Island County > Public Health > Natural Resources > Noxious Weed Control Board > Terrible 10 - Thistles

Terrible 10 - Thistles

Bull Thistle

Canada Thistle

Found in open areas, pastures, roadsides.
Bull thistle is a biennial, tap-rooted, spreads by seed. First year appears as a rosette and in spring of 2nd year puts up erect, spiny, sparsely haired stems 2-5 ft tall. Leaves are 3-6 in long, pinnate, and hairy on top with cottony white below; leaf lobe tips have stout, sharp needles. Fragrant purple/pink flowers are supported by a bulbous, spine-covered bract.
Canada thistle is a perennial, spreads by rhizome and seed; it does not have hairy leaves and the bract below the flower of Canada thistle lacks spines. Canada thistle is not tap-rooted, has spreading rhizomes.
Bull thistle is Eurasian, arrived in contaminated seed.
The sharp spines on these thistles deter animals from grazing; when established in hay fields they have a serious economic impact on value of the hay.

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