Tanacetum vulgare--Common Tansy
About Common Tansy: Common Tansy is a native of Europe, also known as Chrysanthemum vulgare. It was brought to this country for medicinal and horticultural purposes. This perennial plant spreads via an extensive, spreading root system and profuse seed production. It especially favors the disturbed soils along ditch banks, where the water quickly spreads the seeds for miles downstream. Common Tansy is now widespread from coast-to-coast across most northern states and Canadian provinces. Despite extensive infestations along ditches, creeks, and roads, the plant is not yet listed as a noxious weed in many places. Most alarming, you can still buy the seed and grow more!
Common tansy is rich in volatile oils (see Botany in a Day). The aromatic fresh young leaves and flowers may be used as a substitute for sage in cooking. The main volatile oil is thujone, a potent and bitter chemical often used medicinally as a wash to treat roundworm, or internally to expel worms and cause abortions. Excess consumption of thujone for medicinal purposes has caused convulsions and death. It should not be used without medical supervision. The volatile oil can be distilled from the plants and marketed.
Mechanical Controls: Pulling or mowing has little effect on tansy, except to reduce seed production. Most of the big roots are near the surface, so it may feel like you can pull up smaller plant roots and all, but it almost always comes back again and again. Tansy regenerates from root fragments, so cultivation could expand the size of an infestation.
Fire: Dense patches of dried tansy stalks burn very hot and fast. Controlled burns in the spring can help reduce fire danger later in the year. Removing the dead vegetation also makes the plants easier to target with herbicides or grazing animals, although repeated burns may increase weed habitat.
Grazing: Horses and cows sometimes browse the tender young leaves of the common tansy, but they leave it alone as it matures. Tansy could be toxic to these animals in excess. Sheep and goats however, eat the plant with great enthusiasm. My town in plagued by extensive infestations of tansy, and we have begun working with sheep to control the weed. It is truly amazing to set the animals loose in a tansy patch and watch it all disappear. The sheep remove the tansy, allowing light to the grasses below. As a result of our sheep project I realized we did not have a weeds problem so much as a lack-of-sheep problem. If we can increase the number of sheep in the community then we will never have to worry about tansy again!
Chemical Controls: Tansy is relatively easy to control with common herbicides like 2,4-D, or a blend of 2,4-D and clopyralid (Curtail®), or picloram (Tordon® 22K), however the weed patches must be monitored and retreated to kill any tansy that regenerates from the roots. Herbicides are impractical in many cases where the weed is inter-mixed with other desirable plant species. Special care must be taken along ditch banks and creeks to avoid contaminating the water.
Important: Most "weed problems" are really "people problems" from poor land management and a lack of ecological insight. It is easy to reach for a tool like fire, mowing, or herbicides to attack an out-of-control weed, but often those tools do little to get to the root cause of the weed infestation, and sometimes make the problems worse. Please read more about range ecology, desertification, and invasive weeds on this website before applying any tool of weed control Go to: Desertification and Invasive Weeds.
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