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.