thistles

Thistles are a group of flowering plants in the family Asteraceae characterized by prickly leaves and stems and often tall, robust growth. They typically produce globe- or cone-shaped flower heads with spiny bracts, and many species are annual or perennial. The plants are valued by some pollinators for nectar, especially in early flowering.

The common name “thistle” spans several genera, notably Carduus, Cirsium, Onopordum and Silybum. They are native to Europe, Asia, and North Africa but have been introduced worldwide and readily establish in disturbed habitats, grasslands, and pastures. Their appearance ranges from compact rosettes to tall, branched forms with dense spines.

Ecology and reproduction: Thistles are usually insect-pollinated, attracting bees and other pollinators with their bright heads. Seeds are typically dispersed by wind via a tuft of hairs (pappus), aiding rapid spread. Many species form dense stands that compete with crops and native vegetation, and they are regarded as weeds in agricultural and rangeland settings. Some have become invasive outside their native ranges due to high seed production and limited natural enemies.

Human uses and management: Milk thistle (Silybum marianum) has traditional medicinal uses and contains the compound silymarin. Other thistles provide limited forage but are often avoided by livestock because of spines. Management includes mechanical removal when plants are small, mowing or grazing to reduce seed production, and herbicides in agricultural contexts. In some regions, biological control has been explored but requires careful evaluation to avoid non-target impacts.

Notable species: musk thistle (Carduus nutans), Canada thistle (Cirsium arvense), cotton thistle (Onopordum acanthium), milk thistle (Silybum marianum).