Identification

Common tansy is an erect herb that grows from two to five feet (0.6 to 1.5 meters) tall. It has dark brown, stout, creeping rhizomes. Rhizomes of common tansy plants in Gallatin County measured 1/2 - 3/4 inch (5 to 19 millimeters) thick. Buds along the rhizomes can grow into shoots or roots. The roots are coarse, wiry, and have minimal development of fine roots or root hairs. The roots are shallow but extensive, and can occupy most of the upper two feet (60 centimeters) of the soil profile beneath a plant making common tansy competitive for soil moisture and nutrients.

Basal and stem leaves are large, up to one foot long (30 centimeters) and six inches wide (15 centimeters), deeply pinnately dissected, the sections (pinnae) pinnately dissected again with deeply lobed or toothed segments (pinnules) so the leaf appears fern-like (see Figures 2 and 3). The central leaf vein (rachis) is winged with leaf tissue (see Figure 3).

Figure 2. Common tansy leaf, flowering stem, and seed depiction (USDA-NRCS PLANTS Database / Britton, N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. Vol. 3: 522).

Figure 3. A common tansy leaf showing the toothed pinnules on the deeply lobed pinnae along the winged rachis.

The leaves are hairless (glabrous, or nearly glabrous), glandular, dotted with minute depressions (punctate). Stems have numerous leaves (up to 20) arranged alternately from short petioles or sessile. When crushed, the leaves have a strong and distinctive menthol- or camphor-like smell from volatile oils. The flowerheads are numerous (commonly 20 to 200) and arranged in a dense compound inflorescence that is somewhat flat-topped with the outer flowerheads developing before the inner flowerheads (corymb, see Figure 4).

Figure 4. The “button-like” flowerheads of common tansy blooming in mid-August.

Individual flowerheads are made up of yellow disc flowers. There are no petal-like ray flowers. The flowerheads are about 1/4- to 3/8- inch (5 to 10 millimeters) wide and often described as button-like. The bracts below the flowerheads are green with dry, thin, membranous, and translucent (scarious) margins and tips. The bracts overlap each other like shingles on a roof (imbricate).

The individual flowers on the flowerhead are numerous (sometimes over 100) on a flat to low conic, naked (without bracts, bristles, or scales) receptacle. The flower has a short, five-lobed, yellow corolla tube with a short, crown-like pappus (often absent) at the base (see Figure 2, the pappus is absent in the illustration). The outer flowers on the flowerheads produce seeds and the inner flowers produce both pollen and seeds. The achene (seed) is very small (1.2 to 1.8 millimeters), five-ribbed with scattered sessile, transparent, non-mucilaginous glands on the outer surface (epicarp).

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Last Modified: 02/27/2008