Why is Canada thistle such a problem in organic cropping systems?
Canada thistle (also called creeping thistle, California thistle, field thistle), Cirsium arvense (L.) Scop. (family: Asteraceae (Compositae), is a vigorous, competitive perennial weed that is difficult to control because it can establish from seed or wide-ranging, deep creeping roots. An individual plant may produce over 40,000 seeds a year. Canada thistle produces chemicals that inhibit growth of neighboring plants. Canada thistle is adapted to a wide range of habitats and soil types. (View photos of flowering plant and mature seed.) A small portion of seeds (0.2%) can disperse 0.5 mile or more from the parent plant, thus Canada thistle is difficult to prevent even if you are using best management practices. Canada thistle seeds float and are easily spread by flooding or in irrigation water. The seeds can also be dispersed in manure. Canada thistle seed germinate best in the top half-inch of soil at temperatures between 68 and 86oF. Approximately 90% of seed germinate within one year
