Cirsium arvense (L.) Scop.
Also known as creeping thistle
Note: This information is based on experience in the Mid-Atlantic states; recommendations in other regions may vary.
Canada thistle is an upright perennial that is 2 to 5 feet tall at maturity. Plants are often found in patches due to an extensive creeping root system that extends up to 3.5 feet into the soil. Shoots begin to emerge in early-May and plants start bolting 3 to 4 weeks later. Flowering occurs from mid-June to early-July. Although the majority of seeds germinate the following year, buried seed may remain viable for up to 20 years. Plants will also regenerate from root fragments as small as 1 inch. The lack of spines on flowers and stems helps distinguish this species from bull and musk thistles.
Canada thistle cotyledons are dull green, thick, and club shaped. Young leaves are covered with short hairs and leaf margins are wavy with spines. Stems are grooved, branching at the top, and become pubescent at maturity.
A single plant produces an average of 1,500 wind-dispersed seeds.
No herbicide resistance has been reported for Canada thistle in the United States; however, resistance to 2,4-D and MCPA has been reported in Europe.
Canada thistle is a major problem in pastures and rangeland, but it is also a problem in row crops. The key to its control is to exhaust the root system’s energy reserves. Successive mowing every 3 weeks can help deplete root reserves, and mowing should occur before seed set. Sheep, cattle, and goats will graze on young, tender plants, but they tend to avoid dense patches of older plants.
Canada thistle shoots require high light levels to survive. Cover crops such as cereal rye and hairy vetch can provide a competitive edge over emerging plants in the spring. Planting crops in narrow rows can also exclude the amount of light reaching the soil surface. Canada thistle completes its life cycle in crops such as corn and soybean, thus rotating with winter wheat can aid in control.
Glyphosate may be applied as a burndown treatment to control emerged weeds in row crops. There are no preemergence herbicide programs that will provide control of plants originating from rootstocks. Complete control of established populations of Canada thistle often requires multiple applications within a single season for several seasons. Fall applications of glyphosate (or other systemic herbicide) are not effective for controlling Canada thistle since only a few shoots will be present at that time of the year. Preemergence control of plant’s developing from seeds can be achieved with clomazone, metribuzin, atrazine, and cloransulam.
Stinger, Callisto, and glyphosate-containing products will provide effective postemergence control. Multiple or split applications may be required to control under-ground root systems.
Suppression of Canada thistle may be achieved with Basagran, Basagran plus atrazine, dicamba, Peak plus dicamba, and Yukon. Spot treatments of 2,4-D, dicamba, glyphosate, or Stinger are an option for small, localized patches of Canada thistle. Always follow label instructions when using dicamba around sensitive crop species.
Glyphosate-containing products are effective postemergence options, but multiple applications may be necessary for complete control. Engenia or XtendiMax in Xtend soybean or Enlist herbicides in Enlist soybeans are effective options to control emerged plants. Basagran can help to remove aboveground vegetation, but a second application 7 to 10 days later may be necessary to kill the remainder of the top growth. Always use a crop oil with Basagran on soybean.
Earlier plantings can help establish a dense crop canopy to suppress growth. Spring applications of Harmony Extra plus surfactant can be effective in the spring when plants are 4 to 8 inches tall with 4 to 6 inches of new growth. Control can be improved with the addition of 2,4-D ester. 2,4-D application in small grains should be made between the tillering and jointing stages. Glyphosate may also be used as a preharvest treatment once small grains have reached the hard dough stage. A 7-day preharvest interval should be observed when using glyphosate to allow for maximum translocation in the plant.
The standard approach of 2,4-D and dicamba has been shown to be ineffective against established stands. Milestone will provide control of thistles and legumes while not harming established grasses. These products should not be used where desirable trees are present. Spot treatments of glyphosate-containing products will also provide control. Since Canada thistle is not tolerant of shade, proper maintenance of forage species will provide suppression.
Fallow or Spot Treatment
Spot treatments may be made using 2,4-D, Crossbow, glyphosate, metsulfuron, or Stinger. 2,4-D, Crossbow, and Stinger can volatize and drift, causing harm to sensitive crops. Always consult label instructions for proper application procedures.
This content was presented in the “Mid-Atlantic Field Crop Weed Management Guide” (2020), Penn State Ag Communications, AGRS-136