This article is a summary of Nebraska Extension Resources Extension Circular EC 171 Noxious Weeds of Nebraska: Canada Thistle, and the Extension Circular EC 130, 2021 Guide for Weed, Disease, and Insect Management in Nebraska. For photos of this plant see https://go.unl.edu/canadathistle.
Canada thistle was probably introduced to America around 1750 and the state of Vermont enacted noxious weed legislation against it in 1795. It was proclaimed a noxious weed in Nebraska in 1873. Obviously it has a long and tenacious history in the United States.
Canada thistle is a perennial that reproduces from both seed and buds that can develop on its extensive horizontal roots. These roots can grow as much as 9-18 feet laterally and 6-9 feet deep in a single growing season! If roots are cut by cultivation or tillage, segments as small as 1 inch in length can survive and establish a new plant. Both male and female plants are needed for cross pollination and seed production to occur. Female plants are the most prolific, producing up to 5,000 seeds per plant! These seeds can remain viable in the soil for up to 20 years. These characteristics make the plant very challenging to control.
In the rosette stage, Canada thistle can be difficult to distinguish from other native thistles. The underside of the leaves can be hairy or hairless. As the plant grows, it usually will be 2 to 3 feet tall with alternate, dark green leaves that vary in length. The stems of the plant are erect, branching and grooved with flowering heads that form in clusters. Flowers tend to usually be pink or purple, but may also be white. There are a number of different varieties or subspecies of Canada thistle that can vary by differences in their leaves. When Canada thistle plants are detected in a location, plan to return to those sites in the spring and fall to check to see if new rosette plants are establishing in those areas.
Canada thistle is very challenging to control and success usually occurs through utilizing a variety of methods. The first step is to limit establishment by minimizing the risk of the introduction of seed to open ground and to aggressively control individual plants or patches before spreading occurs.
There are a number of control methods that can be utilized to manage Canada thistle. Competition from the establishment of perennial forage grasses or alfalfa can be an effective cultural control method. The key is to first suppress the plant with herbicide, tillage or mowing and then plant species that will be competitive with Canada thistle and keep it from establishing. Healthy, vigorous stands of grasses and legumes can be competitive with Canada thistle plants that may germinate from seed.
There are a number of biological agents that can damage Canada thistle. These include insects, and plant pathogens. Two non-native insects that feed on Canada thistle have been studied as biological control agents. The first is a thistle stem weevil. This insect emerges as an adult in the spring, feeding on thistle leaves and depositing eggs. Larvae migrate into the stems, where they mine the pith, eventually exiting into the soil, where they pupate. Although the larvae may feed on the roots briefly, the damage is mostly confined to the shoots, so the plants easily recover. The second insect is a thistle stem gall fly that lays eggs on the plant. The presence of larvae causes the plant to form a woody gall on the stem, which directs nutrients away from the growth of the plant. However, like the weevil, the gall fly inflicts little lasting damage and has not reduced Canada thistle infestations substantially when used alone.
Canada thistle rust fungus is a fungus that specifically impacts Canada thistle and can be effective at reducing and in some cases eliminating the thistle in a location over time. This fungus is naturally occurring and is present throughout the United States. The fungus is currently being studied to identify ways to propagate the fungus and introduce it to plants in locations where it currently is not present.
Grazing of Canada thistle with goats, sheep, and cattle can reduce plant vigor when repeated grazing occurs to prevent flowering. Goats are the preferred grazing animal, followed by sheep and cattle. The advantage that goats provide is that they are willing to graze older plants that are in the bud and flower stage of production which can be an advantage when seeking to reduce seed production and deplete root reserves. Sheep and cattle prefer to graze this plant when it is young before spines develop. Grazing is most effective when repeated during the season and for multiple seasons to prevent seed production and to deplete root reserves. Targeted, repeated grazing can be challenging to administer without also impacting desirable, competitive plants. Most information suggests best results are achieved when grazing is combined with herbicide treatments.
Mechanical control through the use of mowing at one month intervals through the summer can help to deplete carbohydrate reserves in the plant. Mowing stimulates new Canada thistle shoots to develop from its root system. This reduces plant carbohydrate stores and can make the plant more susceptible to the use of chemical control, especially in the fall of the year after the first frost. Frequent mowing can also reduce plant vigor of surrounding plants that may compete with Canada thistle.
Herbicides have been used with varying degrees of success with Canada thistle. Because there are numerous strains of Canada thistle, these strains can vary in their susceptibility to herbicides. If an infestation has been treated repeatedly with limited success, consider rotating to a different herbicide.
The two most effective times to apply herbicide to Canada thistle are at the flower bud stage of growth in the spring, which is often in the May-June time period, and in the fall. Fall application of herbicides is generally most effective shortly after frost, when Canada thistle is in the rosette stage of development and is storing nutrients in its roots for the winter. For more information on herbicides, rates and the sites where they can be used, consult the herbicide label or the Nebraska Extension publication, Guide for Weed, Disease, and Insect Management in Nebraska, EC 130.
Success in control of Canada thistle is usually achieved with persistence of the use of multiple control methods. Early detection and rapid response are important to identify Canada thistle before establishing and producing seed and to minimize the opportunity for it to get a foothold in a range or pasture setting.