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3.5 PREVENTION <br />Avoid driving vehicles or equipment through mature patches as seed-heads can become at- <br />tached and spread over long distances. Avoid overgrazing of pastures and rangeland and main- <br />tain litter cover to prevent seed germination and establishment. <br />4.0 CANADA THISTLE (CIRSIUM ARVENSE) <br />Canada thistle (see Figure 2), a perennial thistle native to southeastern Eurasia, is difficult to <br />control due to its extensive root system and the fact that it reproduces by both sexual and asex- <br />ual means (i.e., via both seeds and vegetative spread). It has invaded rapidly across North <br />America largely due to the lack of natural competition from other plant species and through her- <br />bivory, but grows best in the northern regions of North America where temperature and rainfall <br />are moderate. It is considered to be naturalized in the northern Great Plains. <br />Canada thistle is adapted to a wide range of soils and can tolerate salt content as high as 2 per- <br />cent. It produces deeper roots in clay or muck soils than in sand, gravel, or limestone soils and <br />is most competitive in deep, well-aerated, cool, and high-nutrient soils. It usually occurs in 17- <br />to 35-inch annual precipitation zones or where soil moisture is comparable, and is less common <br />in light, dry soils. <br />The roots of Canada thistle may penetrate soil to depths more than 15 feet and extend laterally <br />up to 15 feet every year. Root buds occur randomly along the roots and initiate new shoots <br />whenever environmental conditions, particularly soil moisture, are favorable. This is particularly <br />a problem when an infested site is disturbed by tillage or herbicides, but these impacts can be <br />controlled by land management. The greatest flush of root-derived plants occurs in spring, but <br />another flush occurs in fall. Seeds can remain viable in the ground for up to 20 years, and deep <br />burial encourages seed longevity. <br />Canada thistle has been classified into several varieties. Within these varieties are many eco- <br />types which differ in growth characteristics, response to photoperiod, and susceptibility to herbi- <br />cides and cultivation. Some ecotypes show variable responses to herbicides and/or cultivation. <br />7