Weed of the Month: Marestail

  • marestailCommon name: Marestail, horseweed, Canada fleabane
  • Scientific name: Conyza canadensis 1
  • Grass or broadleaf: Annual broadleaf
  • Germination timing: Marestail can begin its growing season in fall or spring. Plants that emerge in the fall can survive the winter and continue growing as the row crop emerges.2
  • Marestail begins its growth cycle as a small rosette between one to four inches across. While scouting, it is important to look beneath debris because the plants may otherwise remain hidden.3
  • Many states in the Midwest are fighting the ever-growing population of marestail that is resistant to glyphosate and ALS-inhibiting herbicides.3

Fast facts

  • Marestail can grow up to six feet tall. Once a plant grows up to five inches, control becomes much more difficult.2
  • Each plant can produce 200,000 seeds per plant—which are spread primarily by wind—that can travel to neighboring farms and even nearby states.2
  • According to WeedScience.org, the first confirmation of glyphosate-resistant horseweed was in Delaware in 2000. Since then, resistance has spread across the Midwestern states.
  • Corn and small grains can help control marestail. The canopy of corn can suppress growth, while small grains can be highly competitive. 2

Weed management tips

  • Use a herbicide in the fall: an application of a burndown herbicide in the fall season can aid in marestail control. It is best to apply the herbicide with temperatures above 50 degrees and with low wind.1
  • Scout your fields: By checking the fields before or immediately after harvest, farmers can make plans to control marestail with a fall or spring herbicide.1
  • Plant a cereal rye cover crop: This crop can provide a mulch that will suppress marestail emergence.1
  • Apply a herbicide in the spring: This practice can control weeds that have grown throughout the winter and suppress emergence of any new weeds. Use a burndown herbicide mixed with a residual herbicide for best results.3

Resistance statistics*

  • According to WeedScience.org, herbicide-resistant marestail has been documented in almond, blueberry, corn, cotton, grapes, rice, soybean and wheat production in 25 states: Alabama, Arkansas, California, Delaware, Illinois, Indiana, Iowa, Kansas, Kentucky, Maryland, Michigan, Mississippi, Missouri, Montana, Nebraska, New Jersey, North Carolina, Ohio, Oklahoma, Pennsylvania, South Dakota, Tennessee, Virginia, Wisconsin and West Virginia.

*Resistance confirmation does not include all weeds and may vary among different areas of each state.

Corteva Agriscience, Agriculture Division of DowDuPont, offers the following weed control solutions:

Corn Herbicides

DuPont Cinch® ATZ herbicide
Durango® DMA® herbicide
Elevore® herbicide
Enlist Duo® herbicide, as part of the Enlist weed control system
Enlist One herbicide, as part of the Enlist weed control system
FulTime® NXT herbicide
Keystone® LA NXT herbicide
Keystone® NXT herbicide
DuPont Realm® Q herbicide
Resicore® herbicide
SureStart® II herbicide

Soybean Herbicides
Durango® DMA® herbicide
Elevore herbicide
Enlist Duo® herbicide, as part of the Enlist weed control system
Enlist One herbicide, as part of the Enlist weed control system
DuPont EverpreX® herbicide
FirstRate® herbicide
Sonic® herbicide
Surveil® herbicide
DuPont Trivence® herbicide
DuPontEnlite® herbicide
DuPontAfforia® herbicide
DuPont™ FeXapan™ herbicide Plus VaporGrip® Technology


1Integrated Crop Management News and Iowa State University Extension and Outreach 2017. Fall Marestail (Horseweed) Management. https://crops.extension.iastate.edu/cropnews/2017/10/fall-marestail-horseweed-management
2 Penn State Extension 2017. Marestail (Horseweed) Management. https://extension.psu.edu/marestail-horseweed-management
3Nebraska Institute of Agriculture and Natural Resources 2016. Fall is Optimal for Marestail Management. https://cropwatch.unl.edu/2016/fall-optimal-marestail-management


®™℠Trademarks and service marks of Dow AgroSciences, DuPont or Pioneer, and their affiliated companies or their respective owners. Cinch ATZ, FeXapan Plus VaporGrip Technology and Keystone LA NXT are Restricted Use Pesticides. Durango DMA, Elevore, EverpreX, FulTime NXT, Keystone NXT, Realm, Resicore, Sonic, SureStart II, Surveil and Trivence are not registered for sale or use in all states. FulTime NXT, Keystone LA NXT, Keystone NXT, 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. Enlist Duo and Enlist One herbicides are not registered for sale or use in all states or counties. Enlist Duo and Enlist One herbicides are the only 2,4-D products authorized for use in Enlist crops. DuPont™ FeXapan™ herbicide plus VaporGrip® Technology is a Restricted Use Pesticides. DuPont™ FeXapan™ herbicide plus VaporGrip® Technology is not registered for sale or use in all states. IT IS A VIOLATION OF FEDERAL AND STATE LAW TO MAKE AN IN-CROP APPLICATION OF ANY DICAMBA HERBICIDE PRODUCT ON SOYBEANS WITH Roundup Ready 2 Xtend® technology, OR ANY OTHER PESTICIDE APPLICATION, UNLESS THE PRODUCT LABELING SPECIFICALLY AUTHORIZES THE USE. Contact the U.S. EPA and your state pesticide regulatory agency with any questions about the approval status of dicamba herbicide products for in-crop use with soybeans with Roundup Ready 2 Xtend® technology and follow all pesticide product labeling. Soybeans with Roundup Ready 2 Xtend® technology contain genes that confer tolerance to glyphosate and dicamba. Glyphosate herbicides will kill crops that are not tolerant to glyphosate. Dicamba will kill crops that are not tolerant to dicamba. Roundup Xtend®, VaporGrip®, Roundup Ready 2 Xtend® (Monsanto). VaporGrip® Technology is used under license from Monsanto Technology LLC. Always read and follow label directions. ©2019 Dow AgroSciences LLC