Life Cycle, Identification and Growth:
Canada thistle (Cirsium arvense) Weed.
In mid- to late-spring, Canada thistle emerges from the soil from either vegetative root buds or germinating seeds. Although Canada thistle seedlings grow slowly and are very susceptible to competition, vegetative buds can be formed on seedling roots seven to eight weeks after germination. Seedlings form a rosette with irregularly lobed spiny leaves. If the rosette is formed early in the season, the plant gradually produces an upright, elongated stalk capable of flowering. Plants that are formed late in the summer or fall do not produce upright stems that year, but remain rosettes until a hard frost causes them to die back to the ground.
Mature plants can develop stems up to five feet tall. A 16-hour day length initiates shoot elongation and flowering. Shoots that emerge in August or later when the photo-period is shorter than 16 hours will remain low-growing rosettes. Research has shown that herbicides applied during shoot elongation will only provide short-term suppression of Canada thistle because most of the toxic ingredient is moved to the shoot apex instead of the roots. The oblong or lanceshape leaves found in these mature plants are four and eight inches long, lack petioles, and are divided into spiny-tipped-irregular lobes. Occasionally, plants with spineless and smooth leaves can be found. Leaves are alternately arranged on the stem and lateral branches grow from buds in the leaf axils.
Distribution Map:
Canada thistle (Cirsium arvense) Distribution in North America shown in green.
Biology and Identification:
Despite its name, Canada thistle is native to Europe, parts of North Africa and Asia, including Afghanistan, Iran, Pakistan and China. This species was introduced to North America from Europe in the 1600′s as a contaminant of grain seed. By 1795, Vermont enacted noxious weed legislation
Close up of Canada thistle (Cirsium arvense) Weed.
against Canada thistle and, in the early 1900′s, the currently named Noxious Weed Act gave a person the right to eradicate this species wherever they found it without fear of trespassing. It was first identified in Montana in 1881 in Musselshell County, and by 2000, it had been identified in every county in the State. Today, Canada thistle is prevalent all over the State of Montana covering 1.5 million acres with the potential for much more.
Damage:
In alfalfa stands grown for seed production, Canada thistle can reduce yield by 48 percent. An extra ten percent yield reduction can occur in alfalfa seed production due to seed cleaning. In pastures, Canada thistle reduces productivity by crowding out forage species with spiny leaves that deter cattle from grazing. In non-cropland ecosystems, Canada thistle can crowd out and replace native grasses and forbs limiting land’s recreational use. In gardens, flower beds, and lawns, Canada thistle’s extensive root system makes it a hassle to control. Mowing or pulling this weed is not effective because it grows again from vegetative buds on the roots. In fact, improper cultivation can even worsen Canada thistle problems!
Management Alternatives:
Because of its extensive root system, complete elimination of Canada thistle usually takes persistent control over several years. Fortunately, there are several cultural, mechanical, biological, and chemical practices that can be combined to exhaust the nutrients stored in the root systems of Canada thistle. These methods should be adapted to the specific conditions of the infested area.
