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How to identify and manage some of the biggest problem plants

Understanding Invasives

Invasive species of plants wreak havoc on rights-of-way throughout the United States and Canada. The following are some of the biggest current threats, as well as suggested vegetation management practices for each plant pest.


Marestail (Conyza Canadensis)

 Marestail (Conyza Canadensis) can be found throughout the United States and most of Canada along roadsides, railroad tracks, fencerows, fallow fields, orchards and open areas.

Also known as horseweed, it can germinate nearly any time of the year. Making matters even worse, heavy reliance on glyphosate for weed control has resulted in the development of glyphosate-resistant biotypes and serious concern about the long-term control of this species.

Native to North America, marestail is an annual weed, winter or summer, depending on location. It is an upright-growing plant that can reach 6 feet. Marestail grows in a basal rosette and typically flowers from June to September, with plants producing up to 200,000 seeds. The seeds are small achenes with a pappus of tan to white bristles, which allows the seed to be dispersed by the wind.

Mature plants have leaves that are alternate, linear and simple, with entirely or slightly toothed margins. Small, yellowish flower heads grow at the end of branched stems. Marestail tolerates drought conditions well, and continues to grow and produce seed under conditions stressful for many other plant species.

In light of glyphosate-resistant marestail biotypes, there are a few options offering residual control and a different mode of action. Aminopyralid can be applied alone in solution or as a tank mix partner when a wider-spectrum of control is needed. It is recommended that 4 to 7 fluid ounces per acre of aminopyralid, or a tank mix including aminopyralid, be used to get adequate control of marestail.


Sulfur cinquefoil (Potentilla recta)


Photo by Ohio State Weed Lab Archive, The Ohio State University, Bugwood.orgSulfur cinquefoil is a perennial species that adapts well to a wide range of environmental conditions. It occurs in open grasslands, roadsides, abandoned fields, open forest sites and other shrubby areas. Native to Eurasia, it was brought to North America from Europe around 1900. By 1950, the plant was well established in the eastern United States and Canada. Spreading west, it is now present in all states except Utah, Arizona and New Mexico.

Sulfur cinquefoil produces several erect stems with some branching at the top, reaching up to 2.5 feet in height and producing up to 1,600 seeds per plant. There are numerous leaves on the stems, which have long stiff hairs that stick straight out from the surface. Each leaf has five to seven distinctly toothed edges and is 2 to 4 inches long. The pale yellow, or sulfur-colored, flowers can be 0.6 to 1 inch in diameter, and each flower has five petals surrounding a darker yellow center. Sulfur cinquefoil spreads both by seed and by new shoots emerging from the edges of its root crown, which makes mowing alone ineffective for control.

In areas where livestock and wildlife feed off the natural vegetation, sulfur cinquefoil is unpalatable; therefore, it reduces forage for livestock and wildlife where it has created a monoculture. Along with its highly competitive nature, sulfur cinquefoil shares the same characteristics and geographies as other problem invasive plants such as yellow toadflax, spotted knapweed and quackgrass.

Control efforts of this invasive plant should be carried out shortly after discovery to reduce the chance of spread and establishment. Herbicide applications should take place during the pre-bud stage of plant development for optimum control. Milestone VM specialty herbicide at 5 fluid ounces per acre has shown high efficacy on sulfur cinquefoil. But take other site characteristics into account — such as other species present, desirable surrounding vegetation and tank-mix choices — before choosing which herbicide to use.

Because of its ability to send up new shoots from the root system, sulfur cinquefoil may re-establish itself over a few years. Monitoring and repeat applications may be necessary for long-term control.


Cogongrass (Imperata cylindrical)


Photo by John D. Byrd, Mississippi State University, Bugwood.orgKnown as one of the world’s 10 worst weeds, cogongrass (Imperata cylindrical) was brought to this country from Southeast Asia as experimental forage, and has been spread through its use as packing material, erosion control, and livestock forage. It spreads primarily from rhizomes and windborne seeds. The plant is highly flammable, but the roots and rhizomes are remarkably resistant to controlled burns.

