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January 2018

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Invasive Watch focuses on marestail

What to look for
HorseweedMarestail, also commonly known as horseweed, coltstail, Canadian fleabane and several others, is a winter or summer annual that grows from 1 to 5 feet tall. Plants start out as a rosette and generally begin to bolt in April. Flowering occurs from June to August and plants disperse seeds from August to October. Stems are erect and unbranched below, but often branched above, and most times covered with short, white hairs — though in some cases will be nearly hairless.

Upper leaves are alternate, lance-shaped, crowded on the stem, bristly with hairs and sessile. Lower leaves are spatulate and sparingly or coarsely toothed. Leaves grow in an alternate spiral up the stem, and the lower ones tend to wither early. The inflorescence is branched with slender flower stalks, and flower heads are inconspicuous with white and yellow flowers.

Where it's found
mapMarestail can be found in every U.S. state and is easily one the most common weeds in the eastern and central part of the country. Native to North and Central American grasslands, but now found throughout Eurasia and Australia, marestail is frequently found along roadsides and rights-of-way, pastures, meadows and in waste areas.

Marestail is a prolific producer of seeds, which are only about 1/16 of an inch long with a white and bristly pappus. Each plant is capable of producing up to 200,000 seeds, and most can germinate right off the plant. Seeds are also known to travel long distances in the wind.

How to treat it
Control of marestail can be difficult, especially with several populations shown as resistant to glyphosate and ALS-inhibiting herbicides. In fact, it was the first weed to develop glyphosate resistance, as reported in 2001 in Delaware. However, there are several very effective herbicide treatment options.

The most effective treatment is a foliar broadcast application of 1.5 to 2 ounces per acre of Opensight® specialty herbicide.

Another excellent foliar treatment is 4 to 7 ounces per acre of Milestone® specialty herbicide. The most consistent marestail control is typically achieved when the weed is treated while it’s small and in the rosette stage. Use lower rates when weeds are small and actively growing, and increase rates as the season progresses and plants become more mature.

Additionally, Cleantraxx® herbicide at 2 pints per acre is effective on marestail as a fall preemergence foliar treatment.