ProCrop

Fall Herbicide Applications for Canada Thistle Control

Canada thistle infestations become very obvious in mid-July and August. Dark green stems that were hiding in the crop or pasture now become all too obvious as the purple and rose colored flowers emerge. Many people want to treat Canada thistle in mid-summer, but that is probably the least effective application timing.

The best approach to Canada thistle control in cropland should include a herbicide treatment in crop to suppress Canada thistle growth, minimize crop yield losses, and prepare the thistle for a fall post-harvest treatment. Fall-applied treatments provide the most effective long-term control. The best herbicide to use will vary depending on crop rotation (See NDSU Extension Bulletin W-799 Perennial and Biennial Thistle Control  http://www.ext.nodak.edu/extpubs/plantsci/weeds/w799w.htm .)

However, the control program must be uninterrupted for two to three years if the infestation is to be reduced. 

Canada thistle growing in pasture and wild lands can be controlled with a variety of herbicides: Clopyralid plus 2,4-D (Curtail), picloram (Tordon), picloram plus 2,4-D amine, dicamba (Banvel), triclopyr plus clopyralid (Redeem), or 2,4-D. Control is greatest when applied in the fall to plants in the rosette growth stage. More herbicide (2 to 3 times) is translocated to the root system of Canada thistle when the treatments are applied to rosettes compared to bolted Canada thistle.

**NOTE** - Research at North Dakota State University has shown that clopyralid alone will provide better Canada thistle control than when clopyralid is formulated with 2,4-D (Curtail) or triclopyr (Redeem). For instance, Redeem at 2.7 pt/A contains 4 oz/A of clopyralid, costs $22/A and provided 50% Canada thistle control 12 months after treatment, while clopyralid applied alone at 4 oz/A (Transline or Stinger) provided 87% control and cost $27 to $40/A depending on formulation.

Tordon may be the most cost-effective herbicide to use for Canada thistle control in pasture, rangeland, and non-crop. Tordon at 6 to 8 oz/A generally provides 90 to 100% Canada thistle control 1 year after treatment and costs $16 to $21/A. However, Tordon cannot be used in wet areas where Canada thistle is often found. Generally, Redeem or Stinger/Transline can be used near water.

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