March 19 2021 06:55 AM

Beat summer weeds with preemergent strategies before they become a problem.


As the weather starts to warm up, landscape and irrigation professionals are getting ready to get back to their clients’ lawns. But they’re not the only ones eager to make their mark on a developing yard this spring. Summer weeds are ramping up just as quickly, and the early months of the season are key to keeping the grass clear of these invaders. Here are a few common opponents, including how to stay ahead of them.

Crabgrass

Landscapers and homeowners alike are familiar with crabgrass, as it’s one of the more common backyard invaders in much of the U.S. As the grass grows in, it has a light green or bluish green color, generally without stolons or rhizomes, says Bert McCarty, professor of turfgrass science and management at Clemson University, Clemson, South Carolina. A telltale sign is the seed head, which can spread out with what look like fingers, part of what earned it the name Digitaria.

“Most homeowners recognize that seed head,” says McCarty. “That might be good, that could get you a new customer.”

Dealing with crabgrass can be a challenge, but preemergent applications can help keep it under control. Look to put that down when soil temperatures at a 4-inch depth reach about 53 degrees for a full day, which is when crabgrass will really begin to germinate, McCarty says. In his region, that’s usually about the third week of March, but that will differ by climate. Some of the more commonly used products include active ingredients such as dithiopyr or prodiamine. Different active ingredients provide different modes of action in taking care of the weed, so try to switch between multiple options with repeated applications. Follow the manufacturer’s instructions for application, including watering in the product if necessary. Prodiamine will need a repeat application in about 75 days. If the weather throughout summer tends to be more rainy, another application might be necessary.

Management practices can be helpful in crowding out crabgrass with more healthy grass, and research has shown that taller fescue can reduce the amount of crabgrass that can compete, says McCarty. When mowed at a height of 1 inch for the summer, the fescue patch was 80% crabgrass. That dropped to 70% at 2 inches and 25% at 3 inches. At 4 inches, only 5% of the patch was crabgrass. But convincing a homeowner to let grass grow a little taller to block weeds might be a challenge.

Wild violet

In the Midwest, wild violet is one perennial wildflower most landscapers are familiar with as a common yard invader. It thrives in moist, shady areas but can definitely tolerate drought conditions as well. It’ll do especially well anyplace where grass is thin and sparse, says Gared Shaffer, extension weeds field specialist at the South Dakota State University Extension, Aberdeen, South Dakota. Look for curled, heart-shaped leaves growing in bunches and for it to spread like other wildflowers through root systems. But the purple flower is what will set it off from other plants in the landscaped area.

There are multiple chemical options for handling wild violet, including products with mecoprop, dicamba or triclopyr. Most of those will take repeated applications, as wild violet can be a tough weed, especially once it’s fully established, Shaffer says. After an application, check back on the weed in about two weeks. If it’s still showing signs of growth, it could be time for another application.

Maintaining healthy, dense turf is one of the best approaches for crowding out wild violet. Keep an eye on patches of grass that seem to be a little weaker, as those will be where perennials like these will target, says Shaffer. Pulling the weeds by hand here with a small shovel is another solid option, as long as you’re able to get the entire root system.

Dandelion

Dandelions are another weed that homeowners can pick out from a distance, with the signature yellow flower and white puffball seed head. But even before flowering, it has distinctive, serrated leaves, says Mc- Carty. It can have a prostrate growth habit along the ground coming out of a central terminal bud with leaves that flop over.

There aren’t really preemergent herbicides that will do much to cover dandelions, which are perennials and will generally be a nuisance year-round unless you really go after them, says McCarty. When you do notice them in a lawn, there are several three-way active ingredient mixes that can be effective. It can be helpful to include a herbicide that will put some extra pressure on the weed like fluroxypyr.

For good coverage of dandelions, it will often take two applications of a herbicide mix about 10 days apart. That can be tough for lawn care operators, as many clients don’t want to have crews making multiple trips and applications to the yard. In those cases, it might be helpful to move up the rate slightly, he says.

“That may not kill the dandelion, but it’ll knock it down to a point that the homeowner is generally pretty happy with it,” McCarty says. That works especially well if the client will tolerate slightly taller mowing heights, which will both help hide the dandelion and crowd it out with more healthy grass.

Sandbur

In the southern U.S., sandbur is a common summer annual weed that has a very distinctive look, with painful, problematic seed pods that the grass gets its name for. As the weed comes up from the ground, it will have a maroon or purple base, says Zach Howard, extension program specialist at Texas A&M University, College Station, Texas. At shorter heights, the stem is fairly flat in comparison to other grasses, and it grows from a crown, unlike other turfgrasses that spread. If an area has had trouble with sandbur in the past, make certain to check it during green-up in the next year, as it’s likely to continue.

“Typically there are burs in that area from years past, and they don’t break down easily,” says Howard. “A lot of times what you’re finding is seed from two or three years ago that’s actually germinating.”

Sandbur will favor areas that are already receiving fertilizer and regular irrigation, so most landscaped areas are targets for the weed, he says.

