World of Weeds: Marestail

Stacy Campbell, K-State Extension
Stacy Campbell

Marestail (Erigeron canadensis), known as horseweed to weed scientists, is a troublesome weed in several cropping systems in Kansas and beyond. It is classified in the Aster family, which is a very large group of plants that also includes several marestail “look-alikes”. Table 1 briefly compares marestail with one of those look-alikes, dwarf fleabane.

Marestail features

Ecology and identification of marestail

Marestail is native to North America and grows throughout the Great Plains. It can be found in fields, rangeland, lawns, and other disturbed sites. There is considerable variation in identifying features among marestail populations, which can make identification troublesome.

Marestail is an annual plant that typically emerges in late fall or early spring and flowers throughout the summer. Marestail begins as a rosette, and the stem elongates to about 1.5 to 3 feet prior to flowering, although some plants may reach heights of greater than 6 feet. Stems are covered with coarse hairs. Leaves are oblong in shape with margins that range from entire in the rosette to toothed as the stem elongates. Leaf surfaces range from smooth to covered with course hairs and are lighter in color on the lower surface. Leaves are generally crowded together on the stem in an alternate arrangement, but they are less crowded near the top of the stem. Leaves may be attached by a short petiole or may be attached to the stem without a petiole.

Marestail flowers are found in a branched inflorescence at the top of the plant that is said to resemble a mare’s tail. They have white to pinkish ray florets that surround yellow disk florets. Each inflorescence is about ¼ to 1/3 inch in diameter and surround by leaf-like bracts. Each seed is enclosed in an achene, appearing somewhat like a small dandelion seed with white bristles at one end.

Marestail possesses a strong, pungent smell and may cause skin irritation in humans and livestock. Marestail is known to be allelopathic, inhibiting the germination and growth of some plant species.

Management

Marestail is most problematic in reduced or no-tillage fields. Marestail populations in Kansas have developed confirmed resistance to glyphosate and ALS-inhibiting herbicides. In addition, application timing is key. Marestail plants that are approximately 4 inches tall are better controlled by herbicides than either rosettes or 8-inch plants. Some herbicides that effectively control marestail are listed in Table 2. Be sure to consult herbicide labels for use rates appropriate for your crop and application timing.

Marestail herbicides

For more information on controlling marestail, please consult the 2021 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland, K-State publication SRP-1162. Information provided by Sarah Lancaster, K-State Extension Weed Control Specialist.

Stacy Campbell is an Agriculture and Natural Resources agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact him by e-mail at scampbel@ksu.edu or calling 785-628-9430.