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Canada Thistle <br />Pag <br />The greatest flush of root-derived plants occurs in spring, but another flush occurs in fall. <br />A flush can occur anytime during the growing season when soil moisture is adequate. <br />This is particularly a problem when Canada thistle growth is disturbed by tillage or <br />herbicides. This feature can be manipulated to the land manager's advantage. <br />Plants that germinate from seed do so at about the same Nme as root-derived shoots. <br />Seedlings grow slowly and are sensitive to competition, particularly if shaded. Canada <br />thistle seedlings develop a perennial habit (the ability to reproduce from their root <br />systems) about seven to eight weeks after germination. <br />Reproduction and spread. Canada thistle begins to flower in late spring to early <br />summer in response to 14- to 16-hour days. Plants are male or female (dioecious) and <br />grow in circular patches that often are one clone and sex. Female flowers produce a <br />sweet odor and insects readily pollinate different sexed patches up to 200 feet apart. <br />Canada thistle develops seed sparingly. It may produce 1,000 to 1,500 seeds per <br />flowering shoot. Generally, vegetative reproduction from its root system contributes to <br />Iocai spread and seed to long distance dispersal. Seed may be transported long distances <br />by water, or attached to animals, clothing, farm equipment and other vehicles, and in <br />contaminated crop seed. Also, wind may help disseminate seed, but most often, the <br />feathery pappus breaks off, leaving the seed attached to the parent plant to be <br />disseminated by other means. Seed can remain viable in soil up to 20 years, and deep <br />burial promotes survival longevity. <br />Canada thistle allocates most of its reproductive energy into vegetative propagation. New <br />shoots and roots can form almost anywhere along the root system of established plants. <br />Tillage segments roots and stimulates new plants to develop. Shoots emerge from root <br />and shoot pieces about 15 days after disturbance by tillage. Small root pieces, 0.25 inch <br />long by 0.125 inch in diameter, have enough stored energy to develop new plants. Also, <br />these small roots can survive at least 100 days without nutrient replenishment from <br />photosynthesis. <br />Management <br />The key principle to Canada thistle control is to stress the plant and force it to use stored <br />root nutrients. Canada thistle can recover from almost any stress, including control <br />attempts, because of root nutrient stores. Therefore, returning infested land to a <br />productive state occurs only over time. Success requires a sound management plan <br />implemented over several years. <br />Cultural control. Grasses and alfalfa can compete effectively with Canada thistle if their <br />growth is favored by good management. Maintain fertility and, if possible, moisture at <br />optimum levels to favor grass or alfalfa growth. Soil analysis can easily determine fertility <br />needs. Be cautious with nitrogen fertilizers, because excess available soil nitrogen may <br />favor weed growth. <br />These are essential management steps to ensure optimum desirable plant growth and <br />competition. However, competition alone seldom is effective against Canada thistle. <br />Chemical control. Read the label, follow directions and use precautions. Research at <br />Colorado State University shows that Tordon 22K (pictoram), Curtail (clopyralid plus 2,4- <br />D), Transline (clopyralid), Banvel/Vanquish/Clarity (dicamba), 2,4-D and Telar <br />(chiorsulfuron) are effective against Canada thiste. These herbicides are most effective <br />when combined with cultural and/or mechanical control. <br />Banvel/Vanquish/Clarity, and 2,4-D may be used on pastures, rangeland and non-crop <br />areas. Tordon, Curtail, Telar and Transline may be applied on noncrop areas only. <br />Colorado State University data indicates that Banvel/Vanquish/Clarity or Telar are <br />effective when combined with 2,4-D as asplit-season application. <br />Apply 2,4-D, 2 quarts per acre (A), in spring when Canada thistle is 10 to 15 inches tall, <br />in pre-bud to early bud growth stages. Re-treat in fall with Banvel/Vanquish/Clarity (2 <br />http://www.ext.colostate.edu/pubs/natres/03108.htm1 <br />6/20/2005 <br />