Canada Thistle

is a creeping perennial that reproduces from seed and vegetative roots.
can colonize in an area 3 to 6 feet in diameter in one year. Horizontal roots may extend 15 feet or more and vertical roots may grow 6 to 15 feet deep.
develops seed sparingly, 1,000 to 1,500 seeds per flowering shoot, transported long distances by water, animals, clothing, farm equipment, and vehicles. It is also known to be a common crop seed contaminant. Wind may help disperse the seed, but most often the feathery pappus breaks off, leaving the seed attached to the parent plant.
seed can remain viable in the soil up to 22 years. Deep burial of seed can prompt seed survival to be even longer.

Mowing has a tendency to stimulate new shoots to develop from the root system. If conducted often enough, mowing will prevent seed production however, plants will continue to spread through the soil by root.
Disking will prompt patches to grow larger and spread quickly to non-infested areas. Each dissected root segment will generate new plants.
Non-crop, grass-friendly products like Milestone, Redeem, Curtail, Transline, and Telar have proven to be the most effective. Late season treatments of Telar may temporarily injure cool season grasses. Other products such as those containing 2,4-D or Dicamba have also been used. Applications made when the weed is at the bud through flower stage will translocate better to the roots, however, annual treatments may be necessary. Always read and follow the label when using any herbicide.
Controlling Canada thistle in an irrigated broadleaf crop is difficult. It is best to consult your field advisor to carefully plan control options based on your specific crop rotation schedule.