Tansy ragwort is classified as a biennial herb. It can complete its life cycle as a winter annual and occasionally as a perennial, depending on environmental conditions. As a biennial, tansy ragwort spends the first year in the rosette stage, with petiolate (stalked) dark green, basal leaves. The leaf underside is somewhat hairy, and appears whitish. The overall rosette has a ruffled appearance, due to deeply indented and blunt toothed lobes of the leaves. The basal leaves are often deciduous. The size of the rosette may indicate the potential for flowering, with larger rosettes producing more flowers. During the second year, one or several flowering stems bolt, with the overall plant being one to four feet high. The leaves found on the flowering stem are alternate, and sessile. The flower heads are in flat topped clusters. Each flower head is composed of yellow, daisy-like flowers. Each flower head is a composite of many disc flowers surrounded by (usually) 13 ray flowers. A distinguishing characteristic is the 13 ‘petals’, which are ray flowers. Tansy ragwort has a taproot, and often a large woody rootstock. Initial infestation is by seed. 

Habitat:  Tansy ragwort will establish in disturbed sites that includes roadsides, pastures, and forested areas recently harvested for timber (Sweeney et al. 1992). Tansy ragwort does not show a preference for soil texture or acidity. Soil moisture may play a role in the distribution of this species. (Harper 1958 as cited in Sharrow et al. 1988). In Britain and western Europe tansy ragwort is considered a climax species in coastal sand dune communities. However, the major distribution of tansy ragwort is as a pioneer to mid seral species to newly disturbed sites. (Harper and Wood 1957 as cited in Sharrow et al. 1988).

Growth and Development: As a biennial, most tansy ragwort seeds germinate in the fall. The first year is spent in the rosette stage with dark green and ruffled basal leaves. The flowering stalk bolts during the second year. Flowers are produced from late summer into the fall. After seed production, individual plants generally die. However, the crown and the root system from a flowering plant can produce new rosettes (Forbes 1977, McEvoy 1984c as cited in Sharrow et al. 1988).While a biennial life cycle is typical, tansy ragwort will behave as a perennial if the flowering stalk is cut, mowed, trampled, or mechanically injured in any way while flowering. Vegetative regeneration can then occur from crown buds, root fragments or intact roots (Baker-Kratz and Macquire 1984; Black 1976). When flowers are removed prior to seed set, the plant is able to reflower later in the same season. Defoliated rosettes will continue to grow indefinitely as vegetative perennials (Forbes 1977 and McEvoy 1984c; Harper 1958 as cited in Sharrow et al. 1988). Seeds require light for germination, but they can remain viable in soils for 10 - 16 years (Thompson and Makepeace 1983 as cited in Sharrow et al. 1988).

Reproduction:  The number of seeds per plant can range from 5,000 to 200,000 (Cameron 1935 as cited in Sharrow et al. 1988). Tansy ragwort flowers from July through September, and the seed matures and disperses during the flowering season. Pioneer invasion is by seed. Tansy ragwort patches can establish when root and crown sprouts vegetatively produce new rosettes.

Detrimental: Tansy ragwort is toxic, and can be lethal to cattle and horses, to a lesser extent goats, and seldom sheep. These toxic properties remain in cut plants found in hay. The toxic properties are also be a threat to humans, as a possible contaminant to the human food chain