Identification

Tansy ragwort is a winter annual, biennial, or short-lived perennial typically growing between 8-36 inches (20-80 centimeters) tall but can attain heights greater than 6 feet (175-200 centimeters) under optimal conditions. Rigid stems grow singly or in groups from an upright caudex (a short, woody, persistent stem at or just below the ground surface) with only the upper half of the stems branching at the inflorescence. Tansy ragwort additionally perennates via fragments of the numerous (50-100/crown) soft, fleshy roots that grow out horizontally and penetrate the soil to a maximum depth of 12 inches (30 centimeters).

Rosettes, either seedling or root bud in origin, are formed from distinctive stalked basal leaves 2.7-7.9 inches (7-20 centimeters) long, deeply and pinnately lobed with ovate, obovate, or narrow coarsely-toothed segments (see Figure 2).

Figure 2. Leaves, inflorescence, and flowers . Diagram or Graphic; Britton, N.L., and A. Brown. 1913. Illustrated flora of the northern states and Canada. Vol. 3: 542. Available from USDA PLANTS Database.

Leaves are either glabrous (without hairs) or with loose wooly down on the underside, especially during early development. Rosette leaves generally die back at or before flowering. Stem leaves are deeply bi- or tri-pinnatifid, arranged alternately on the stem and are reduced in size higher on the stem. Middle and upper stem leaves lack a petiole and slightly clasp the stem.

Bright yellow, showy flowers arise only on the pedicel (branch) ends; outer pedicels are progressively longer than inner pedicels and form flat- or round-topped, dense and compact corymbs (see Figure 3).

Figure 3. Tansy ragwort flowers. Photo by Michael Shephard, USDA Forest Service, available from Bugwood.org

Capitula (flowerheads) 12-25 millimeters in diameter and numbering 20-60 per corymb have 13 dark-tipped bracts 3-4 millimeters long, and are composed of both ray and disc florets. There are 13 achene (small fruit) producing ray florets with rays (petals) 8-2 millimeters long. Disc florets are numerous and produce both pollen and achenes. The number of capitula and achenes produced per plant is highly variable.

Single seeds are contained within each achene; achenes produced from ray florets shed their pappus, are glabrous, and have a thick pericarp (skin), whereas achenes produced from disc florets retain their pappus, are pubescent with short hairs (trichomes) along prominent ribs and are lighter in weight than ray achenes. The pappus, composed of numerous white 'hairs' 6 millimeters in length, is attached to the top of the achene; its resemblance to a white beard explains the origin of the genus name, Senecio, derived from the Latin senex for "old" or "old man".

Tansy ragwort can be distinguished from native Senecio congeners by its comparatively larger size and exaggerated pattern of leaf dissection. Although tansy ragwort superficially resembles common tansy (Tanacetum vulgare), it differs significantly in flower composition, producing capitula with both ray and disc florets whereas common tansy flowers are composed solely of disc florets.

Other common names for tansy ragwort include stinking willy and staggerwort. In the United Kingdom Senecio jacobaea has formed hybrids with Senecio aquaticus, Senecio cineraria, and Senecio alpinus.

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Last Modified: 06/18/2009