Robust biennial 3 to 6 feet in height. Stems have vertical rows of prominent, spiny, ribbon-like leaflets or wings. Basal leaves have sharp yellow spines, are up to 2 foot long and 1 foot wide. Upper and lower leaf surfaces are covered with fine wooly hairs, giving the foliage a grey green appearance. Plants flower in mid summer with globe or urn shaped flower heads, in groups of 2 or 3 at branch tips. Flower heads are up to 2 inches in diameter, with long, stiff, sharp bracts at the base. Flowers range from dark pink to lavender. Seeds are smooth, slender, plumed and may remain viable for 20 years, however seeds suffer high rates of loss to insect, bird and rodent predation. Scotch thistle reduce forage production and large spiny plants present a barrier to livestock and wildlife, preventing animals from grazing or accessing water.

Scotch thistle as with all biennials can be controlled by mechanical means. You need only sever the rosette below the growing point, about 1 inch below the soil surface. As it only spreads by seed, stopping that production is key to success and good results are seen using 2,4-D or Glyphosate. Clopyralid chemistry and Aminopyralid are highly active on thistles. Herbicide treatments in the first year of growth are most effective. Persistence will quickly yield results with rapid reductions in density within a couple of years.
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Modified from Selected Noxious Weeds of Eastern Oregon, Gary Page, Malheur County Weed Inspector.