Canada thistle is a problematic perennial weed that reproduces mainly by creeping horizontal roots. It is shocking how aggressively these roots spread. It has been reported that Canada thistle roots can spread more than twenty feet in a single year and produce a total network of underground root growth measuring 1600 ft. When one thinks of the amount of water and nutrients that this type of root mass is capable of soaking up it is no wonder that our crops are unable to out compete it. It is estimated that Canada thistle costs prairie farmers in excess of $320 million each year.
The best strategy for Canada thistle control in crop may involve taking advantage of the biology of the weed itself. In the springtime, Canada thistle plants are focusing on seed production and are sending nutrients upward in the plant for vegetative growth. This means that herbicides will move with the nutrients to the top of the plant, thus providing good top growth control, but will have limited effectiveness with respect to root kill. In the fall however, the plants at bud stages are shifting from reproductive stages and are starting to prepare for winter by providing food down into the roots. By applying systemic herbicides at this time you trick the plant into moving the herbicide down into the root along with the food and actually kill the plant.
Research has shown that the most effective method of controlling Canada thistle involves an in-crop application of a systemic herbicide followed by pre-harvest glyphosate application. Clopyralid products are very effective systemic herbicide options as they have a greater ability to move into the root system and provide control during the growing season. Application of both in-crop clopyralid and pre-harvest glyphosate provides significantly greater Canada thistle control than either single product. However there are higher costs when considering application of both products.
If in crop control is not an option then fall applications of glyphosate can work quite well to keep Canada thistle under control. There are considerations for both pre-harvest and post-harvest applications to be successful.
Pre-harvest glyphosate should be applied when the crop is at a seed moisture level of thirty percent or less. This is about the hard dough stage in wheat, barley and oats, and when most seeds have turned in canola. It is also important to remember that crops treated with pre-harvest glyphosate must not be saved for seed as traces of glyphosate can significantly reduce seedling emergence and vigor.
Post-harvest glyphosate applications targeting Canada thistle are also popular; however in northern regions these are less effective than pre-harvest applications. Harvest activities remove most of the thistles' biomass and a waiting period of as long as six weeks is required for the plants to regrow prior to spraying. Once the glyphosate is applied it then takes about two weeks of weather free of heavy frosts for the herbicide to effectively move to the root. These conditions are often not met in the northern areas of the province.
For those with fallow or unseeded acres this year, a silver lining to this situation may be the opportunity to control difficult to manage weeds, including Canada thistle. When managing Canada thistle in chemical fallow it is ideal to allow the weeds to grow until they reach the bud stage. The bud stage is a weak point in the plants' lifecycle and application of a systemic herbicide, such as a glyphosate product, at this time can provide good control. Allowing the thistles to grow this long also has the added benefit of having them soak up some of the excess water in the field.
The use of tillage alone in fallow fields is not an ideal strategy for Canada thistle control as well over half of the thistles' roots are below the depth of tillage. Tillage can also break up the roots and these fragments are then moved throughout the field and can establish new plants. Wet conditions like the past couple years are ideal for establishment of the root fragments, and this could turn a small patch of Canada thistle into a much wider spread problem.
For more information on this, or other crops related topics, contact Sherrilyn Phelps at 306-446-7475 or the Agriculture Knowledge Centre at 1-866-457-2377.
- Phelps is Regional Crops Specialist, North Battleford Regional Services Branch
