Field Scouting Guide: Horseweed


Horseweed is almost completely resistant to glyphosate, making this weed tricky to control.

This month’s guide concentrates on Conyza canadensis (horseweed, sometimes called marestail). Each month, we bring you
a different crop protection issue, ranging from weeds and diseases to insects and wildlife.

We’ve reached out to pathologists to learn how to spot and treat this weed. This month, our contributors are Douglas Doohan, The Ohio State University; Mark VanGessel, University of Delaware; and Bernard H. Zandstra, Michigan State University.



  • Scientific name: Conyza canadensis
  • Common name: Horseweed is the most accepted name, but it has also been called marestail, Canada fleabane.
  • Geographical range: Widespread across North America
  • Crops affected: All no- and low-till annual and perennial crops.


VanGessel: This is a problem in Delaware, and it is found throughout the state. The economic impact of horseweed is that it requires additional herbicides, and growers must resort to tillage. As horseweed begins to bolt, it becomes more difficult to control.

Zandstra: Horseweed is a problem in Michigan. The economic impact is competition and interference with human activity. Horseweed is a problem at all stages of growth and during all seasons.

Doohan: The big issue with horseweed is in long-term, plastic-mulch-covered fields. At least in Ohio, many growers keep plastic down for two and sometimes three years. Basically, you have a no-till (or at least reduced-till) field, the ideal environment for the weed to grow.


Doohan: Because horseweed is almost completely glyphosate resistant, Roundup is no longer an option for burning it off, and that is critical. This is because planting schedules dictate that some fields may go unplanted for many weeks, even months during the growing season. You should control the weeds at this time. Paraquat is only marginally effective on horseweed. Glufosinate is the best option for controlling glyphosate-resistant horseweed, but it cannot be used prior to planting most vegetables with a sufficiently short pre-plant interval. The IR-4 Project has been working with the registrant (Bayer) to shorten these intervals so that it can be a useful tool for growers.

VanGessel: It depends on the crop, but fall treatments will control early emerging populations. Early spring also is an important time for control. Traditional chemistries like 2,4-D, dicamba, Sharpen (BASF) metribuzin, or atrazine in combination with paraquat, Rely (BASF), or Liberty (BASF) can be used to treat the issue. On the organic side, tillage or row cultivation and cover crops are the best options.

Zandstra: For preemergence, atrazine, oxyfluorfen, saflufenacil, and terbacil are good picks. For post emergence, growers should use clopyralid, 2,4-D, dicamba, mesotrione, oxyfluorfen, topramezone, and fluroxypyr. Keep in mind that the weed has multiple herbicide resistance, as well as annual, biennial, and winter-annual growth habits.


Courtesy of Virginia Tech


In the rosette stage of growth, horseweed might resemble other weeds that have this rosette habit, such as shepherd’s-purse or Virginia pepperweed (Lepidium virginicum).

You can find horseweed throughout the U.S. in agronomic crops, pastures, orchards, fallow fields, waste areas, and roadsides.

  • Seedling: Seedlings develop a basal rosette, and cotyledons are oval, 2 to 3 mm long. Young leaves are egg-shaped with toothed margins that become hairy.
  • Stems: Erect, solid, hairy, reaching 6.5 feet in height.
  • Leaves: Mature plants have leaves that are entirely without petioles (sessile). Leaves are 4 inches long, 10 mm wide, alternate, linear, entire or more often toothed, crowded along the stem, and hairy. They become progressively smaller up the stem.
  • Roots: A short taproot with a secondary fibrous root system.
  • Flowers: Many small inconspicuous flower heads occur at the top of the central stem. Individual flowers are 5 mm in diameter with white or slightly pink ray flowers and yellow disk flowers.
  • Fruit: A 1-mm achene, tapered from the base to the apex, with many small, white bristles that allow wind dispersal.
  • Identifying characteristics: Erect plants with mature leaves are entirely without petioles. When mature, this weed is easily identifiable. However, in the rosette stage of growth, horseweed might resemble other weeds that have this rosette habit, such as shepherd’s-purse or Virginia pepperweed (Lepidium virginicum).