Thistles are primarily of the plant family Asteraceae, though one plant commonly referred to as Russian thistle is actually from the Chenopodiaceae family. They are remarkable for their prickly form on leaves, stems and flower heads. They inhabit disturbed land along roadsides, overgrazed pastureland and agricultural areas that are constantly tilled. While they provide numerous benefits to insect and plant wildlife, they also overcrowd native plants, harm farm animals and cause painful wounds on humans who come into contact with them.
Artichokes and their cousins, the cardoon, are both edible thistle plants. The artichoke head that is available at the supermarket is actually the flower head. Once you get to the heart of the artichoke, the spindly parts are petals of the underdeveloped flower.
Bull thistle flowers range from pink to purple with weblike hairs. The top and bottom of all the long, pointed leaves are covered in spines, though the spines of the upper leaves are longer.
Burdock is a biennial in the same family as thistle and is often referred to as thistle, though its flowers have hooked bracts instead of straight ones like most thistle plants. The leaves are also broader and come up in a rosette formation.
This perennial thistle does not have spines at the tops of the leaves and is lobed instead of smooth on the edges. It spreads by seed and rhizome.
Milk thistle is a Mediterranean herb commonly used as an herbal remedy for liver issues. The flowers range from red to purple, and the plant grows from 4 to 10 feet tall in full sun.
Musk or Nodding Thistle
The flowers of musk thistle are pink, purple or white and are often held on stems bent with the weight of the 2-inch heads atop spiny brachts. The leaves are thin and lobed.
Pasture or Tall Thistle
Pasture thistle is a common name for thistles found in pasture land in the U.S., including Canada thistle, musk and bull thistle. In some parts of the U.S., it refers specifically to tall thistle, a variety that isn't much taller than its cousins but has smaller flowers.
Russian thistle is not related to any other thistle and has only tiny white flowers with no spine bracts. The leaves have no spines, but the tips of them are prickly. The plant dries up and breaks off at the roots, rolling over grassland as tumbleweed.
Scotch or Cotton Thistle
Scotch thistle resembles artichokes and cardoons with its wide, bluish-grey, hairy leaves. The flowers are red-purple and often occur in clusters.