Invasive Watch featuring Canada thistle (Cirsium Arvense)
What to look for
Canada thistle is an erect perennial plant that grows from 1 to 4 feet tall. It has tremendous leaf variability, varying from light to dark green, oblong or lanceshaped, deeply cut, often with spiny-toothed margin, and slightly hairy below. Flower heads typically appear from June to October and are small, bristly clusters that range from light lavender to deep rose purple or sometimes white.
Canada thistle has extensive, fleshy, creeping roots that rapidly spread to form dense colonies. Each plant can spread up to 12 feet through the soil by root growth in a single season. Vegetative shoots arise from adventitious buds located on the plant’s roots. Plants are both male and female and spread by seed, with each shoot capable of producing more than 1,000 seeds.
Where it is found
Canada thistle, despite its name, is native to Europe and is generally thought to have been introduced into the United States in the 1600s through farm seed shipments. It is listed as a noxious weed in at least 43 states, with the only exceptions being the most southern states, such as Florida and Georgia.
This noxious weed invades a variety of dry to moist habitats such as cultivated fields, roadsides and other noncrop areas, wet meadows, and rangelands and pastures. It can often be found along stream banks or other waterways.
How to treat it
There are several effective herbicide treatments to control Canada thistle. The most effective is a foliar broadcast or high-volume hand spray application of Milestone® specialty herbicide at 5 to 7 fluid ounces per acre. Other highly effective foliar or high-volume treatments are Capstone® specialty herbicide at 6 to 9 pints per acre or Opensight® specialty herbicide at 2.5 to 3.3 ounces per acre.
If you’re treating Canada thistle under trees, use 0.67 to 1.33 pints per acre of Transline® specialty herbicide or selective control.
If you’re treating minor thistle infestations or are performing follow-up spot treatments using a backpack sprayer, the recommended spray mixture is Milestone at 0.5 to 1 teaspoon per gallon of spray solution. When working with such low use rates, proper calibration of spray equipment is required to determine the exact amount to add to the backpack. Transline at 0.25 to 0.33 fluid ounce per gallon of spray solution can also be used.
For all treatments, a nonionic surfactant containing 80 percent or greater active ingredient, at 1/4 to 1/2 percent v/v, is recommended to improve wetting and penetration.
When to treat it
Canada thistle may be treated in spring or fall. For spring applications, apply herbicide after all plants have fully emerged (some may be budding) until the oldest plants are in full flower stage. Good control is obtained up to the full flower stage but decreases at the fuzz stage (when many blooms are setting seed).
In the fall, Canada thistle plants translocate sugars to the roots, and this is an excellent time for herbicides to be translocated down to kill the roots. Fall treatments can be made even when plants are starting to senesce and after a light frost if there is some green foliage. Use higher rates for older, denser stands.
Control of Canada thistle requires follow-up treatment of all new plants. Soil disturbances will enhance new plant growth, while the development of a thick cover of desirable plants, especially grasses, will reduce re-invasion and seed germination. The treated sites must be checked each year and new plants treated.
Benefits of control
When Canada thistle is allowed to form dense monocultures, it can negatively impact desirable grasses and forbs, which, in turn, reduces beneficial forage for wildlife and nesting areas for birds. However, successful control can result in increased grass and improved habitat for wildlife.
The graph below summarizes the percentage canopy cover of several grass species present at a research location at North Dakota State University. The baseline data shows a very similar composition before treatment for treated and untreated plots. The second set of data shows the grass response 10 months after treatment (10 MAT). The grass response after an application of Milestone to control Canada thistle was outstanding, averaging a 46 percent increase compared with treated plots.
In addition, Milestone, with its low toxicity to birds, fish, mammals and aquatic vertebrae, is an excellent herbicide option in natural areas where wildlife is present and can be applied to seasonably dry wetlands and up to the water’s edge.