Canada thistle is a strong competitor for soil resources and can result in drastic yield losses due to its extensive root system. This characteristic also makes it difficult to eliminate mechanically and enhances the likelihood of weed re-growth. Depending on the severity of the infestation or the invasion, there is potential for 100% yield loss if the required weed control measures are not in place. Increases in reduced tillage across Canada have favoured the further establishment of this noxious weed.
Canada thistle is a perennial plant that over-winters through rhizomes (roots). Early identifiable features are irregularly shaped leaves with indentations, with a shape from round to triangular as the distal end is reached (refer to picture). The colour of the plant is dark green, occasionally with a bluish colouration on the surface of some leaves. Leaf margins are covered with sharp spines, which makes difficult to collect this weed.
Cotyledons are oval shaped and the root system grows extremely fast. Normally, four months following germination, the plant rhizomes could be up to 100 cm in depth.
The defining features of the Canadian thistle are its pinkish, purple flowers which can be present from the beginning of June through to August. The species, C. arvense, has an extensive root system which can reach up to 450 cm underground, making Canada thistle extremely difficult to control. At maturity, this weed can reach 150 cm in height. At this growth stage, chemical control can be very difficult.
Optimal growth conditions
Optimal conditions for the growth of Canada thistle are moist to dry transition areas and conditions of heavy rainfall ranging from 50-75cm annually. It also grows well in grazed lands, crop land and roadsides.