Bull Thistle

Cirsium vulgare
Thursday, June 24, 2010

Bull thistle is an aggressively invasive weed native to Europe, western Asia, and northern Africa. It was introduced to the United States during the colonial period, and is now present throughout most of North America.

It is a biennial or sometimes an annual, forming rosettes in its first year and bolting in its second. The rosettes will grow from seeds, which germinate in the fall or spring. The event of fall germination makes a plant an annual, as it will likely flower the next spring. The rosettes grow until the next spring, sometimes reaching over three feet in diameter. Those rosettes that are not big enough to bolt that summer may wait another year for their chance. 

Those that are ready, however, produce flowering stems that are erect and can reach a height of six feet. They have sharp spines coating the fleshy stem as well as at the points of all their deeply lobed leaves. The leaves have thick, coarse hairs on the top, and light, wooly hairs on the underside. This, combined with their spines, make them undesirable to herbivores from mammals to insects. Their flowers grow at the apex of the plant, and are usually around 1 to 2 inches tall and a bright magenta or purple color. Spines also coat the base of the flower heads. The flowers are pollinated mainly by insects (especially butterflies), but sometimes by hummingbirds as well, who drink the nectar. They flower from June to September, during which time they also seed, producing new rosettes. They produce achenes, wind-carried seeds, which are in this case also called thistledown. Many birds and small mammals also consume the seeds.

Bull thistle, like many invasive species, mainly inhabits disturbed areas such as roadsides, logged areas, pastures, and riverbanks. It prefers bright sunlight, but can live in a variety of conditions.

Although bull thistle does not generally invade undisturbed areas, it can outcompete native species inhabiting disturbed areas. Bull thistle can be controlled most effectively by cutting the root several inches below the ground. Cutting off only the flower stalks will merely make the plant sprout another shoot; also, cut flower stalks left on the ground are likely to produce viable seeds, even after they are cut. All cut stalks should be destroyed in order to prevent seeding. At Mountain Lake, thistles have been uprooted most years from the dam around the pond; but that is still where they are most prevalent. Although mainly considered a weed, bull thistles do attract many beautiful and interesting pollinators with their striking flowers.

The thistle stalk was collected near Maphis; the flower came from a plant growing on the pond dam.

Article by Hazel Galloway