Canada Thistle

Photo: Canada ThistleIdentification

Small purple to white flowers in clusters. Extensive deep seated root system spreading horizontally. Large seed production but low percentage of viable seeds.


A lack of control will result in a reduction in crop production of up to 25% in heavily infested ground. New infestation can be spread from seeds, but are more often caused by redistribution of roots by tillage practices.

Chemical Control

Varies depending on location of infestation, stage of growth, and time of year. Several herbicides are available. Contact your local weed control supervisor, or chemical consultant.

Biological Control

Three bio-agents are presently established in Oregon. A stem gall fly has been established in Umatilla County, additional releases will be made in the future. Check with local weed control program for availability.

Plant Characteristics

Canada thistle is one of our most common and also one of our most pernicious weeds. It is a deep-rooted perennial broadleaf that can expand rather rapidly; one study reported lateral extension of 20 feet in one season. A 60-foot diameter circle was created in 3 seasons--from one plant.

It, of course, is highly competitive with our crops, and it interferes with harvest. Imagine what fun it is to pick strawberries in a C. thistle patch! It can harbor certain insects (bean aphid, stalk borer) and is an alternate host for some pathogens. Grazing animals may be scratched and develop infections. Small buds may be harvested along with the crop and be difficult to separate, as in canning peas.

It can spread by seeds, mostly for short distances. A high percentage of the floating plumes have no seed attached. One study counted only 0.2% of the plumes had seed 0.6 mile from the source, but, of course, it would not take very many seeds to start a new infestation. Fresh seeds germinate well, and germination is high even after 6 months submerged in water. One study with buried seed indicated that a few seeds were viable after 21 years in the soil.

Seedlings and new sprouts appear above the soil when the soil and air temperatures warm, probably when the mean weekly temperatures reach the low 40s F. A study in Iowa involved the root system in soil frozen to a depth of 20 inches from mid-December to mid-March. The aerial and subterranean shoots were killed back to the roots when the soil froze. During January new underground roots and shoots began to develop on the old roots and 2- to 3-inch shoots were present in early February. In early March, the new roots and shoots were larger and after the soil thawed in mid-March, development proceeded rapidly and leaves appeared on the surface of the soil in mid-April. Seedlings can reproduce vegetatively when they are 7 to 9 weeks old.

C. thistle seedlings first develop a tap-fibrous root, and soon the main root thickens and produces lateral roots , which spread horizontally. After growing a few inches, the horizontal roots bend downward, growing toward the water table. At the point of bending, a new horizontal root develops and continues the horizontal spread. Aerial shoots develop from the original vertical roots or from adventitious buds on the horizontal system. As with field bindweed and a great many broadleaf perennials, the plant creeps horizontally by roots, not rhizomes.

C. thistle is the only Cirsium I know that is functionally dioecious; i.e., the male and female flowers are borne on separate plants in most cases. Technically, all flowers are perfect, but the pistils abort in some flowers and the stamens in others. Occasionally, a male flower will produce a seed, so the separation is not absolute.


So, how do we control it? If we were any good at it, the weed would have disappeared long ago. Still, there are some approaches that can help. Work at OSU back in the late 1930s showed that tillage every 3 weeks could kill out established C. thistle in one season. Other states reported that C. thistle was not as serious a weed as field bindweed or white top because they could control it easier through tillage.

Chemically, several herbicides work to one extent or another. Tordon is very effective where it can be used. One of the most effective compounds is clopyralid (Stinger, Curtail, Transline). It is registered in many cropping situations--the Curtail (containing 2,4-D) in grass crops and range, the Stinger (clopyralid only) in mint and sugarbeets. It is selective in strawberries, but registration has not yet come through.

Virgil Freed opined back in the 1940s and 50s that MCPA is better on C. thistle than is 2,4-D, and my observations would agree with that. Dicamba, Roundup, Glean/Ally, amitrole, and Basagran can all help hold C. thistle back where their use is appropriate. Remember, we are trying to get the herbicide into the roots, so we need to time the application when sugars are moving downward. This generally means at bud to early bloom, and shortly before frost in the fall. The only exception is with clopyralid. Work at OSU agrees with the label that it should be applied as soon as all shoots are above ground. The only reason I can think of as to why that should be true is that waiting until bloom is always a compromise. Sugars are moving downward best at that point, but the cuticle on the leaves continues to thicken. Maybe clopyralid is influenced more by the cuticle than by the sugar movement, I don't really know.

Timing is Critical

The other best time to treat is in the fall. Some work in Canada with Banvel dicamba indicated better control when treatment was made to C. thistle regrowth in the fall than in the summer at the early bloom stage. The use of clopyralid for thistle control in mint uses both timings. The best approach is to treat with Stinger in late September or so with 2/3 pint (0.25 lb ae/A) and follow that in the spring with 1/3 pint/A when any surviving thistles have emerged.One fact should be noted. C. thistle is quite a variable species, so plants remaining after a herbicide treatment may have survived because they are different genetically. There are a considerable number of observations indicating that a resistant population of thistle has been selected out after a few repeated treatments of a particular herbicide. The most dramatic demonstration of this genetic variability occurred a few years ago when someone found that a fungicide, Dyrene, I think it was, could kill C. this-tle. We tried it but it didn't work, but several growers were pleased--the first year. They got at least 80% kill in some cases. But, of course, the remaining 20% were resistant and results the second year were essentially nil. I mentioned this to Jesse Hodgson at Montana State. He had a collection of several types of C. thistle. He later told me that he had sprayed one type with a high rate of the fungicide. It did absolutely nothing, but a tiny amount of drift turned the adjoining type to dust.

The moral of the story, as usual, is to avoid depending entirely on herbicides, and especially on one type of herbicide. Attack with as many different approaches as you can.

Source: Arnold Appleby, OSU Crop & Soil Science

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