Control customers’ weeds year-round with chemical applications in fall.

Hedgehog / Adobe Stock


Many landscape contractors look ahead to fall as one of the most effective times to apply herbicides to handle problem weeds. Customers likely won’t be as willing to wait for that window, however.

Imagine assuring your clients that you’ll handle the dandelions, the creeping thistle, the clover, the annual bluegrass and the chickweed in the early fall. You could encourage them to sit back, relax and embrace a plethora of weeds throughout the spring and summer months, when they’re most likely utilizing their outdoor living spaces and admiring their beautiful grass and flowering landscapes. David Gardner, PhD, professor of horticulture and crop science at The Ohio State University, admits it can be a very challenging sell.

Purple deadnettle
Photos: The Ohio State University

“Agronomically, it always makes more sense to apply herbicides in the fall, but if you’re a businessperson, it is hard to say to your clients, ‘Enjoy these dandelions in the spring and summer, and I’ll control them in the fall,’” Gardner says. “When you spray in the spring to get rid of them, it is usually only temporary.”

In fact, when Gardner begins academic research trials in the fall, he says the areas often remain weed-free until the summer months. Sometimes, the weeds stay at bay for an entire year. That’s because applying herbicides in the fall allows more herbicide to get into the plant structures below the ground, which causes the whole plant to die, Gardner says.

Henbit

In stark contrast, trials beginning in spring typically translate to between 50 to 90 days of good weed control, depending on factors like the specific herbicide used, the rate at which it’s applied and the weather. That’s because you’re only killing the top growth while leaving roots underground that will grow new plants within a matter of months.

So begins a vicious game of catch-up, and according to the experts, it is one that you’ll be hard-pressed to win. In the proverbial game of weed control for client landscapes and lawns, the early bird definitely catches the worm.

“There are a complement of weeds that are emerging in late summer into fall, which will regrow in the spring,” says Mark Loux, PhD, a professor and extension weed scientist at The Ohio State University. “The goal is interrupting their life cycle in the fall, which is the ideal time to control them. If you wait until spring, they’ll regrow and you’ll have to deal with them in the spring when they’re bigger and harder to control.”

Winter annual weeds

By late summer, summer annual weeds have set seeds, but those seeds will die after the first frost. While summer annual weeds are no longer a concern, winter annual weeds like common chickweed, henbit, purple deadnettle and hairy bittercress are just beginning their life cycle. These winter annuals produce seeds that germinate late in the year, when temperatures begin to cool. Then, they persist as tiny vegetative plants as they survive the winter. When spring arrives, the plants become larger, more visible and much more challenging to combat.

“By the time they are visible, they’ve overwintered,” Gardner says. “The tissue is hardened off due to cold temperatures, and it becomes more difficult to control those weeds effectively with herbicides.”

Common chickweed

For severe infestations, Gardner recommends a preemergent herbicide with active ingredients like dithiopyr or prodiamine in late August or early September, but as these applications are typically more costly than a post-emergent, their best use is for more severe infestations. In late October, when the plants are young, a three-way post-emergent herbicide is an effective choice.

When spreading a post-emergent herbicide in October, landscape professionals should make sure it is at least 60 degrees. If it’s cooler, Gardner says he has had good success controlling weeds using an ester-based formulation in temperatures down to the low 40s.

Winter annual weeds in a landscape bed are a different story. If they germinate and become entwined with desired plant material, there are no herbicides on the market that will selectively kill the unwanted weed growth. In this case, a preemergent herbicide is the best option. In landscapes where spot spraying is possible, glyphosate is another option. While glyphosate has been reviewed to be non-carcinogenic, be certain to use proper protective gear and follow label instructions when using it. As a contact herbicide, it kills most plants it comes into contact with, so be very precise in application, even when spot spraying.

“In a landscape, you can’t use residual herbicides,” Loux says. “You’re going to wait until they come up, then hit them in mid to late fall so they’re not there in the spring.”

