Thistle

Thistle is a common name for a group of flowering plants in the Asteraceae family, characterized by prickly leaves and stems and heads of small, tubular flowers. The name is applied to several genera, most notably Cirsium and Carduus, with Onopordum also included. Plants are typically herbaceous perennials or biennials, often forming a rosette in their first year and sending up a spiny flowering stem in subsequent years. Flowers are usually purple or magenta, though white or yellow forms occur in some species.

Description and distribution: Thistles are adapted to open, sunny habitats and disturbed soils. Leaves are stiff and lobed, with sharp spines along the margins and sometimes on the leaf surface. The flower heads are composite, with many florets arranged in rounded or elongated inflorescences. Native ranges include Europe, Asia, and North Africa; many thistles have been introduced elsewhere and are considered invasive in parts of North America and other regions. They spread mainly by wind-dispersed seeds bearing a fluffy pappus.

Ecology and uses: Thistles provide nectar and pollen for insects and serve as forage for some grazing animals, though the spines limit use of leaves. Some species have economic value: milk thistle (Silybum marianum) is used medicinally for liver protection; globe artichoke (Cynara scolymus) is a cultivated edible thistle. The thistle is a national emblem of Scotland and has appeared in heraldry and culture.

Management: Many thistles are considered weeds in pastures and croplands. Control strategies emphasize preventing seed production, mechanical removal before flowering, mowing, grazing with spiny-resistant animals, and, in some regions, chemical herbicides. Biological control has been explored in limited cases using thistle-feeding insects.