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Scotch Thistle

Onopordum acanthium

Scotch thistle (Onopordum acanthium) is a biennial or perennial plant that can form dense, inpenetrable stands and competes with field crops and forage plants.

Considered regionally noxious under the BC Weed Control Act, Scotch thistle is a major concern in the North Okanagan region, and otherwise occurs at the lower elevations of BC's roadsides, irrigation ditches, rangelands and disturbed areas. Scotch thistle continues to be grown in gardens around BC. There is concern that impacts will increase as it escapes controlled garden environments and invades natural areas.

Growing up to 3 m tall, Scotch thistle is identifiable by its large, bright violet to reddish flowers that are supported by large spine-tipped bracts. Woody stems also have spine-edged wings that run up the sides. Leaves are irregularly-lobed, have sharp yellow spikes, and are covered in woolly hairs that give the foliage a grey-green appearance.

Scotch thistle reproduces through thousands of seeds that remain viable in the soil for over 30 years. Seeds spread mainly with the wind, but also in hay and water, and by attaching to clothing or animal fur. Seeds also contain a water-soluble germination inhibitor, making it successful in moist areas such as nearby streams and in riparian areas.