The window for controlling weeds in a landscape is a bit wider. Loux says that you have until Thanksgiving to attack that growth, which is late enough in the season that most other plant material has gone dormant

White clover

Annual bluegrass, or Poa annua, is one winter annual that presents a formidable challenge. Once annual bluegrass, with its characteristic canoe-shaped leaf tips, begins to grow, it is resistant to post-emergent products, leaving preemergent herbicides as the best option for control.

“It’s just a pain,” says Berni Kurz, an extension educator at the University of Arkansas. “You think you have it whooped, and then in February or March, it comes back.”

Kurz says there are at least six or seven preemergent herbicides that will conquer annual bluegrass, including active ingredients such as trifluralin, prodiamine, pendimethalin, dithiopry and oryzalin. Many of the options are granular, which makes for convenient spreading. It is also harder to overdo a granular application. But once again, getting the preemergent down early is the key to effective control.

“We recommend applying it in August because here in Arkansas, we typically get the cold front in late August or early September,” Kurz says. “So if you get it on in early to mid-August, the preemergent material is ready and waiting for the grass seeds to germinate. There are a slew of chemicals that work, but the key is getting them on early. If you miss the window for application, you’re in trouble and you’ve got a lot of catching up to do.”

Perennial weeds

Perennial weeds are flowering plants that reproduce by seeds or grow back from roots or tubers. They do not die when the frost rolls in. Instead, they lay dormant through the winter, only to come back with a vengeance when spring arrives. For perennial weeds like dandelion and white clover, the same three-way post-emergent herbicides used to combat winter annual weeds will work effectively. Florasulam is another choice for use in temperatures down to 45 degrees.

The addition of more stubborn broadleaf weed varieties like creeping thistle, ground ivy and wild violet present more of a challenge. In this case, the addition of a protox inhibitor like carfentrazone or sulfentrazone will be more effective, says Gardner.

Creeping Charlie

As for timing, the strategy is the same for both perennial weeds and winter annuals. In the fall, perennials are getting ready for winter by storing carbohydrates underground. Applying an herbicide in the fall makes it more likely that it will travel underground and affect the tissue that makes the plant a perennial, Gardner explains.

In addition to stopping weeds in their tracks, the application of herbicides in the fall also allows time for lawns to fill in and become thick and healthy, a natural form of weed control.

“Anywhere the lawn is thin is where weeds come in, and anything you spray in mid to late fall will take most of that out,” Loux says. “You’ve taken care of them. They won’t be there in the spring and that gives the lawn time to fill back in. In thin areas, that is your chance to put more grass seed in there.”

Loux also recommends avoiding one common, and detrimental, mistake: cutting the lawn too low. When cutting, be sure to cut only the top 1/3 of the grass blades at one time. Cutting too aggressively, also known as scalping the lawn, allows weeds to creep in.

Anything that keeps the lawn healthy will also help prevent weeds from taking root, Loux says. Dethatching and aerating will keep a lawn in prime shape, along with two fall fertilizations — one in September and one in early November. A decent lawn fertilizer with nitrogen, phosphorous and potassium should do the trick.

Lauren Sable Freiman is a freelance writer based in Cleveland and can be reached at laurensable@gmail.com.

The organic approach

While chemical applications are the most effective way to control weed outbreaks, the options for organic weed control are not so robust.

“There are some organic products, but they are just not consistent enough for me to go out on a limb to recommend those products,” says Berni Kurz, an extension ed ucator at the University of Arkansas. “But mulch, we know that works. I never let my mulch get depleted. That is my weed control in my landscape.”

Landscape professionals can’t go wrong by encouraging clients to keep a healthy layer of mulch in their landscape beds. Kurz says a minimum two-inch layer of organic mulch alone does an amazing job of weed prevention. If a few weeds manage to poke out of the mulch, the growth is so minimal that hand-pulling isn’t difficult.