This introduced perennial plant is 2-4' tall, branching occasionally. The stems are glabrous or sparingly hairy. The alternate leaves are up to 12" long and 4" across, and more or less ovate in outline. They are double or triple pinnately lobed, which provides them with a fern-like appearance. In var. crispum, as revealed in the photograph below, the margins of these lobes are curled, whereas in the typical variety these margins are flat. The smallest lobes are dentate along the margins. The leaves have flat petioles and they are largely hairless.
In the past, Common Tansy was used to eliminate parasitic worms from the digestive tract. Sometimes it was also used as an aborticide, sometimes with fatal results. Because of the toxic substances in the foliage, it is probably unwise to ingest herbal preparations from this plant. Common Tansy is often associated with death, in part because its flowering stems were used at funerals to repel flies from the deceased. There are native Tanacetum spp.
(Tansies), but they don't occur in Illinois. The native Tansies are often smaller plants with flowerheads that are larger in size, but fewer in number. Their foliage is flat or curled, depending on the species.
Range and Habitat in Illinois
Common Tansy occurs occasionally in the northern half of Illinois, but it is uncommon or absent in the southern half of the state. It was introduced into the United States from Europe as an ornamental plant and medicinal herb. Habitats include edges of prairies and fields, fence rows, pastures, weedy meadows, old homestead sites, landfills and soil piles, and areas along roads and railroads. The rhizomes can survive earth-moving operations to produce new plants. This old-fashioned plant is still grown in flower gardens, although it is less common than before. Because of its ornamental foliage, var. crispum is often grown in gardens, while the more typical Common Tansy with flat leaves is the variety that is usually found outside of gardens in natural and disturbed habitats. Sometimes this plant spreads aggressively in pastures because cattle are reluctant to eat the foliage.