What does it look like?
Common tansy (Tanacetum vulgare L.) is a perennial that reproduces and spreads mainly by seed and to a lesser extent by rootstocks. Tansy flowers look like small yellow buttons grouped together in flat-topped clusters. There can be 20 – 200 flower heads per plant. The leaves are deeply divided into toothed segments that are dotted with small glands. It is an aromatic plant with somewhat woody stems forming dense patches. Clumps of tansy can grow as tall as 1.5 – 2 metres.
Its weedy nature…
Tansy is a non-native plant of European origin. It is a very effective competitor and is found in non-crop areas, roadsides, low areas and on stream banks. It can quickly take over pastures where competition by desired species is reduced, such as in an overgrazed pasture situation. Seeds can remain viable in the soil for up to 25 years, and tilling alone will expose this seed bank in a heavily infested area. Not only is tansy unpalatable to livestock, but in some cases toxicity has been reported.
In the case of tansy, seed dispersal and seedling establishment are more critical than vegetative spread. Cropland and frequently tilled areas do not normally become infested with tansy as this plant does not survive under cultivation.
Management of tansy is often a long process that requires mowing and treating the regrowth with an appropriate herbicide over a period of years.
Residents of smaller properties can repeatedly mow the tansy and treat the regrowth with Killex or Roundup. Be aware however that Roundup will also remove grasses. This reduces competition for tansy which can then grow back. For larger areas, the regrowth can be treated with Escort or Ally, a herbicide registered for pasture, rangeland and non-crop areas. Grazon or Tordon can also be used in specific situations to treat tansy infestations. Following label instructions is extremely important.
Mowing once a season will not control existing tansy plants as the plant will react by producing more shoots. However one mowing operation, done low to the ground, before July, will halt that season’s seed production. It is important to remember that a seed bank from many years previous will still exist in the soil.
A note on fertilizer use…
Dense stands of grasses and legumes compete well with tansy. The addition of fertilizer, as much as 100 lbs/acre nitrogen, and phosphorous, potassium and sulphur to soil test specifications in your pasture can significantly reduce tansy numbers. This increases the competitive ability of the non-weed plants in the pasture.
Photographic credits to Alberta Agriculture, Food and Rural Development, the British Columbia, Ministry of Agriculture, Food and Fisheries, Field Guide to Noxious and Other Selected Weeds of British Columbia and Strathcona County, Transportation and Agriculture Services.