LEFT: Mature marestail grows in a field. RIGHT: Young marestail plants emerging from soil. A dime is placed near the plant to demonstrate its scale.

Marestail (also known as horseweed) is a native plant to the United States and is considered either a winter annual or biennial species. In the Dakotas, the marestail population will germinate in the fall and bolt in the spring.

The most successful treatments for control of marestail is in the fall, but if the species exists in your field in the spring then options are available if field conditions allow application, Gared Shaffer, South Dakota State University Extension weeds specialist, said in a news release.

The first leaves of marestail have a broad, round end and a whorled leaf arrangement that forms a rosette that is often difficult to identify. Small plants may be purple or green during cool weather.

Marestail bolts in the spring, and leaves are alternate, hairy, 1 to 4 inches long, linear in shape and attached directly to the stem.

Not letting marestail produce seed is of upmost importance because they can produce up to 200,000 seeds per plant. According to research, 20 to 91% of those seeds that germinate in the fall can survive through the winter.

A cost-effective fall burndown after harvest before a hard freeze could include a dicamba product, glyphosate, 2,4D, Atrazine, Salflufenacil, Flumioxazin or a mixture of these depending upon what cash crop will follow in the spring.

Atrazine, 2,4D, Salflufenacil, Flumioxazin and glyphosate work satisfactorily under cool temperatures. Dicamba products do not work well under cool temperatures. Make dicamba applications when high temperatures are at least in the mid-50s.

Controlling marestail in the spring burndown, pre-emergent or post-emergent applications in cash crops can be challenging for producers. Often these plants escape the burndown or pre-emergent applications and are not noticed until bolting.

South Dakota State University has not reported marestail glyphosate resistance, but North Dakota State University has, so it is a possibility to be considered. Resistance can be slowed by rotating crops and herbicide programs.

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