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Noxious weeds

King County, Washington

To offer a suggestion or report an error on the King County Noxious Weeds website, please contact Sasha Shaw, education specialist.

Tansy ragwort
Senecio jacobaea

Tansy ragwort flowers closeup - click for larger imageTansy ragwort is an invasive, toxic biennial weed from Europe most often found in pastures and along roads and trails. Although animals tend to avoid it, they may eat enough of it to become ill and even die.  The highest risk is after the plants have been cut or when mixed in with hay, because the plants are not as bitter then and just as toxic.  In spite of efforts to control it, tansy ragwort is widespread in the Pacific Northwest.

Tansy ragwort is often confused with an even more widespread weed called common tansy (Tanacetum vulgare), also a European species and somewhat toxic, but not generally consumed by livestock because of its strong odor and very bitter taste. The two "tansies" are most readily distinguished by their flowers. Tansy ragwort has outer ray petals on its blooms and common tansy just has button-like blooms with no outer petals.

Legal Status in King County, Washington

Tansy RagwortPublic and private landowners are required to control infestations of tansy ragwort that occur on their property in King County. Tansy ragwort is a Class B Noxious Weed in Washington, first listed before 1988.  Because it is widespread in the county, it has not been designated for required control by the Washington State Noxious Weed Control Board. However, because of the risk to livestock, it has been selected for required control by the King County Noxious Weed Control Board and it is on the list of Regulated Class B Noxious Weeds for King County.

For more information, see Noxious Weed Lists and Laws or visit the website of the Washington State Noxious Weed Control Board.

This species is also on the Washington quarantine list (known as the prohibited plants list) (external link) and it is prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts, seeds in packets, blends or "wildflower mixes" of this species, into or within the state of Washington.

History and impact

An invader from Europe, tansy ragwort was first seen in seaports in the early 1900's and is often spread in contaminated hay. When prevalent, tansy ragwort is one of the most common causes of poisoning in cattle and horses, caused by consumption of the weed found in pasture, hay or silage. Milk produced by affected cows and goats can contain toxins. Stock does not reject or avoid it in hay or silage; its poisonous alkaloids are unaffected by drying. Honey from tansy ragwort also contains the alkaloids.

Biology and morphology

tansy ragwort rosetteThe plant's stem is stout, erect or slightly spreading, and may be branched; often groups of stems arise from the plant crown. A biennial plant, tansy ragwort usually germinates in fall or early winter, lives through the next year as a rosette, then dies the following year after producing flowers and seeds. Its leaves are dark green on top, whitish-green underneath, and have deeply cut, blunt-toothed lobes with a ragged/ruffled appearance. Flower clusters develop on stout, leafy elongated stems that grow up to 6 feet tall; each flower cluster is composed of many bright-yellow flowers with (usually) 13 petals. Its seeds have a white pappus and are wind-carried, resulting in rapid spread of tansy ragwort infestations. A single large plant may produce 150,000 seeds, which may lie dormant in the soil for as long as 15 years. The plant's fibrous system of coarse, light colored roots spreading from the crown can produce small adventitious shoots when stimulated by mechanical destruction or pulling.

Tansy RagwortAdditional information on tansy ragwort

What to do if you find this plant in King County, Washington

Please notify us if you see tansy ragwort growing in King County. Our program staff can provide the property owner or appropriate public agency with site-specific advice on how best to remove it. We map all known locations of regulated noxious weeds such as tansy ragwort in order to help us and others locate new infestations in time to control them.

Additional tansy ragwort photos - click thumbnail for larger image

Tansy ragwort leaf closeup - click for larger image tansy ragwort rosette - click for larger image Tansy Ragwort Tansy Ragwort
Tansy ragwort in a woodland - click for larger image Tansy Ragwort tansy comparison - click for larger image tansy ragwort flowers - click for larger image

Report tansy ragwort in King County, Washington

Locate tansy ragwort in King County, Washington

Related information

Related agencies


Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact staff, see the Noxious Weed Control Program Directory, send an email, or call 206-477-WEED (206-477-9333).