Thistle is the name given to a group of plants that have sharp spines or prickles. Thistles are often troublesome weeds. They grow in many places throughout the world. The most common thistles are the Canada thistle, the bull thistle, the tall thistle, and the pasture thistle. The first two come from Europe, and the last two are native to North America. They grow in pastures and grain and hay fields, and along roads, where the soil is rich. The most troublesome, the Canada thistle, is a perennial. The other three species are biennial.
Thistles have tough, fibrous stems, prickly leaves with many lobes, and soft, silky flowers, usually purple or pinkish-purple in color. The flowers usually grow in round heads that form large, downy seed balls after the blossoms wither. The wind scatters the seeds, and this helps the thistles multiply rapidly. Some kinds have strong roots and are hard to uproot. Root pieces left in the soil may produce new plants. Thistles are hard to remove from grain fields. Biennial species must be cut down before the flowers bloom. Chemicals that kill thistles but do not harm crops also can be used.
A number of plants similar to thistles are sometimes called thistles. Included among these is the Russian thistle or tumbleweed. This plant has become a serious pest in large areas of North America.
