Scotch thistle (Onopordum acanthium) is an extremely large biennial or winter annual thistle, commonly growing 8-10 feet tall, depending on the amount of available moisture. It is usually well branched with a large amount of soft white hair on the stalks, upper and lower leaf surfaces. Leaf margins and midribs, as well as flower buds, are covered with very sharp, yellow-tipped spines. The stems of Scotch thistle are “winged”, having a soft, thin tissue attached like a ribbon along the sides of the stalks. Purple flowers are born either singly or in clusters of 2 to 5 and bloom from June to September.
A prolific seed producer, each plant can produce 8,400 to 40,000 seeds. Scotch thistle forms large infestations that can crowd out more desirable forage species, prevent livestock from accessing water, and has the potential to invade extensive acres of pasture land. Individual plants are so large that they shade out surrounding vegetation, using the water and nutrients that would have gone to more desirable species.
When a Scotch thistle plant dies, it leaves abundant litter that can smother surrounding plants. While
Scotch Thistle is drought tolerant, it thrives with high soil moisture, causing it to be a threat to most areas in the Pacific Northwest.