Canada Thistle: Bane of Gardeners
Posted: May 9, 2015
Usually I find the work satisfying because it results in a healthy, more beautiful garden. However some weeds are pernicious and thwart normal means of control. Then the task is very frustrating. Canada thistle (Cirsium arvense) is one of the worst. You probably have this pervasive weed in your yard, and if you don’t, you likely soon will. It thrives in sunny disturbed ground, such as cropland and gardens, and is exceptionally aggressive.
Canada thistle is a cool-season perennial which spreads by both seeds and creeping roots. (Most thistle varieties are annuals and therefore easier to control.) It emerges from the ground as a rosette and then grows to a height of 1-3 feet, with hairy, branching stems, and spiny, lobed leaves. It produces lavender flowers about 1 inch in size. Eventually the flowers form white feathery pods containing about 50 seeds which are easily transported by wind, water, or animals. Seeds can survive in soil for as long as 20 years. Turning or tilling the soil often brings weed seeds such as Canada thistle to the surface, where they germinate when reached by sunlight.
Seeds allow the plant to spread to distant areas, but reproduction from its root system is responsible for infestations. One plant can propagate an area 6 feet in diameter in a year or two. Canada thistle produces roots that extend horizontally up to 17 feet and vertically to a depth of 20 feet. The plant emerges from this root system in the spring and grows very quickly. It tolerates a variety of soils, but is especially aggressive in deep, cool, well-cultivated ground. It is less common in dry soils and shady areas. If undisturbed, plants tend to become less active during the heat of July and August, but new growth from the roots begins again as the weather cools in late summer and fall.
Trying to eliminate Canada thistle by hand-pulling or tilling stimulates the extensive root system to develop new shoots, making control extremely difficult. It is best controlled by applying an herbicide, such as glyphosate (e.g. Round Up), which is transported from a leaf application into the root system. Repeat treatments are usually necessary and are best applied in early June as the flowers are forming and again in September or October when re-growth occurs. The weed will die only after top growth is killed and can no longer provide nutrients to the roots, and the roots have used up their extensive store of nutrients. Control may require a few years of treatment.
For additional information about Canada thistle, see the following websites:
- Luzerne County Master Gardener