Common tansy was imported as a medicinal herb but has since been shown to be carcinogenic. It is an extremely aggressive, rhizomatous perennial which often forms monocultural stands in pasture and waste land. It is often confused with tansy ragwort because of misuse of the common name rather than any similarities between the plants. Common tansy is an aromatic perennial without ray flowers. Tansy ragwort is a non-aromatic biennial with ray flowers.