Tanacetum vulgare aka Common Tansy

Tanacetum vulgare, or common tansy, belongs to the aster family. This herb has soft stems and lives for many years. It comes from parts of Europe and Asia with temperate climates. Its native range spans Britain and Ireland. Settlers brought it to North America in the seventeenth century. It now grows wild across northern states and southern Canada. It grows by roads, streams, and railways, and on disturbed ground.

In gardens, tansy grows about 2–3.3 feet [0.6–1 metre] tall. Extension records note that some plants reach 5 feet [1.5 metres]. Reports give garden widths of about 1.6–2 feet [0.5–0.6 metres]. Its upright stems branch and turn brown to reddish brown. Green leaves grow in turn on each side of the stems. Each pinnate leaf has narrow parts with toothed edges. The leaves smell of camphor when crushed. The plant sheds its leaves at the season's end.

Dense groups of yellow flower heads top the stems. Each small head holds disc flowers but lacks outer ray petals. The round heads give tansy its button-like look. Flowers bloom from July to September and may last into autumn. Small brown fruits called achenes follow the blooms. They ripen from late summer onward. One plant may yield about 50,000 seeds each year. Rhizomes, or stems below ground, also help it spread. Pieces of root help it spread too. These traits let it form thick stands and crowd out native plants.

Tansy prefers full sun but also grows in part shade. It favours moist soil rich in humus that drains well. It also copes with dry, poor soil and drought. Sand, loam, clay, and chalk can all suit it. It grows in acid, neutral, and alkaline soils. The RHS gives it a UK hardiness rating of H7. Its scent and flowers suit herb gardens and informal plantings. Yet its spreading roots make it a poor choice for small borders. Some American states restrict or ban it as a noxious weed. Removing dead flowers helps limit new plants from seed.

Few serious pests or diseases trouble tansy. Spider mites can still occur. Touching the plant may cause a skin rash. Wear gloves and long sleeves when handling it. People once used it to repel insects and make dyes. They also used it to treat worms in the gut. Records also note the use of leaves and flowers in food and tea. Those uses do not make it safe to eat. Taking too much of its thujone can cause convulsions or death. It can poison people, horses, and cattle.