Field Facts: Marestail

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Small marestail

While most yield-robbing weeds stop growing during the fall months, this is when marestail is just getting started. In fact, it’s estimated that 75% of all marestail seedlings germinate in the fall.1 Plants that emerge in the fall can survive the winter and continue growing as the row crop emerges the following spring.

  • Common names: Marestail, horseweed, Canada fleabane
  • Scientific name: Conyza canadensis 2 
  • Cotyledons: Small, round
  • Leaf: Lance-shaped  
  • Stalk: Central, flowering with hairs
  • Flowers: Clustered, small and white 
  • Reproduction: Seeds

Fast facts

  • If left to flourish, marestail can rob soybean yield up to 80%.1 
  • Marestail can grow up to 6 feet tall, and each plant can produce 200,000 seeds. Seeds are spread primarily by wind, traveling to neighboring farms and even to nearby states.3 
  • Marestail begins its growth cycle as a small rosette between 1 to 4 inches across. While scouting, look beneath debris because the plants may otherwise remain hidden.
  • Small weeds are easier to control, so controlling marestail at the rosette stage will be more effective than waiting for the bolting stage. Bolting is when the central, flowering stalk forms.
  • Many states in the Midwest are fighting ever-growing populations of marestail that are resistant to glyphosate and ALS-inhibiting herbicides.
  • According to the International Herbicide-resistant Weed Database, the first confirmation of glyphosate-resistant marestail was in Delaware in 2000. Since then, resistance has spread to crops in 25 states.

Control tips

  • A herbicide program approach that includes burndown applications, multiple passes, several modes of action and residual activity is good for controlling marestail in both corn and soybeans. 
  • Planting corn and/or small grains in rotation can help control marestail and complement your customers’ herbicide programs.
  • Fall-planted cover crops, such as cereal rye, also have been used to suppress marestail.

1Jha, P., and R. Yadav. 2021. 2020 Summary of Herbicide Evaluations for Marestail (Horseweed) Control in Soybean. https://crops.extension.iastate.edu/cropnews/2021/02/2020-summary-herbicide-evaluations-marestail-horseweed-control-soybean

2Werle, R., C. Proctor, and J. Miller. 2016.Fall is Optimal for Marestail Management. https://cropwatch.unl.edu/2016/fall-optimal-marestail-management


The transgenic soybean event in Enlist E3® soybeans is jointly developed and owned by Corteva Agriscience LLC and M.S. Technologies L.L.C. Enlist Duo® and Enlist One® herbicides are not registered for sale or use in all states or counties. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your area. Enlist Duo and Enlist One herbicides are the only 2,4-D products authorized for use with Enlist® crops. Consult Enlist herbicide labels for weed species controlled. Elevore®, EverpreX®, Resicore®, Sonic® and SureStart® II are not registered for sale or use in all states. Resicore and SureStart II are not available for sale, distribution or use in Nassau and Suffolk counties in the state of New York. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.