Cogongrass is a non-native, fast-growing perennial grass. Its leaf blades are narrow-erect with a prominent white midrib and sharp, pointed tips. It produces fluffy, white, plumelike seed heads in early spring. This spring flowering is contrary to most summer grasses, which generally flower later in the season.

Cogongrass also has been documented to initiate flowering at other times of the year in response to disturbance such as herbicide application, fire, mowing or the first hard frost. Seed heads range from 2 to 8 inches in length and may contain as many as 3,000 seeds. Each seed has silky, white hairs that aid in wind dispersal.

Able to invade and overtake disturbed ecosystems, it forms a dense mat of thatch and leaves that out-competes other plants. Cogongrass often attains a height of 3 to 4 feet late in a growing season. It often displaces a large variety of native plant species used by native animals as forage, host plants and shelter. Some ground-nesting species have been known to be displaced due to the dense cover that cogongrass creates.

Mix 5 to 6 quarts of Accord XRT II herbicide with 16 ounces of Arsenal herbicide and a non-ionic surfactant. Treat when the plants are at least 18 inches tall and actively growing in summer or fall. For cogongrass growing under hardwood trees or near other desirable vegetation, apply 8 quarts of Accord XRT II per acre. Foliar treatments are most effective between June and early October, as long as the plants are actively growing and not under drought stress.

Early applications also may be made to prevent seeding. Apply 1 to 2 quarts of Accord XRT II per acre after green-up but before flowering. This low rate will prevent cogongrass from flowering and producing seed but will not control existing plants.


Tree-of-heaven (Ailanthus altissima)

 Tree-of-heaven (Ailanthus altissima) is a rapidly growing deciduous tree that grows in disturbed soils, fields, roadsides, fencerows, woodland edges, forest openings and rocky areas. Also known as Ailanthus, tree-of-heaven grows between 80 and 100 feet tall and up to 6 feet in diameter. Native to China, tree-of-heaven was introduced to North America in 1748 by a Pennsylvania gardener using it as an ornamental. It was widely planted in cities because of its ability to grow in poor conditions. Today, it occupies 42 states.

Tree-of-heaven reproduces rapidly by both root sprouts and by wind- and water-dispersed seeds. Once established, it can quickly take over a site and form an impenetrable thicket. In addition to its prolific vegetative reproduction, Ailanthus also produces toxins that prevent the establishment of other plant species, which may consequently inhibit succession.

With its smooth stems, pale gray bark and large compound leaves, tree-of heaven is sometimes confused with black walnut, butternut or some species of sumac, so proper identification is crucial.

The most effective control method for tree-of-heaven can be achieved with basal bark or cut-stump treatments. Keep in mind that it is relatively easy to kill the above-ground portion of Ailanthus trees, but control of the root system is essential to prevent or limit stump sprouting and root suckering.

For individual plant treatments of tree-of-heaven, use a mixture of 20 percent Garlon 4 Ultra specialty herbicide plus 5 percent Tordon K specialty herbicide in basal oil applied as a basal bark treatment for control. For basal bark applications, herbicides should be applied to stems less than 6 inches in basal diameter in a manner that thoroughly wets the lower 12 to 15 inches, including the root collar area, but not to the point of runoff. To control re-sprouting of cut stumps, spray the root collar area, sides of the stump, any exposed roots and the outer portion of the cut surface, including the cambium, until thoroughly wet, but not to the point of runoff.

If the Ailanthus infestation is widespread, and individual plant treatments aren’t possible, a high-volume foliar mixture of Garlon 3A, Tordon K and Escort XP herbicides in water, plus a nonionic surfactant, should be applied. Rates of each vary according to region of the country, so product labels should be referenced to get the proper herbicide rates.


Article provided by Dow AgroSciences. For additional information on these species, visit the U.S. Department of Agriculture’s National Agricultural Library or www.invasiveinstitute.com


Accord, Garlon, Milestone and Tordon are trademarks of Dow AgroSciences LLC Tordon K is a federally Restricted Use Pesticide. State restrictions on the sale and use of Accord XRT II and Garlon 4 Ultra apply. Consult the label before purchase or use for full details.

Arsenal is a registered trademark of BASF.

Escort is a registered trademark of DuPont.

Always read and follow label directions.

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