A perennial, it reproduces with seed and freely sprouting, horizontal root stalks. Established plants spread laterally by producing creeping horizontal roots. Canada thistle also spreads by seed with the help of pappi attached to the seed. Branching horizontal and vertical roots contain an abundant food reserve.
Carbohydrates stored in the roots fuel the growth of new shoots each season.
The seedling has oblong cotyledons. The leaves are oval to spear-shaped. The margins have numerous triangular indentations and sharp spines. The seedlings are weak competitors if they are shaded by the crop. The leaves are alternate, oblong, or lance shaped.