Jacobaea vulgaris • Class B

Tansy spends its first year in the rosette stage with dark green ruffled basal leaves.  Rosettes usually overwinter and produce one or two flowering stalks in the next growing season.

The plant grows to heights of between 2 and 6 feet. It develops a stout taproot from which numerous fibrous roots grow about 1 foot deep. 

The leaves are light to dark green and deeply lobed.  The upper part of the stem is highly branched and bears up to 250 bright yellow, daisy-like flowers. Each flower has about 13 petals which helps to distinguish it from two other weeds that are similar in appearnce; St. John’s Wort, which has only 5 petals per flower, and Common Tansy, which has button-like flowers with no outer ray petals. 

It is highly invasive and difficult to control. Tansy Ragwort is toxic to livestock.  Its toxic alkaloids pose a threat to humans through food chain contamination.  

Tansy ragwort is a problem in grasslands, disturbed areas, forests, pastures, rangelands, and clear-cuts. Tansy ragwort occurs on many different soil types. 

Tansy Ragwort contains pyrrolizidine alkaloids which lead to liver failure. This weed is toxic especially to cattle, horses, deer, pigs, and goats.

• The most effective control is prevention. Above all else, prevent plants from going to seed. 

• Hand-pulling is effective on small infestation sites. Tansy can re-grow if the roots are broken off, or anytime the plant is cut. Covering the site with mulch will help prevent new germination from the disturbed site. 

• Maintain desirable vegetation. Overseed with desired grass or pasture seeds in fields, especially on bare ground and disturbed soil areas after mechanical control (hand-pulling).

• Mowing is not recommended. Mowing will prevent seed production; however, any damage to the stalk will cause the plant to keep growing as a perennial. 

• Spot spraying with an herbicide containing the active ingredient glyphosate may be used effectively during the period of time from rosette stage to full flower. Applications made after the petals turn brown are ineffective. Herbicide application should take place when plants are actively growing and before seeds are produced. Spray each plant thoroughly on the stems and leaves, enough to be wet but not dripping. Be aware, glyphosate is non-selective and will injure any plants that it comes in contact with, including grass. 

• For selective control of Tansy ragwort in agricultural settings (pastures, hayfields, etc.): an herbicide containing the active ingredient aminopyralid may be applied while the plant is actively growing, before seeds are produced. It is also effective in the fall before a killing frost. Aminopyralid products will not harm grass and can be used around livestock (provided all label precautions are followed). 

• When using herbicides, read and follow all label instructions and obey all label precautions. (Note: pesticide product registration is renewed annually and product names and formulations may vary from year to year.) 

• To minimize any harmful impact on bees and other pollinators, timing is important.  Ideally treat plants before blooming.  If treatment after blooming is necessary, do control work early in the morning, or in the evening when bees are less active.

• For information about the biological control of Tansy Ragwort, see the WSU Extension Integrated Weed Control Project 

 Download our Flyer or visit Washington State Noxious Weed Control Board Here. Photo by Leo Michels