Canada thistle was introduced in North America as a seed contaminant in both French and British colonies. The first legislation to control the weed was passed by Vermont in 1795. It is a long-lived perennial that usually grows 2 to 3 feet tall and bears alternate, dark green leaves that vary in size, usually deeply cut with spiny, toothed edges. It has an extensive underground root system that may penetrate the soil to a depth of 10 feet or more, and can grow laterally 12 to 15 feet per year. Root buds occur randomly along the roots and initiate new shoots whenever environmental conditions are favorable. Root segments as small as 0.6 inch can initiate shoot growth and become established.
Canada thistle has the potential to form dense infestations rapidly through vegetative reproduction, and the spread of these clones may continue indefinitely, crowding out and displacing native grasses and forbs.
Flowering occurs from June to September. Male and female flowers are produced on different plants, so cross-pollination is necessary for seed production. Flowers produce from 40 to 80 seeds per head. Seed distribution can be done via the downy thistles that blow in the wind, although most seed remains in the bud which break off the stem during the winter and roll on top of the snow. Seeds mature rabidly and are able to germinate 10 days after pollination.
Control in non-crop lands:
Herbicide: Numerous herbicides are available to control Canada thistle. Curtail, Tordon, Milestone, ForeFront, Chaparral, Cimarron and Redeem are approved for partical reimbursement of costs through the Landowner Assistance Program. Control is greatest when applied to at the early bud growth stage, (early summer) or in he fall to plants in the rosette form.
Cultural: Repeated mowing will reduce Canada thistle infestation. Mow whenever the plants are in the early bud growth stage to prevent seed-set. Several mowing a year are needed because plant populations vary in maturity. Mowing should be combined with a chemical control program for best results.
Biological: Two biological control agents have been introduced for Canada thistle control. The Canada thistle stem mining stem mining weevil, Ceutorhynchus litura. The adults overwinter in the soil and emerge in early spring as the first thistle rosettes begin to appear. The adults lay their eggs in a thistle rosette leaf. The larva hatch an tunnel through the leaf into the lower stem and root collar. The larva mine the inner stem tissue which stresses the thistle plant. The combined stress of the feeding and entry/exit holes in the stem which allow other pathogens to attack the thistle plant eventually kill the plant.
The Canada thistle gull fly, Urophora cardui, causes meristematic galls. The galls (enlarged areas in the plant stem about the size of a golf ball) prevent plant nutrients from reaching the flower buds above the gall preventing seed formation. This will not kill the plant or reduce established stands but will reduce seed production.