Horseweed (marestail)

Photo

Young horseweed seedling with lobed leaves beginning to form a rosette.

Mature horseweed plant in mid-summer. Photo: Penn State University

Mature horseweed plant in mid-summer. Photo: Penn State University

Horseweed (also called marestail) is of rising concern in many states due to its rapidly spreading herbicide resistant populations. It is a winter annual, and its seeds are wind-dispersed. It should be controlled when seedlings are small in order for herbicide applications to be effective, and not allowed to go to seed. Because seedlings emerge in both the fall and the spring, fields should be scouted for horseweed in the fall and treated with a fall burndown and/or a fall cover crop to prevent those seedlings from re-emerging in the spring.

Resistant populations:

  • Group 2 (ALS)
  • Group 9 (glyphosate)
  • Group 2 + Group 9
  • Group 22
  • Group 9 + 22

Locations of resistant populations:

Arkansas, Delaware, Illinois, Iowa, Maryland, Mississippi, Missouri, Nebraska, North Carolina, Pennsylvania, Tennessee, Virginia

Resources:

Biology and Management of Horseweed – Part of the Glyphosate, Weeds, and Crops Series

Herbicide recommendations for horseweed in no-till soy – Iowa State University

Management of Herbicide-Resistant Horseweed in No-till Soy – Take Action

Horseweed Management in No-till Soybeans – Penn State University

Control Horseweed Prior to Soybean Emergence – Penn State University

Late Season Horseweed Control – Penn State University

Controlling Glyphosate-Resistant Horseweed in No-till Corn – Cornell University

Horseweed-Weed of the Year – North Dakota State University