April 29 2020 06:00 AM

Stay ahead of summer weeds by knowing what to look for.

Spring is here, summer’s coming and this is likely your busiest time of the year. While trees, turf and bedding plants salute the warming temperatures with a burst of growth, unfortunately the weeds are right there with them. The good news is there are plenty of options to help you keep your landscapes clean and thriving. Here are a few of the most dastardly devils with tips to keep them at bay.

Dandelions

Dandelions attract bees and the edible plants have many purported medicinal uses, but every plant has its place. “If you’ve got kids playing in the yard, you don’t want bees everywhere,” cautions John Kaminski, PhD, turfgrass science professor and researcher at Penn State University.

Taraxacum officinale is a Eurasian herbaceous perennial with a long, tough taproot. The jagged leaf edges resemble canine teeth, and the common name is modified French for “lion’s tooth” (dent de lion). The leaves form a low-to-the-ground rosette after the plant plunges its deep root down. By the time the leaf rosette is visible, the plant is well established, which is one of the reasons it is so difficult to eradicate by hand. Just a small piece of root left behind will eventually become a reproducing plant.

We’ve all seen the “puffballs” dandelions produce, allowing seeds to spread literally hundreds of miles, depending on the wind. They prefer sunny spots but will settle for shade. They are tough and can establish themselves in almost any soil under almost any conditions, including wet or dry compacted soils, clay or sand. Individual plants can live five to 10 years and grow up to 20 inches across.

Fortunately, dandelions are very susceptible to the active ingredient 2,4-dichlorophenoxyacetic acid. The two most common formulations are amine salts and esters. Basically, esters are absorbed more quickly and are more efficient, but amines are nonvolatile and less likely to drift to desirable species.

“There are several ‘three-way’ mixes that will do the job,” Kaminski says. “In the spring, look for amine solutions. They are safer and can cause less potential damage than ester formulations.” Kaminski recommends ester formulations for fall control.

Because of its biology, the dandelion is extremely difficult to control in turfgrass without herbicides. In landscaped areas, repeated hand-pulling and heavy mulching can aid in control, but the landscaper must be diligent and consistent.

Wild violet

The Viola family includes many desirable ornamental species, with pansies and Johnny jump-ups especially treasured by home gardeners. The scent of violets was especially beloved during Victorian times, and they were planted and cherished throughout the world. Unfortunately, this diminutive sweet flower can become a raging monster in a home lawn.

There are several species of wild violets, including the Eastern U.S. native Viola sororia, but all are equally as difficult to control in turf. The plant spreads by rhizomes, which are swollen underground stems with a waxy cuticle that resists herbicides. Like the dandelion tap root, if even a small piece is left behind it can result in vigorous new plants. Low-growing rosettes of heart-shaped leaves with serrated margins grow up to about 3.5 inches long.

You’ll see and smell violets flowering in April and May. The flowers can be purple, violet, blue or white and are less than an inch across. The flowers are self-pollinating and produce a seed capsule that gets dropped and released on the soil surface. Ants can harvest and spread the seeds. “Seeds germinate under cool, moist conditions and give rise to new plants,” says Peter Landschoot, PhD, professor and researcher at Penn State. Between rhizomes and seeds, the plant can rapidly overtake a landscape.

The low-growing perennial is best controlled with herbicides containing triclopyr. “Repeated application over the course of the growing season and over multiple years may be needed for effective control,” cautions Landschoot.

Triclopyr is usually sold in combination with other herbicides, and some of those mixes are available in either an ester or amine formulation.

Persistence and regular treatment are the key, as the thick, waxy cuticle of the rhizome slows herbicide intake. Unfortunately, preemergent herbicides do not provide suppression or control. There are few or no nonchemical controls as well.

“Improving turf density through fertilization, regular mowing and use of turfgrasses well-adapted to site conditions will help to slow the spread of this weed but may not provide effective suppression once wild violet is established,” says Landschoot.

And if herbicides are banned in your community? “No real advice, other than learn to love it,” Landschoot says. “At least it has pretty flowers.”

Spurge

As an annual weed, spurge is remarkable for its prolific seeds. Researchers have noted that some spurges can complete their lifecycle in as few as two weeks and can produce up to 500 seeds per square foot. Seeds are dispersed by ants and wind. Seeds can also stick to shoes and tools, so be mindful about spreading it between jobs.

Spurge is a low-growing pest in the Euphorbiaceae family, which includes poinsettias and several drought-tolerant desert plants including succulents. Euphorbias are known for their milky sap, which can often be irritating. The plants form thick mats that compete and crowd out desirable plants.

