Tansy is a bitter herb that had more followers in times past than now. A European native, Tansy was mainly used as a medicinal herb or an insect repellant. 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. The upper stems terminate in flat clusters of 20-200 yellow flower-heads. Each flower-head is about 1/3" across, consisting of numerous yellow disk florets and no ray florets (or insignificant ones). Each disk floret is narrowly tubular and has 5 lobes that are upright, rather than spreading, when it is fully open. The outer disk florets bloom ahead of the inner disk disk florets. Each flower-head is flat-topped and shaped like a button. A single series of overlapping floral bracts surround the base of the flower-head. Each bract is green, oblong-linear in shape, and often has a papery upper margin. The blooming period occurs from mid-summer to early fall and lasts about 1-2 months. Whatever fragrance the flower-heads may possess is overwhelmed by the strong odor of the foliage. This odor is camphor-like and somewhat rank. Each disk floret is replaced by an oblong grayish brown achenes that often has a tiny crown of scales at its apex. The achenes never have tufts of hairs. The root system is fibrous and produces rhizomes. This plant often forms vegetative colonies.
Sources: Common Tansy (Tancetum vulgare crispum), http://www.illinoiswildflowers.info/weeds/plants/cm_tansy_cr.htm (accessed January 20, 2017). Tanacetum vulgare Tansy Plant - Mountain Valley Growers, https://www.mountainvalleygrowers.com/tanvulgare.htm (accessed January 20, 2017).