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cause of its substantial root nutrient stores. Breaking up the roots by plowing or disking only <br />increases the number of plants (-15 days following such disturbance), since new plants can es- <br />tablish from roots as small as % X '/8" as long as 100 days after cutting. <br />The best management strategy is to integrate chemical and cultural or mechanical control, with <br />an objective of stressing the plant and forcing it to use stored root nutrients. Therefore, returning <br />infested land to a productive state occurs only over time. Success requires management efforts <br />implemented over several years. <br />4.1 MECHANICAL CONTROL <br />Occasional hand-pulling is not an effective strategy for Canada thistle control. Repeated re- <br />moval of all Canada thistle shoots will provide control if repeated as soon as new shoots ap- <br />pear. <br />Although new plants can form from tiny shoot segments, repeated - and deep - cultivation will <br />prevent carbohydrate storage in the root system and, if repeated frequently enough, stress and <br />eventually kill the plant. <br />Mowing in combination with chemical and cultural controls may be effective, particularly in habi- <br />tats where root growth may be restricted such as in an area with a high water table. Mowing <br />should begin when Canada thistle is 12-15 inches tall and should be followed at approximately <br />one-month intervals. Mowing followed by fall applications of herbicides (see Table 2) has been <br />shown to be a successful control program for two reasons: (1) after mowing and some re- <br />growth, proportionally more weed foliage is present to intercept herbicide, and (2) the plant <br />draws on root nutrient stores after mowing to develop new shoots, so these stores are some- <br />what depleted. <br />4.2 CULTURAL CONTROL <br />Cultural control generally is insufficient to adequately control Canada thistle, despite some re- <br />search indicating that crop or perennial grass competition alone can suppress the weed. Roto- <br />tilling before planting desirable species will improve perennial grass establishment; although, as <br />noted above, disking and rototilling alone will actually increase the thistle's density. Grasses <br />and alfalfa are good competitors, especially if moisture and nutrients are optimized, although <br />9 <br />