It’s not flashy, but pulling up sandbur by hand is the best way to deal with it and give the surrounding lawn the chance to outcompete it. One chemical control for the weed is indaziflam, as long as it’s applied early in the season to problem areas as sandbur is germinating, he says. Look for a soil temperature approaching 70 degrees as a sign that it’s time to watch for new growth.

Sandbur does provide lawn care operators with one effective way to deal with it, thanks to its own strategies for spreading. “You can use the fact that they’re burs to your advantage,” says Howard. If an area is infested with sandbur, take a piece of carpet or a material that they can hook onto and roll it or lay it across the grass. Lift the carpet away, and that should pull a good number of the burs along with it. Black landscape cloth can be useful as well in keeping the burs from finding a place to germinate. It’s also important for lawn care operators to check their own clothes as they work around areas with sandbur to make certain that they’re not carrying it to other parts of the lawn or even other properties throughout the day.

Lespedeza

Common Lespedeza is a nuisance that sometimes shows up in yards using centipedegrass, a lowmaintenance, warm-season grass in the South. Lespedeza isn’t just another grass like some other weeds, but a legume that can make its own nitrogen. One of centipedegrass’s defining characteristics is that it doesn’t need much nitrogen to thrive, where many other plants like having more of it around. In a low-nitrogen situation, however, Lespedeza brings its own supply and can carve out a niche where it can be a strong competitor to centipedegrass, says McCarty.

Lespedeza leaves are ovate in shape, meaning slightly egg-shaped with the broader end toward the base. McCarty says one telltale sign is parallel venations on the leaf that come off at 45-degree angles to the midrib. As the plant matures, its stems are tougher and more wood-like, and the plant produces a light purple flower.

Going after Lespedeza early is key to keeping it under control, he says. As it gets into summer and the stems grow tougher, it can be difficult to knock back even with herbicide applications. In May or June, products with metsulfuron can be effective in controlling it. Once the plant has matured somewhat, it will also likely take a product with fluroxypyr in combination to keep it in check.

Raising the mowing height on the centipedegrass can be helpful, but its relative low need for inputs like nitrogen can be a double-edged sword when dealing with Lespedeza, says McCarty. Adding more nitrogen can make the weed less competitive, but it can make the centipedegrass more susceptible to winter damage. Stick to about 1 to 2 pounds of nitrogen per 1,000 square feet per year on those areas to provide some growth.

Ground ivy

Ground ivy, another perennial broadleaf weed seen in several areas of the U.S., goes by multiple names. Where Shaffer is from in South Dakota, it often gets called creeping Charlie. It tends to like moist, rich soils in well-fertilized and irrigated areas with some shade or full sun, producing roundish, green leaves and a purplish flower from a square stem. It spreads through an aggressive stolon across the top of the ground, and even with lower mowing heights, it can tolerate the damage and come back just as strong, he says. It prefers areas where the lawn has seen some damage or is already weakened and can grow into a dense mat to crowd out other plants.

Maintaining a taller mowing height will help keep control of ground ivy, but there are also multiple chemical options to handle it. Look for active ingredients such as flumioxazin or isoxaben to suppress it from spreading. Sulfentrazone will also knock it back effectively, but watch for eventual chemical resistance after repeated applications. While that’s more common on heavily manicured grass like golf courses, it can be a problem on home lawns as well.

Slender aster

Slender aster is a broadleaf weed that shows up in the southern U.S. that tends to favor well-fertilized and irrigated areas, says Howard. Early on in germination, look for a cluster of leaves coming up that have a lance shape. As it grows, it develops a more spindly shape, with tougher, wiry stems that end in a white or purple flower with a yellow center.

Multiple chemical options cover slender aster, including both indaziflam and pendimethalin, Howard says. Applying on an annual basis in late spring or early summer should help keep the weed under control. But like others, one of the best ways to deal with it and make certain it stays out of the area is to pull it up by hand. Landscaping cloth with mulch over it can also help keep it out of flower beds as well.

The author is editor-in-chief of Irrigation & Green Industry and can be reached at kylebrown@igin.com.



By the numbers

Chemical products, including fungicides, fertilizers, herbicides and insecticides, are some of the most important tools for landscape professionals in dealing with the common pests that plague lawns. We asked our readers for feedback to uncover the larger trends of how they choose which inputs to purchase and apply.

Our survey tracked contractors across specialties, with slightly more covering residential properties than commercial. Across the board, the total annual budget for chemical products generally wasn’t a major chunk of the company’s overall spend, with a quarter dedicating $2,500 or less. A total of 33% spend between $5,000 and $25,000 annually on chemical products, and slightly fewer respondents (28%) spend more than $25,000.

Authorized distributors and dealers of chemical products should feel proud of their market saturation, as 85% of respondents go straight to them for what they need. Big box stores might seem like a potential threat when it comes to purchasing equipment, but they only pulled 6% here.

In choosing the right chemical product for the job, respondents most often look for quality (92%) and reliability (91%) as the most important factors. Rounding out the top three is the product’s safety at 81%.

While chemical regulations have been a struggle for a fifth of respondents, three-quarters have found ways to continue operations alongside those rules. That’s encouraging, as chemical regulations continue to be enacted across the country with different levels of stringency. Just 5% of respondents are facing no regulations on chemical products at all.