Invasive Watch: Marestail

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Marestail is a tough winter annual broadleaf weed that is cruitial to treat early.

Marestail, or horseweed, has proven to be one of the most challenging weeds vegetation managers have to deal with. It’s almost everywhere, it spreads rapidly and is expanding in its resistance to many herbicides. The more it’s allowed to grow, the harder it becomes to control, making it crucial to treat marestail early.


Marestail, also commonly known as horseweed, is a winter annual weed. It primarily emerges in fall as a rosette but is also known to emerge in early spring. Fall-emerged marestail becomes dormant over the winter, then reemerges in spring. Seedlings have small, lobed leaves that grow into rosettes 1 to 4 inches tall. The fast-growing rosettes sprout erect stems that produce up to 200,000 seeds per plant — which can be spread long distances by the wind — and reach heights of 6 feet. Stems usually have short, white hairs but can also be nearly hairless. Its many leaves are alternate and lance-shaped. Marestail typically blooms from June to August with numerous tiny, inconspicuous flowers.


Marestail thrives in locations where soil is undisturbed, making roadsides and bareground areas such as railways or utility substations more susceptible to infestations. When that happens, it can lead to line-of-sight and safety concerns, issues with erosion, as well as the destruction of desirable native habitat. This species is native to North America, and, while once considered mainly a Midwestern problem, marestail has spread to cause significant issues throughout the United States.


It’s crucial to treat marestail early, during the rosette stage, because once plants bolt to 5 inches in height, they become much more difficult to control. Marestail also has a well-documented history of herbicide resistance, including resistance to glyphosate and diuron.

For best results, use 2.0 to 2.85 ounces of TerraVue herbicide per acre during early stages of active growth. Use a higher rate when growing conditions are less than favorable, when weed foliage is tall and dense, or when optimum longer-term residual control is desired. TerraVue also provides preemergence control of germinating seeds or seedlings of susceptible weeds following application.



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