Tansy (Tanacetum vulgare L.)
Growth: This coarse, aromatic perennial has many tall stems, 0.7 to 1.2 metres (two to four feet), growing from rhizomes.
Leaves: The leaves are pinnately divided (several segments occurring at regular intervals along opposite sides of a common mid-vein) with each dark green leaflet being narrowly toothed. Some of the lower leaves reach 30 cm (12 in.) in length. There are numerous glands on the leaflets which emit a strong, rank odour when crushed.
Stems and Flowers: The slightly ridged stems are topped by numerous deep yellow button-like flowers that are approximately six mm (1/4 in.) in diameter. Between 20 and 200 flowers make up the flat topped cluster. Flowering occurs in August.
From seeds and, to a limited extent, short rhizomes slowly expand the clump.
Boiled and/or steeped, tansy was used for laxative purposes, in topical dressings and for its internal healing properties. Tansy was used in small doses to treat jaundice, dyspepsia, colds, fevers, hysteria, worms and nervous disorders for women. Externally, compresses of tansy tea were considered excellent for swelling, tumours, inflammations, sciatica, bruises, freckles and sunburn. In large doses, it can be toxic.
In earlier years, it was used by Newfoundland women as an insect repellent. They would tuck leaf pieces under their head scarves to keep insects from biting while they worked outside.
References report tansy has been used commercially as an insecticide.
Throughout Canada with many locations throughout the Prairies. Tansy is prevalent along fence lines. Since it was, and still is to some extent, a garden plant, it is commonly spotted in abandoned farm yards, hedgerows, fence lines, waste ground and roadsides.
Although generally not a problem in cultivated crops, it will behave as an increaser in overgrazed pastures. Once in pastures, it can lead to abortions or death if livestock eat it due to a shortage of suitable plants. There appear to be many "chemotypes" of common tansy with varying chemical contact. Those with the chemical thujone are the chemotypes that cause the most toxicity concerns.
Tansy does not tolerate frequent disturbance, and is therefore rarely a problem in annual crops. Keeping pastures and forage stands in good condition will help prevent tansy invasion. It is suspected that mature stems remaining erect through the winter can aid tansy spread as seeds shatter and drift over packed snow. Cutting tansy immediately following flowering will help to reduce spread to new areas.
Several potential biological control agents have been documented in Europe. A consortium of Canadian provinces and U.S. states have accessed funding to investigate their viability for introduction to North America.
For herbicide control consult the provincial Guide to Crop Protection.