Several species of thistle originated in Europe and Asia. Easily identified by their spiny leaves and shaggy purple blossoms, thistles have enough beauty and allure to land themselves on the pages of a history text while also serving as some of the world's worst weeds. Thistles native to the Old World have spread to the Americas and Australia, where they often establish in abundance in yards, gardens and agricultural fields.
The thistle has served as an important emblem for Scotland since the 13th century, when, according to legend, an attempt at a Norwegian invasion was thwarted when the invaders trod on the spines of a thistle and cried out, alerting the Scots in time to muster a defense. In the 16th century, King James V of Scotland established an Order of the Thistle akin to the English Order of the Garter, designed to honor his most loyal knights. The thistle remains the national symbol of Scotland.
With the arrival of international trade, the prolific seeds of the thistle traveled to new lands. Today, Old World thistles, such as the Scotch thistle (Onopordum acanthium), inhabit the New World as well. Tall, sturdy plants, Scotch thistles can grow up to 8 feet high. Fine white hairs cover the spiny leaves, and the round, purple-pink blossoms form in late summer, changing into the prolific, fuzzy seeds that allow this plant to become such a nuisance.
Thistles are biennials, meaning that it takes the plant two years to grow, flower and produce seeds. Biennial plants develop leafy growth in the first year. The thistle in its first year produces a rosette of thorny leaves close to the ground. In the second year, the first year's growth sends up a flower stalk, from which seeds develop.
Although an attractive plant to many people, the Scotch thistle -- among other Old World thistles, such as the musk thistle -- creates a serious problem in areas where it does not grow natively. These thistles can often tolerate dry, inhospitable climates and produce thousands of seeds per plant. Non-native thistles can quickly overtake agricultural land, where they compete with crops or exclude livestock from grazing. Remove and destroy thistles when you find them in your garden, preferably during the first year of growth, before they form seed.
If you appreciate the bold beauty of the Scotch thistle or similar Old World species, consider using native thistle species in your garden instead. These plants are better adapted to your local environment and are less likely to become invasive. The Native Plant Information Network lists 36 thistle species native to the United States and Canada.