Submodule 5: Most Common Western Rangelands Weeds

Canada Thistle

Canada thistle (Cirsium arvense), a perennial forb, grows up to 4 feet (1.3 meters) tall, with stems that are erect and generally smooth. It has extensive roots and small flowers. Since it is a dioecious species (male and female flowers are found on separate plants), it is possible to have an entire population that is one sex. The stems are branched and have irregularly lobed leaves with spines and toothed margins. The first leaves are rounded with smooth margins, while later leaves have spines and are serrated (have a saw-toothed edge) and lobed. Upper leaves are noticeably smaller than lower leaves. Round composite flowers, about 0.5 inches (1.25 cm) in diameter and surrounded by spineless bracts, are produced on several branching stalks from about June through October. They are composed of numerous white, rose, or lavender florets.

Canada thistle (left, courtesy Larry Howery; right, courtesy nps.gov)