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Canada thistle [DRAFT] (Cirsium arvense)
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Canada thistle (Cirsium arvense) is an invasive herb that can aggressively colonize rangelands pastures, agricultural fields and natural areas. It thrives in disturbed habitats and is a pernicious weed in temperate agricultural land across four continents.
Canada thistle, despite its common name, is an introduced species native to Europe and Northern Asia. It was introduced to North America in the early 1600s as a contaminant in agricultural seed and has since spread throughout the United States and Canada.
Canada thistle is a clump-forming perennial with a robust, spreading root system which can extend to ten feet below the soil. Plants expand clonally along radial, lateral roots that can grow up to 4.5 m annually. Plants can regenerate from small root fragments which can survive for over 100 days in the soil.
Seedlings and shoots emerge in mid to late spring. Seedlings grow slowly and form a basal rosette before bolting later in the season. After only 7 to 8 weeks seedlings are able to regenerate from root tissue. Perennial shoots develop as a rosette early in the season and elongate as day length increases in early summer. Mature shoots flower from July to August. Canada thistle is dioecious, with individual plants producing only male or female flowers; female plants can produce up to 5,000 seeds per stem. Seeds are wind dispersed and, though most germinate during the first year, can survive for up to 20 years in the soil.
Seedlings have long thick, oblong cotyledons that reach 1cm in length. Early leaves are covered in short hairs and have wavy margins with sharp spines.
Mature plants have erect, grooved stems that are smooth or sparsely hair and branch towards the apex. Leaves are alternate, waxy, dark green above and light below, and have an irregularly lobed margin with sharp spines. Flowers are pink to purple, surrounded by spineless bracts, and occur at the apex of stems and branches in clusters. Fruits are achenes (2-2.5cm long) containing a single seed and have white to brown-red pappus.
Canada thistle has been declared a noxious weed in 33 states. It is an aggressive competitor for light, nutrients and moisture, and infestations in agricultural fields can reduce crop yield dramatically. The spiny foliage is unpalatable to most livestock, allowing Canada thistle to thrive in and reduce the value of pastures and rangeland. In natural areas like old fields and meadows Canada thistle can displace native grasses and forbs.
There are a number of approved biological control agents for Canada thistle in the United States, including the Canada thistle stem weevil (Hadroplontus litura), the Canda thistle gall fly (Urophora cardui). There is little evidence that either insect has had a substantial impact on Canada thistle (McClay 2002).
Mechanical control of Canada thistle is possible, but to be effective control efforts must often be repeated to exhaust the substantial energy reserves of the root system. Mowing at least twice a year can be effective, particularly if the first mowing is in early June when flowering buds are beginning to form and root reserves are at the lowest (Jacobs 2006).
Broadleaf herbicides such as glyphosate can be used to control Canada thistle. As with mowing, applications should usually be made early in the season before the plants have begun to flower. While cultivation alone generally ineffective —it fragments roots and leaves potential propagules in the soil—it can be useful when combined with late season applications of glyphosate on rosettes (Hunter 1996).
Hunter, J.H. 1996. Control of Canada Thistle (Cirsium arvense) with Glyphosate Applied at the Bud vs Rosette Stage Weed Science 44:934-938
Jacobs J, S. J., and Menalled F. 2006. Ecology and Management of Canada thistle [Cirsium arvense (L.) Scop.]. Montana Invasive Species Technical Notes, No. 5.
McClay, A.S. Canada Thistle pp. 217-228 In: Van Driesche, R., et al., 2002, Biological Control of Invasive Plants in the Eastern United States, USDA Forest Service Publication FHTET-2002-04, 413 p.