July 22 2019 06:00 AM

Weed prevention can seem like a losing battle, but with the right tools and strategies, we can increase our odds against these nasty little invaders.

When an army unit sets out to fight a battle, it needs provisions, mainly food and water. The last thing those soldiers need is to have the enemy find its cache of provisions, consume them and then settle in for a long siege.

Landscape maintenance contractors and lawn care service operators are enmeshed in a permanent state of war with weeds, an enemy that constantly seeks to invade and suck up the provisions you’ve carefully set out for your clients’ lawns and landscape beds. Fortunately, you can take measures to keep these unwanted plants from infiltrating the territory of the ones you want to protect and nurture.

We may never totally defeat weeds. “If you look at any herbicide label, it’ll never claim to eradicate weeds, only control them,” says Jack Mell, sales manager at Plant It Earth’s Plant City, Florida, branch. Even if it’s a war we’ll never completely win, we can still employ strategies that will make the enemy’s invasion plans more difficult.

Pre-emergents versus post-emergents

Weed control products fall into two major categories: pre-emergents, the primary tool for preventing weeds from coming up in the first place; and post-emergents, which zap them once they pop their ugly little heads up.

Lush Lawn, a company that specializes in lawn care, tree care and weed and pest control services at five locations in Michigan, uses both methods. “I’d say it’s about 50/50,” says Aaron Samson, founder and CEO. “It’s important to do a mixture of both pre and post.”

The season with pre-emergent treatments, mainly to keep the crabgrass from germinating. “It’s important to put it down before the second or third week of May before it gets warm enough for the seeds to germinate. It also helps keep out some of the summer annuals and broadleaf weeds as well,” Samson says.

When to apply herbicide depends on your climate zone. “In warmer markets, you’d have to put down the crabgrass pre-emergents twice a year, but in our market, once a year is usually enough,” Samson continues. “And then the rest of the year we’ll blanket a lawn with the post-emergent products, to take care of dandelions, ground ivy and white clover. Throughout the summer we might see summer annuals like oxalis or speedwell popping up. They’ll usually be in isolated pockets, so we can get by with spot-spraying them. Once we get into fall we like to blanket the lawn again with a post-emergent to get the weeds that pop up again, like thistle.”

Read that label!

Whoever came up with the old saying, “When all else fails, read the directions,” could have been thinking about herbicides. Samson says, “The label is your roadmap to proper application and mixing techniques. It’ll tell you how many ounces you put in per gallons of water and tell you just how to mix it, and when the outside temperature might be too hot to put it down.”

Brian Thompson, marketing manager, turf, professional and specialty solutions for herbicide manufacturer BASF Corp., Florham Park, New Jersey, agrees. “You need to really read those labels to gain the best value of these products and the best efficacy.”

He says that a lot of herbicides, such as BASF’s Drive XLR8 or Pylex products, have label language that instructs the user to mix it with an adjuvant such as methylated seed oil that helps the herbicide get into the plant and makes it more effective.

These chemicals aren’t all that forgiving, so precision is required. When a herbicide isn’t applied the way the label specifies, in the proper amount, at the right time and at the right outdoor temperature, grass can be damaged.

“In the southern climates, you have some grasses that you really have to be careful with, for example, St. Augustine grass — my understanding is that it’s very sensitive to weed control chemicals,” says Samson. “In the north our grasses are less susceptible, but you still have to be careful — you could hurt them if you apply this stuff on a day that’s 95 degrees or even 90 degrees, put it down too heavily or mix it wrong.”

Product formulations

Pre-emergents often come in granular form while post-emergents are usually liquid. This has to do with the way the weeds absorb them. Pre-emergents are absorbed through the roots, which lends itself to a granular approach, while post-emergent products use the leaves as their entry points, which is why they are usually in liquid form.

Many lawn care professionals use a combo fertilizer/pre-emergent herbicide product early in the season which is almost always granular.

It seems a bit counterintuitive to be distributing something that feeds plant growth — fertilizer — at the same time as we’re setting out an agent that retards it — herbicide. Aren’t we fertilizing the weeds along with the grass? Not really, says Samson. “You can simultaneously get rid of weeds and stimulate the growth of grass. Because of the selectivity of the herbicides, there is no negative impact on the weed control. It still works.”

Will Jellicorse, owner of Tennessee Turfgrass LLC, Knoxville, Tennessee, doesn’t use fertilizer/herbicide combo products. “I like to spray my herbicide separately — that way I know exactly where it’s going. I’m not worried about flinging fertilizer in a bed or out in the road or down a drain, somewhere it doesn’t belong.”

Protecting bedding plants with landscape fabric

Preventing weeds in landscape beds is a different game than preventing them in turf. That’s where landscape fabric comes in.

Some landscapers don’t like using landscape fabric, saying that it suffocates the soil and kills the beneficial bacteria that lives in it. They may be thinking of older products that weren’t much different than plastic trash bags.

“When I first went into the landscape business, black plastic sheeting was being used,” says Larry DeWitt, CEO of Sikeston, Missouri-based DeWitt Co. “The soil couldn’t breathe, just as you couldn’t if you tied a plastic bag around your head.”

What was needed was a permeable fabric that lets water, air and sunlight through, and today we have it from DeWitt and other manufacturers.

