Introduced and naturalized from Europe, it has established throughout western North America. Flowers through the summer into fall. The specific name means spotted. Knapweed is an aggressive, competitive plant, establishing quickly on disturbed sites and producing a chemical to inhibit surrounding plants. The stem also is very leafy and bears deeply dissected, fernlike leaves that are lightly woolly with a dusty look.
Larger leaves form a rosette at the base.
Commonly grows on dry, gravelly or sandy soils of mountain ridges, hillsides and disturbed sites of mid-mountain to alpine elevations. Distributed from British Columbia to Montana, south to New Mexico and California, it flowers through the summer. The specific name honors botanist David Douglas. Elk thistle has large, spiny, grayish-green leaves attached to a thick stalk, which may stand anywhere from four inches to four feet tall. The light lavender flowers are hidden and clumped among the foliage near the top.