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Canada Thistle Management
by Bob Hartzler

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February 7, 2002 -  Canada thistle is a native of southeast Europe that was introduced accidentally to North America in the late 1700's.  Since that time it has become one of the most troublesome perennial weeds throughout the northern United States.   Canada thistle shares a trait with one of our worst annual weeds, waterhemp.   Both of these species are dioecious, meaning that an individual plant is either male or female.  Since Canada thistle spreads primarily through vegetative rootstocks rather than  seed, infestations in fields and pastures may be made up entirely of male or female plants. 

The persistent nature of Canada thistle is due largely to the extensive root system the plant produces.  In order to eliminate this weed, the entire root system must be drained of energy reserves.  No matter what strategy (herbicides, tillage, mowing) is used, it will take repeated measures to bring established patches under control.   Prior to the development of herbicides, a recommendation for controlling Canada thistle was repeated tillage every 4 to 6 weeks for 2-3 years.  While effective, this was expensive and damaging to the soil.

Canada thistle is capable of colonizing many habitats, but it is probably most difficult to manage in pastures and other similar habitats.  The key to managing Canada thistle in pastures is to use an integrated approach.  Canada thistle is not tolerant of shade, thus the first step is to invigorate competition from the desirable forage species planted in the pasture.  This may involve fertilization, liming, overseeding and adjusting stocking rates.  Overgrazing is a common cause of cause of infestations since the grazing animals avoid eating the spiny thistles.    Simply applying herbicides while ignoring the health of the pasture will generally not provide acceptable results.

A variety of herbicides are available for controlling Canada thistle in pastures.  The standard 2,4-D + Banvel program used to control many pasture weeds generally is ineffective against established stands of Canada thistle.  Herbicide programs containing either picloram (Tordon) or clopyralid (Stinger) have proven most effective against Canada thistle.  While more costly than other treatments, their performance warrants the cost when Canada thistle is the target.   Tables 1 and 2 summarize research conducted in Colorado and Iowa investigating the effectiveness of several herbicide treatments.

In the Colorado experiment the herbicides were applied in the fall and evaluations made one year after final application.  The treatments were applied during two consecutive years.   The authors also investigated the effect of mowing prior to application on herbicide efficacy.  Thistles were mowed 0 to 3 times during the summer prior to application, with the last mowing made 5 to 6 weeks prior to herbicide application in order to allow sufficient time for thistle regrowth.  The mowing treatments did not influence Canada thistle control, so only data of the non-mowed treatments are provided.  Tordon 22K + 2,4-D provided better control than Curtail (a combination product containing clopyralid and 2,4-D).

Table 1.  Effectiveness of several herbicide treatments on Canada thistle. 

Product Rate % Ground Cover Provided
by Canada Thistle
Curtail (clopyralid + 2,4-D) 2.6 pt 54
Curtail (clopyralid + 2,4-D) 5.2 pt 36
Tordon 22K + 2,4-D 0.75 pt + 2 pt 9
Tordon 22K + 2,4-D 1.0 pt + 2 pt 0
No herbicide - 75

Beck, G. and J. R. Sebastion.  2000.  Combining mowing and fall applied herbicides
to control Canada thistle.  Weed Technol. 14:351-356.

Jim Fawcett, Area Extension Field Crop Specialist in eastern Iowa, conducted a trial in Benton county during 1998-99.  Treatments included applications made in the fall of 1998, spring of 1999 and also areas treated in both the fall and spring.   Control of Canada thistle was evaluated one year after the spring applications (Table 2).  Spring treatments generally provided better control than fall applications in this experiment, and there was little benefit to the combined fall and spring treatment.   As in the Colorado experiment, Tordon   provided the most effective Canada thistle control.

Table 2.  Effectiveness of several treatments on Canada thistle in a Benton county, Iowa pasture. 
Jim Fawcett.  2000.

Product Rate % Canada Thistle Control
Fall Spring Fall + Spring
Stinger 0.66 pt 55 65 70
Banvel + 2,4-D 2 pt + 2 pt 43 63 67
Tordon 22K 3 pt 91 98 99
Tordon 22K + 2,4-D 2 pt + 2 pt 78 98 94
Crossbow 6 pt 33 55 53

Canada thistle shoots are relatively easy to kill with herbicides, it's the extensive root system that makes the weed so difficult to bring under control.  The greater control of Canada thistle with Tordon in these two studies may be explained by the persistence and mobility of picloram compared to the other herbicides.   Researchers in Canada  investigated the contribution of herbicide that reaches the soil in controlling Canada thistle (Table 3).  Herbicides were applied so that in some treatments the herbicide contacted both the foliage and soil (typical of what would happen in the field) and in others so that only the foliage was contacted.   Shoots of the treated plants were physically removed after providing time for translocation of the herbicide, and the effect of herbicide and application method on Canada thistle regrowth was determined.

Although the rates of the herbicides applied were lower than typically used when Canada thistle is the target species, some important observations can be made from the studies.  Stinger was more effective than Tordon when the herbicide was only allowed to contact the foliage of the plant (Table 3).   With Tordon, Canada thistle control was improved at all rates when the herbicide was allowed to contact both the foliage and soil, whereas with Stinger foliar applications alone provided complete control at the two high rates.

Table 3.  Regrowth of Canada thistle shoot following application of Stinger and Tordon.  Treated shoots
were removed 3 days after application.  Data is the dry weight of shoot regrowth measured 42 days after
application, expressed as a percent of the regrowth of untreated Canada thistle.

Product Rate
(g ai/ha)
Application Method
Foliage Only Foliage and Soil
Stinger 25 1 66 19*
  50 2 25 10*
  100 4 1 0
  200 8 0 0
Tordon 22K 25 1.5 115 53*
  50 3 106 28*
  100 6 104 1*
  200 9 50 0*

* indicates that application means were significantly different at the 0.05 level of confidence.

Hall, C.J., H. D. Bestman, M.D. Devine and W. H. Vanden Born. 1985.  Contribution of soil spray deposit from
postemergence herbicide applications to control of Canada thistle.  Weed Sci. 33:836-839.

Both picloram (Tordon) and clopyralid (Stinger) are translocated in the phloem of plants, and thus can provide control of the root system of Canada thistle.  However, at any given time of the growing season a significant percentage of the vegetative rootstocks are not directly connected to actively growing shoots.  These disconnected rootstocks are unaffected by foliar applied herbicides.   In addition, translocated herbicides may not be distributed evenly throughout the root system of actively growing plants.  Because of these two factors, herbicides that are active both through the foliage and the soil have a greater likelihood of providing long-term Canada thistle control.  Under field conditions the soil activity of picloram probably contributes more to Canada thistle than that of clopyralid.

While both picloram and clopyralid are persistent in the soil and readily absorbed by roots, picloram is much more mobile in the soil than clopyralid. In field situations, picloram would be more likely to leach to the depth of the Canada thistle rootstocks than clopyralid, and therefore provide additional control of shoots not affected by the foliar absorbed herbicide.  This soil activity is probably why treatments including picloram provided better control than clopyralid in the two studies discussed in this article.  The mobility of picloram is the reason for Tordon products being classified as Restricted Use Pesticides, since picloram can easily move off the target site with runoff water.

In summary, Canada thistle control is best achieved through an integrated program.  Since no single treatment can subdue an established infestation, repeated measures are needed for long-term control.  Treatments containing Tordon have provided more effective long-term control than other herbicides, but users must be aware of restrictions when using this product to avoid off-target movement and injury to neighboring plants.


 Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.