Landscape fabrics today are light-years ahead of that old black plastic. Today, they’re made of polyethylene and come in different forms for different uses: biodegradable, woven, nonwoven spunbonded and nonwoven point-bonded.

Even so, there are advantages and disadvantages to using landscape fabric. It holds moisture in very effectively, but also tends to restrict the movement of earthworms and other beneficial soil mixers, and soil can become compacted beneath it. And weeds can still poke through the barrier after a while.

Still, landscape fabric is a great tool to have in your arsenal for keeping weeds out of beds and planters or slowing them down.

Tackling crabgrass

When we’re talking weed prevention, one of the major targets — and a hardened one at that — is crabgrass. Many landscape pros believe crabgrass is something that you have to treat preventatively, with pre-emergents, or else, as they say in New York, “fahgeddaboudit.”

“That’s not correct,” says Ron Freeman, owner of Arbor Pro Plant Care Experts, a lawn care and weed control business in Rochester, New Hampshire. “You can put down a pre-emergent and a lot of times crabgrass will still come up. It’s actually easier to spray it and kill it after it comes up.”

Samson agrees with the general consensus that it’s best to tackle crabgrass pre-emergently. “Post-emergents are only effective on crabgrass in the first couple stages of maturity, at the seedling or young plant stage. Once it matures it’s very tough to control even with a post-emergent.”

A spreader is used to apply a granular pre-emergent/fertilizer combo.
Photo: Lush Lawn

“The sentiment that crabgrass must be treated pre-emergently may come from its physiological uniqueness,” says Thompson. “Very young and very old crabgrass is easy to control. But for some reason, when it’s between the two- and four-tiller (leaf) stages, it metabolizes the herbicide in a way that makes the herbicide less effective. That’s why post-emergents applied at that stage often don’t work.”

Thompson says using a combination of quinclorac and topramezone will nullify that metabolic process. “Those two chemistries together create a synergism, like a one-plus-one-equals-three effect.”

What is it about crabgrass that makes it such a tough opponent? Samson blames its aggressive root structure and the way it leafs out. “It tends to kill the grass around it or out-compete it for nutrients and sunlight. That’s why it’s really best to use the pre-emergents on it. If you do get crabgrass later in the season, you can use post-emergents, but it’s difficult to control at that point.”

Irrigation and mowing

Cultivation practices can help prevent weeds too. The right amount of irrigation helps keep grass healthy. Freeman says that the mere fact you’re irrigating at all can discourage crabgrass. “The dirt stays cooler, and the cooler the dirt, the better the lawn,” as crabgrass needs hot soil to emerge.

Water management is critical, says Jellicorse, who recommends watering deeply and infrequently, every two to three days, “especially this time of year, when it’s hot and humid — you don’t want to overwater. Fescue will tell you when it’s under drought stress. You’ll see it start turning a purplish-gray color. But that’s fine, you can let it stress out a little bit.”

“Cutting high is very important for weed control,” says Samson. “A cutting height of 3 inches or higher is very helpful for keeping weeds out. The longer the grass is allowed to be, the deeper its roots can go.”

“It’s important to keep the mower blades sharp too, because you get a cleaner cut,” says Jellicorse. “The grass leaves heal faster, which helps keep them strong.”

Resistance and adaptation

An inconvenient truth about weeds is that they’ll eventually develop a resistance to any herbicides we hit them with.

“Within any population of weeds there will always be a small subset of individual plants with a genetic mutation or some other mechanism that allows them to survive an herbicidal attack,” says James Brosnan, Ph.D., associate professor of plant sciences at the University of Tennessee, Knoxville, and the leader of its new Weed Diagnostics Center. He also serves as an advisor to several turf management organizations.

Brosnan has done a lot of research on herbicide resistance. “If the same measures are implemented over time, you’re selecting the same subset of the population. Eventually, that builds to the point where your control measures are no longer effective as there are now more resistant plants than susceptible ones in the landscape.”

“The resistance problem has really grown in turf,” adds Brosnan. “The USDA calls it an epidemic, especially in bluegrass. They’ve funded a $5.2 million project to tackle it in all turfgrass systems — golf courses, sod farms, residential and commercial lawns, and athletic fields.”

But he cautions, the answer to resistance will not come exclusively from a jug. “It’ll be more of an integrated approach where we’re trying to optimize the growing environment. We can do some things agronomically to make conditions more favorable for the turfgrass and less favorable for the weeds.”

One of those agronomic things is remembering that every herbicide has a certain mode of action, the biological system it attacks in a weed in order to kill it. Using a different herbicide with a different mode of action every year helps avoid the resistance/adaptation problem.

But it can be tough to remember which chemical acts as a cellular mitosis inhibitor, a photosynthesis inhibitor or what have you. “There’s an easy way to keep track of this — just look at the group number on the label,” says Brosnan. “That number correlates to what that herbicide’s mode of action is.”

Jellicorse is one of Brosnan’s former students at UT. He must have paid attention in class because he says, “I’m a big fan of rotating my herbicides. To avoid creating resistance, I never use the same thing over and over.”

There’s no magic bullet when it comes to weed prevention. The best method is to maintain a strong, healthy lawn with nice thick grass that doesn’t give weeds any weaknesses to exploit. By building a healthy lawn, you’re building a defensive wall that’s high enough to keep weeds out. Good luck with your battle plans!

The author is senior editor of Irrigation & Green Industry and can be reached at maryvillano@igin.com.