Weed of the month: Marestail

  • marestailCommon name: marestail, horseweed, Canadian fleabane, coltstail, butterweed
  • Scientific name: Conyza canadensis
  • Grass or broadleaf: Annual broadleaf
  • Native to North America (circa 1640), marestail is now found throughout the world.
  • A single female marestail plant can produce approximately 200,000 seeds that are transported by wind, providing for effective spread of herbicide-resistant populations.
  • Seeds germinate in the fall and overwinter in the rosette stage before emerging in two stages: late March and early June. Up to 90 percent of seeds that germinate in fall can survive through the winter.1
  • Mature plants grow erect, reaching heights from 6 to 10 feet tall. The main stem appears unbranched near maturity except for the flowering stems near the top. The central stem is covered with long white hairs; leaves alternate around the stem, appearing whorled.
  • Flowers bloom from June through September; yet unlike other winter annual broadleaf weeds, marestail does not mature until late summer, then it sets and disperses seed from August to October. Up to 86 percent of seeds produced can germinate right off the plant.2
  • Marestail is the first annual broadleaf weed documented to develop glyphosate resistance (14 states in 2000).3

Marestail facts that may surprise you

  • Native Americans used a tea steeped from marestail leaves to treat dysentery.
  • Marestail is a diuretic and can make humans sweat.
  • Marestail has also been referred to as fleabane because the leaves used to be put in pets’ beds to help to get rid of fleas.4

Fast Facts from Jeff Ellis, Ph.D., field scientist, Dow AgroSciences

To correctly identify marestail when scouting, look for the following:

  • Scout for marestail year-round and learn to identify the weed in the small rosette stage.
  • First leaves have a broad, round end and whorled leaf arrangement that forms a rosette.
  • Marestail bolts in the spring. Leaves are alternate, hairy, 1 to 4 inches long, linear in shape and attached directly to stem.
  • Marestail is often misidentified as whitlowgrass, mouseear chickweed or several of the fleabane species, especially annual fleabane.
  • The larger the rosette is prior to winter, the greater the chance of survival into the spring. Spring marestail generally remains a rosette for a relatively short period prior to bolting.

Marestail control/management tips

Ellis says:

  • Timing is critical for marestail control
    • Farmers can find greater success at controlling marestail with more herbicide options when marestail is in the rosette stage of growth.
    • Controlling marestail after it has bolted is more difficult because there are fewer herbicide options.
  • Use glyphosate, 2,4-D, dicamba, ALS-inhibiting herbicides, and/or residual herbicides in fall and early spring burndown in no-till soybeans.
  • For in-crop soybeans, glyphosate tank-mixes with FirstRate® herbicide provide excellent control of bolted marestail. Pending registration, Elevore herbicide will be a new tool for farmers to use in burndown applications. Elevore will control marestail up to 8 inches tall.
  • Combining herbicides with good efficacy on marestail often provides the best control.
  • Use the full herbicide rates, recommended spray adjuvants and adequate spray volumes to optimize herbicide performance.

General tips to manage herbicide-resistant weeds

Take the following steps to manage herbicide resistance issues:

  • Develop an integrated weed management plan that delivers multiple modes of action throughout the season. With resistance increasing, the Enlist™ weed control system allows use of effective modes of action, including Enlist Duo® herbicide — new 2,4-D choline and glyphosate — and glufosinate in soybeans and Enlist Duo and FOPs in corn.
  • Using diverse herbicides or herbicide mixtures with different modes of action will reduce overreliance on any single herbicide and minimize the likelihood of selection pressure for resistance.
  • Use full rates of the herbicides during applications. Do not use partial rates or trim back for any reason, including cost.
  • Spray when weeds are small. Although it can be challenging because of weather and other factors, this is the ideal application timing.
  • Scout fields regularly to identify weeds when they are small and easy to control.

Dow AgroSciences weed control solutions:

SureStart® II herbicide
Resicore® herbicide
Keystone® NXT herbicide
Keystone® LA NXT herbicide 

FulTime® NXT herbicide
Surpass® NXT herbicide 
Durango® DMA® herbicide
Duramax® herbicide
Enlist Duo® herbicide, as part of the Enlist weed control system

Sonic® herbicide
Surveil® herbicide
Durango® DMA® herbicide
Duramax® herbicide
Enlist Duo® herbicide, as part of the Enlist weed control system

Additional information:

More information can be found through these weed science resources:


1Shaffer, G. 2016. Marestail Control. http://igrow.org/agronomy/soybeans/marestail-control/

2Michigan State University. Controlling Horseweed (marestail). http://fieldcrop.msu.edu/uploads/documents/Controlling%20Horseweed%20(Marestail).pdf

3Glyphosate, Weeds, and Crops Group, Purdue Extension Education. 2006. Biology and Management of Horseweed. https://www.extension.purdue.edu/extmedia/gwc/gwc-9-w.pdf

4Deane, G. Horseweed, Mare’s Tail. http://www.eattheweeds.com/conyza-canadensis-herb-fire-food-2/

®™Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. FulTime NXT, Keystone LA NXT and Keystone NXT are federally Restricted Use Pesticides. Enlist Duo herbicide is 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. Duramax, Durango DMA, FulTime NXT, Keystone LA NXT, Keystone NXT, Resicore, Sonic, SureStart II, Surpass NXT and Surveil are not registered for sale or use in all states. FulTime NXT, Keystone LA NXT, Keystone NXT, Resicore, SureStart II and Surpass NXT 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. Elevore is not yet registered with the U.S. EPA. This presentation is intended to provide technical information only and is not an offer for sale of product. Always read and follow label directions. ©2017 Dow AgroSciences LLC