The thistle's reputation is nearly as thorny as the plant itself. Shakespeare called them "rough" and "hateful." Australia issued an Act of Parliament to rid them from the continent, and most states in the United States list them as noxious, invasive weeds. Despite the bad press, parts of the thistle are quite valuable. Also called cardoons, their stems are popular in European cuisine. Their flowers attract goldfinches, butterflies and bees, and seeds may offer medicinal benefits.
Thistle flowers are spiky, with a round or urn-like shape. The prickles are an defensive adaptation designed to protect the blooms from herbivorous animals. The flowers range from about 1 to 2 inches in diameter, and are most often in the purple colour family. However, some thistle flowers are red, white, pink and yellow. The base of the flower is green and often covered in spines, with a bulbous or slightly swollen appearance.
Thistle flowers attract several butterfly species, especially those in the Fritillary genus. The flowers also produce nectar that draws the North American goldfinch. As Scotland's national flower, legend has it that Viking invaders couldn't help crying out when they stepped on thistle flowers, which alerted Scots about an impending attack.
Thistle seeds grow within the spiky, swollen base below the flower. After the flower dries out, its seed-filled base hardens into a seed pod. When the pod cracks open, it sends rice-sized brown seeds flying through the air. The seeds have silky hair (pappus) that easily disperse in the wind, or get attached to animal fur and are transported away from the parent plant. The seeds are rich in oil and protein, making them a favourite with finches and other birds. Long used by herbalists, scientists are now conducting research on milk thistle seeds for their medicinal value, especially to treat liver ailments.
Leaves & Stems
Thistle leaves are long and deeply-lobed, with sharp, thorny-looking points. Some species have wavy foliage, while other types have leaves covered with small, fuzzy hairs that give them a powdery appearance. Thistle leaves range in colour from bright green to mottled grey.
Their green stems are grooved and thorny. Some biannual species can grow up to 10 feet tall, depending on conditions. Perennial thistles, such as Canadian and bull species, reach about five feet in height. Cardoons (thistle stems) are edible and part of French, Italian and European culinary traditions.