A tansy is a plant in the Tanacetum genus, especially T. vulgaris, a popular herbaceous ornamental used in the garden. This plant has historically been used for a number of medical purposes, although modern gardeners usually prefer to grow it as an ornamental. Gardeners should also be aware that it is considered an invasive species in many regions of the world outside its native Eurasia, and some gardening organizations discourage its cultivation for this reason.
Plants in this genus are all generally shrubby in nature, with upright, reddish stems and bright yellow flowers which look like little buttons. Tansy also has a very sharp aroma which is quite distinctive and memorable, and may explain why the plant was used medicinally, as many traditional herbals have strong odors, suggesting that people believed odoriferous plants were more beneficial than others. It grows as a perennial in some areas, and as an annual in cooler regions.
Many gardeners like this plant because it is very easy to grow, and it requires little to no maintenance. However, this can turn into a problem, as the plant will freely proliferate beyond the regions where it is planted, and it is difficult to eradicate. Tansy can also choke out more delicate plants, and overwhelm the garden with its aroma, especially in warm weather or if it encroaches on pathways, where it can be crushed, releasing the strong scent.
Tansy is actually mildly toxic, making it somewhat surprising that its greens and shoots were once eaten and used in medicine. It was also sometimes used as a bitter herb in salads, and the leaves were used to make tisanes and tonics to treat a variety of conditions. Worms in particular were treated with tansy, since the plant has volatile oils which kill intestinal parasites, and some people believed that it could induce miscarriage, while others thought that it promoted healthy pregnancy. Tansy also contains thujone, the active ingredient in traditional absinthe.
In modern gardens, this plant may be used as a form of natural pest control. Tansy tends to repel insects, so it can be planted as a border to protect more tender plants, or grown in containers around the door to dissuade flies and other unwanted insect visitors. Young children should be kept away from tansy, as they may be tempted to chew on the toxic flowers, thanks to their bright color.