This species can be distinguished by, among other traits, the blue-green color and the thick growth of leaf hairs as shown in the detail shot below right. The Second Year , the weed transforms into a coarse branching plant up to eight feet tall and five feet in diameter. Under poor growing conditions, the plants may stand less than a foot tall, but can produce nearly as many seeds as the larger plants. Scotch thistle leaves are deeply lobed with long, stiff spines along the margins. The leaves have a winged appearance that continues down the stems of the plant.
Fine hairs give the plant a grayish appearance.
Purple flowers more than an inch in diameter are produced in the summer. Flower heads remain upright, rather than nodding as Musk thistle flowers do. Stocks supporting the flowers are leafy. Infestations of Scotch thistle often start in disturbed areas such as roadways, campsites, burned areas, and ditch banks. The weed adapts best to areas along rivers and streams, but can be a serious problem in pastures, grain fields, and range areas.