Written by Jason Andersen, Farm Bill Wildlife Biologist, Pheasants Forever
In my experience working with farmers, nothing gets their hackles up quite like Canada thistle. This aggressive plant is difficult to control and is often a source of frustration. However, with proper management, Canada thistle can be controlled in natural areas.
Canada thistle (Cirsium arvense) is a noxious weed that plagues the Iowa landscape. It thrives in fallow areas, ditches, fencerows, and grass plantings. Canada thistle emerges in March through May and typically grows two to three feet tall with sharp spines on its light-green foliage. It can be easily recognized by its bottle-shaped purple flowers, blooming in late-June through July. Canada thistle is a perennial plant (meaning that it comes back from its roots each year) which can spread by seed and underground stem growth (rhizomes). These qualities allow it to aggressively outcompete other plants and form dense colonies when unmanaged.
The good news is that Canada thistle can be controlled with a two-pronged approach to management -- a spring disturbance and a fall herbicide treatment. This approach revolves around the fall treatment, which is vital. Canada thistle goes dormant with the hard frost of fall, typically occurring in early-October. We can use this to our advantage. Spot apply herbicide to the plant body around the first hard frost, or in the few weeks before. This timing allows for the herbicide to be absorbed into the plant and taken down into the root system when the plant goes dormant, maximizing mortality.
We can further increase our fall control by first weakening the plant through disturbance in the spring or early summer. Spot apply herbicide prior to blooming or mow during flower bud formation or within the first seven days of blooming. This will top-kill the plant and rob it of the energy it used to create the flower, weakening the root system, while also preventing the plant from setting seed. A Canada thistle weakened in this way will be more susceptible to herbicide treatment in the fall.
Herbicide selection for Canada thistle control will strongly depend on the adjacent plant community and the risk of overspray or drift. It is important to avoid killing desirable plants, which compete with Canada thistle and assist with control, while also adding to stand health and benefitting wildlife. The best advice is to be as targeted as you can and spot treat whenever possible. In native stands with wildflowers, products with selective active ingredients such as aminopyralid are often used. In stands with heavy grass, such as pastures, use of non-selective broadleaf herbicides such as 2,4-D and clopyralid is common. The application of herbicides should only be performed in accordance with the product label, by personnel trained in the handling and use of chemicals.
Repeat monitoring and treatment will be needed to control Canada thistle. Treatment of dense patches may take several years to get under control. It’s important to evaluate the effectiveness of your treatments and adjust as necessary. The two-pronged approach, spring disturbance and fall herbicide treatment, is a tested and effective approach. Including it into your land management regimen will help you get on top of the Canada thistle problem in your natural areas.