Tansy Ragwort
Senecio jacobaea

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

The 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.

As a Class B noxious weed, control is required in King County. For information on tansy ragwort identification and control, please download our tansy ragwort weed alert (793 KB Acrobat file).

For more information about tansy ragwort, please follow this link to the WA State Noxious Weed Control Board.

If you find tansy ragwort in King County, please notify us through our online infestation form.

To find out where we have records of this weed in King County, use our interactive noxious weed map and search for tansy ragwort.

Tansy ragwort photos - click thumbnail for larger image

For more information on tansy ragwort identification and control, please read the Tansy Ragwort Weed Alert (792.8 KB Acrobat file).

For detailed information on controlling tansy ragwort in King County, please read the tansy ragwort best management practices (46 KB Acrobat file).