What does thistle plant look like? Information on features, living habitat and flowers.
Thistle; any of various prickly leaved plants of the composite family (Compositae). Thistles are usually tall plants with rounded flower heads. Unlike the daisy, which has flower heads of both disk and ray flowers, the thistle has a flower head composed only of disk flowers. In some thistles, bristles or hairs are mixed in with the disk flowers.
Many thistles are troublesome weeds. The few species that are cultivated are usually grown as ornamental border plants. The Scotch thistle (Onopordum acanthium), native to Europe, is also found in North America,, where it is a serious pest in the western United States. It grows 9 feet (2.7 meters) tall and bears small lavender flower heads.
Another important European weed that has become naturalized in North America is the Canada thistle (Cirsium arvense). This perennial species, ranging in height from 1 to 4 feet (0.3-1.2 meters), has an extensive root system with main roots that grow parallel to the surface of the ground. The leaves are vivid green on the upper surface and woolly on the under surface. The purple flower heads of the Canada thistle are only 1/2 inch (13 mm) in diameter. Related to the Canada thistle is the bull thistle (C. vulgare), a biennial weed of pastures and roadsides throughout the western and northern United States. It reaches a height of 5 feet (1.5 meters) and has purplish flower heads about 2 inches (50 mm) in diameter. Many other thistles belong to the genus Cirsium.
Both the musk thistle (Carduus natans) and the Italian thistle (C. pycnocephalus) are Eurasian plants that grow as weeds in North America. A related plant, C. kerneri, is sometimes cultivated. It ranges in height from 1 to 3 feet (60-90 cm) and bears rosy purple flower heads. The yellow star thistle (Centaurea solstitialis) is widespread in California; it is a pesty weed but important as a source of nectar and pollen for bees.
Thistles vary in their reactions to herbicides. For example, the bull thistle is rather susceptible, but the Canada thistle is relatively resistant and the Scotch thistle is extremely resistant.