Revised November, 30, 2006
Introduced perennial from Eurasia. Reproduces by seeds and whitish, creeping rootstocks which send up new shoots every 8 to 12 inches. Plants 2 to 4 feet tall, branched above, with a well-developed, freely branching, fibrous root system. Leaves alternate, simple, oblong or lanceolate, irregularly lobed and spiny toothed, hairy when young, dark green. Flowers white or rose-purple in composite heads grouped at ends of top branches. They are usually dioecious, i.e. male and female flowers on different plants. For viable seed to be produced both male and female plants need to be present. Seeds about 1/8 inch long, smooth, light to dark brown color, oblong, slightly flattened and slightly curved, bear a white hairy pappus (parachute) at the top which helps support the seed in the air.
Prevention of Spread of Canada Thistle
The occurrence of new infestations of Canada thistle can be reduced by planting weed free seed, using livestock feeding materials free of Canada thistle seed and cleaning equipment before leaving infested fields. Close attention should be placed on any feed or seed materials imported from the northern and northwestern states. Quick identification and eradication of Canada thistle plants is essential to prevent its spread.
Canada Thistle Control Practices
Canada thistle control shall mean preventing the production of viable seed and destroying the plant's ability to reproduce by vegetative means.
Cultural Control Practices
First plants to appear should be destroyed by pulling or hoeing before becoming securely rooted. Canada thistles usually appear above ground in early spring. The decline in total food reserves in underground parts proceeds rapidly, then is slower until early summer when the plants bloom and are in their weakest stage. Cultivation begun then is usually most effective. Persistent cultivation, which destroys roots and rootstocks and exhausts food reserves, is effective in eradication. Avoid continuous small grain or row crops.Combination of cultivation, crops and chemicals - One season of intensive cultivation followed by winter wheat or winter rye will eradicate a high percentage of Canada thistle. Bromegrass, established in a thistle infested area, sprayed with 3/4 pound of actual 2,4-D acid per acre over a two-year period is an effective control.
Herbicides Approved for Controlling Canada Thistle
The following herbicides may be used for cost-share with landowners. Other products labeled and registered for use on this noxious weed in Kansas may be used in accordance with label directions but are not available for cost-share. Be sure to follow all label directions and precautions. For additional information consult the current KSU publication of AChemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland@.
2,4-D Low Volatile Ester or Amine). Apply in the spring at early bud stage and in the fall when plants are actively growing. Follow label directions and precautions.
Picloram (Tordon). Restricted use pesticide. Do not apply to cropland. Follow label directions and precautions.
Dicamba (Banvel, Clarity, Vanquish). Apply at early bud stage in the spring and at the rosette stage in the fall. Follow label directions & precautions.
Glyphosate (Roundup). Apply at the pre-bud stage in the spring and when plants are actively growing in the fall. Follow label directions and precautions.
Chlorsulfuron (Telar). Follow label directions and precautions.
Clopyralid (Stinger). Apply from rosette to pre-bud. For most effective control of Canada thistle, apply as a broadcast treatment to the entire infested area. Follow label directions and precautions.
Triclopyr + Clopyralid (Confront). For control of Canada thistle in turf. Follow label directions and precautions.
Clopyralid + 2,4-D (Curtail). Follow label directions and precautions.
Aminopyralid (Milestone). Apply in either spring to plants in prebud growth stage or in the fall to plant regrowth. Follow label directions and precautions.
Glyphosate + Diquat (QuikPro). Apply in prebud growth stage. Follow label directions and precautions.
Diflufenzopyr + Dicamba + Picloram (Overdrive + Tordon 22k). Restricted use pesticide. Do not apply to cropland. Follow label directions and precautions.
Biological Control Practices
There are no biological controls approved for use on Canada thistle at this time.