Look for products that contain oryzalin, dithiopyr, pendimethalin, prodiamine, benefin, isoxaben or trifluralin. Since spurges can germinate any time air temperatures are above 60 degrees F, use these herbicides in the spring and again in the mid-summer according to label instructions.

“I wouldn’t recommend preemergents in turf,” says Kai Umeda, turfgrass specialist at the University of Arizona. “Spurge is usually a problem in bare spots, and it’s better to reseed the grass so it can outcompete and suppress the spurge.”

In landscape beds, a thick layer of mulch can prevent seeding and establishment. Diligent hand-
pulling will eventually diminish the seed bank. Gloves are recommended as protection from the irritating sap.

Nutsedge

Although it’s often called nutgrass, it’s not a grass at all. To add to the confusion, there are two different types, which makes nutsedge even more challenging to control.

The two most common species of nutsedge are yellow nutsedge (Cyperus esculentus) and purple nutsedge (C. rotundus). Yellow nutsedge is extremely hardy and is a problem throughout the nation. Purple nutsedge does not germinate below about 20 degrees F, which limits its spread. However, it widely spreads throughout the South and Southwest, where you can have both.

Nutsedges strongly resemble grasses, but sedges like these emerge from the ground in bundles of three leaves as opposed to true grasses, which emerge with two leaves. In addition, nutsedge blades are thick and triangular. Unlike some grasses, the leaves are hairless and smooth. It grows taller than surrounding turf in the summer.

Yellow nutsedge spreads by both rhizomes and tubers. The rhizomes are thickened stems that move through the soil to spread and establish the plant. The rhizomes end in a tuber, often called a nut or nutlet. These tubers sprout their three leaves and the cycle starts again. A plant can produce multiple rhizomes, resulting in multiple tubers.

Purple nutsedge spreads similarly but also produces chains of tubers along the way. A single rhizome can have several “nutlets,” resulting in chains of weeds. Purple nutsedge has a blunt-tipped leaf blade, while yellow nutsedge blades are sharply pointed.

Nutsedges are perennials. The flowers of yellow nutsedge are golden, while purple nutsedge flowers are dark and purplish. Normally, flowers are rare in turf situations when regularly mowed. Although it can spread by seed, rhizomes and tubers are the biggest concern.

If detected early, nutsedge can be discouraged and eventually controlled by hand-pulling, but it must be diligently repeated. In landscape beds, digging down and finding and removing tubers is recommended. Removing the plant shoots eventually depletes the energy reserves in the tuber. Nutsedge uses about 60% of its reserves to develop the first plant and 20% for the second. Unfortunately, that still leaves 20% reserve in the tuber, which is perfectly capable of growing and thriving and beginning a new infestation.

“Many herbicides are available for sedge control, but application timing is critical to optimize control,” says Aaron Patton, PhD, associate professor of agronomy at Purdue University. “For best results, apply herbicides prior to tuber production.” This means early in the season, when the plants are still growing vigorous vegetation.

“For use in lawns you have lots of options available,” says Umeda. The turfgrass specialist has been conducting trials on various controls. Some effective products include active ingredients like imazaquin, sulfosulfuron, trifloxysulfuron, flazasulfuron, atrazine or pyrimisulfan.

Landscape professionals should be aware that purple nutsedge likes damp soils, and Umeda cautions that it can pop up around leaky drip emitters. As with most plant pests, maintaining a dense stand of turf or a thriving landscape to outcompete invaders is the first step to control.

Helen M. Stone is a freelance writer specializing in commercial horticulture.

Going natural

Herbicide bans are becoming increasingly common in some areas. There are several products touted for chemical-free control, but how do they work? Kai Umeda, turfgrass specialist at the University of Arizona, is currently conducting trials on several products and finds that they can offer a viable, if imperfect, alternative.

“The organic products are up and down,” he says. “When you spray stuff like vinegar or citrus oils or clove oils you have to have good coverage.”

Experiments on golf courses using a traditional boom sprayer did not give satisfactory results. “It doesn’t seem to be as effective as when you are able to douse them completely. The key to using most of these products is to spray to the point of runoff,” Umeda says.

One criticism of these types of products is that they burn the top foliage off, but provide very little root kill. Application is needed on a regular basis until the entire weed succumbs, including the root.

Weeds should be sprayed while young. “You have to get them when they are still small,” he warns.

In addition, patience is necessary. “Results can take a while to show,” Umeda says. “It could take a week, or even almost two weeks, to show that they are working well.”

With increasing consumer interest in chemical-free options, research into natural weed control products will continue, which will lead to new and improved options regularly being brought to the market.