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The Road to Recovery

If ingested by grazing horses or cattle, tansy ragwort destroys the liver, causing cirrhosis-like lesions and eventual failure.

A relative of the sunflower, the poisonous Eurasian weed, tansy ragwort, thrives in coastal Washington, Oregon, northern California, and in the New England States. Likely brought to both coasts of the United States in ships' ballast, tansy ragwort has spread eastward from the Pacific coast. First documented in Oregon in 1922, the ragwort's eastward progression was probably aided by human transportation of contaminated hay and straw. Since a single specimen of this prolific weed can produce up to 150,000 seeds, which can remain viable for three to fifteen years, tansy ragwort can quickly establish a foothold in a given pasture.

Photo of Tansy Ragwort
Poisonous tansy ragwort is an enemy to ranchers and beekeepers alike.

If ingested by grazing horses or cattle, tansy ragwort destroys the liver, causing cirrhosis-like lesions and eventual failure. By the time symptoms of ragwort poisoning appear, it is too late to save the animal. It's a drawn-out death that can occur up to six months after initial consumption. Aside from killing livestock, the weed displaces native grasses and ruins honey produced by bees that gather its pollen.

To control the noxious weed, several of the plant's natural enemies were released- the cinnabar moth in 1960, the seedhead fly in 1966 and the flea beetle in the late 1960's. These three insects work best in concert to control the weed, each feeding on a different part of the ragwort and at a different time of the year. In Oregon, a biological control program reduced the tansy ragwort population by 90% over 17 years. The reduction has saved an estimated five million dollars per year.

But tansy ragwort is still a problem in places, including parts of Idaho, Montana and British Columbia, where the insect species have not been able to establish populations. In these areas, people will have to resort to the more labor-intensive and costly means of control, such as mowing, tilling and applying herbicides to keep their pastures ragwort-free.

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Photos: Nova Scotia Museum

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