The common burdock and the milk thistle are two weedy plants that tend to look alike. You can distinguish between the two plants and properly identify them by studying their various plant characteristics, such as their leaves, stems and flowers.
Common burdock grows from a taproot and reproduces by seed, growing as a biennial plant. Milk thistle is an annual or biennial plant that also propagates by seed. Milk thistle plants can grow 2 to 6 feet tall, while burdock plants grow 3 to 10 feet tall.
Stems & Flowers
Burdock grows upright with a branched form and second-year stems. Unlike the milk thistle, burdock's stems have red striping or striations. Burdock's flower heads are reddish-purple with tubular florets atop short stalks, while the milk thistle's 2-inch-thick disk flowers are purple in the centre, large and spiny. Burdock flowers are much smaller than those of the milk thistle, usually measuring only 3/4 inch in diameter, sometimes varying from pink to lavender or purple.
The milk thistle leaf is perhaps the best identifying characteristic of the plant, measuring up to 2 1/2 feet long and 11 inches wide with yellow spines and lobed or wavy margins. Milk thistle leaves are distinctly mottled with white, particularly at the leaf veins. Burdock leaves, on the other hand, form a rosette at the plant base, consisting of hairy, heart-shaped leaves that resemble those of the rhubarb plant.