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The Dirt | Weed of the Month August 2025 | Tansy Ragwort (Senecio jacobaea)

Tansy Ragwort is a toxic, invasive weed in Oregon, thriving in sunlit areas. Recommended removal: manual digging and herbicides.
Tansy ragwort Weed of the Month flyer.
A dense cluster of vibrant yellow flowers with green leaves in a natural setting.
Tansy ragwort. Photo by Bruce N. Newhouse.
A bush with clusters of small yellow flowers in a grassy field under a clear blue sky.
Tansy ragwort. Photo by Gabriel E. Campbell.
A green, leafy plant with curly leaves grows in dry brown grass.
Tansy ragwort rosette. Photo by Cierra Dawson.
A cluster of curly-leaf plants on grassy ground.
Tansy ragwort rosette. Photo by Cierra Dawson.
A dense cluster of vibrant yellow flowers with green leaves in a natural setting.
A bush with clusters of small yellow flowers in a grassy field under a clear blue sky.
A green, leafy plant with curly leaves grows in dry brown grass.
A cluster of curly-leaf plants on grassy ground.

 

Tansy ragwort is an invader of upland prairies, pastures, and anywhere in full sun. The leaves are distinctly crinkly and lobed. This species exhibits either biennial (<2-years) or perennial (>2-years) lifecycles depending on the suitability of its growing conditions. It begins as a basal rosette of leaves and eventually produces a flowering stem that is purple tinged in color and sparsely hairy. The root system of this species also varies from a single taproot to a branched woody base.

You’ve likely seen tansy ragwort while driving on the highway. It tends to put on a bright yellow display along roadsides when flowering between June-September. Despite its vividness, this species is a noxious weed in the state of Oregon and is extremely toxic to livestock. Left unmanaged, it can completely take over pastures, prairies, and grasslands, which presents both a major hazard to animals and greatly reduces vegetation diversity.

Managing tansy ragwort can be difficult, but it is extremely necessary. This plant reproduces exclusively through seeds which can last up to 15 years in the soil seed bank. Manually removing the plants (including roots) is best for small infestations. If you have large areas infested with tansy ragwort that would be too cumbersome to physically remove, applying herbicide is recommended. There are also three Oregon Department of Agriculture-approved biocontrol agents for tansy ragwort that help keep this plant at bay in Benton County. Read more about tansy ragwort biocontrol here.