Canada Thistle
Cirsium Arvense var: horridum


Provincial Noxious. Perennial.
Canada thistle has purple (sometimes white) flowers, smaller than most thistle flowers, and spiny, dark-green, oblong leaves growing directly from the stem. Mature plants can be 0.3 to 2 metres in height and have both horizontal and vertical roots. This is the only thistle with male and female flowers on separate plants.

This thistle infests crops, pastures, rangelands, roadsides, and riverbanks. It spreads rapidly through horizontal roots (rhizomes) that give rise to shoots and can form dense patches and virtual monocultures. Thistle thickets can restrict recreational access to infested areas.

Canada thistle prefers rich loam soil, ample precipitation, and is somewhat shade intolerant, but it has adapted to a wide range of environmental conditions. It is widespread in all areas of British Columbia, and is considered a major concern in the Omineca and Peace River regions.

Plants average about 1,500 seeds per plant which do not usually remain viable after three years. However, plants spread efficiently through horizontal roots.

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