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Invasive Weed of the Month: Tansy Ragwort posted on 06/30/2023
Local News

Tansy ragwort (Senecio jacobaea) is an invasive weed with a long and deadly history in the Pacific Northwest. In Oregon, it is designated as a Class B invasive weed. It is believed to have been introduced here in the early 1900s through ballast water from a ship. This plant is native to Europe and Asia but is now well-established in Western Oregon.

This bright yellow weed is a familiar sight in our rural communities. It likes a cool, wet climate, well-drained soils, and full to partial sun. Patches of tansy are found in pastures, fields, grasslands, horse trails, and range land. It also grows along roadsides and in vacant lots, waste places, riparian areas, forested areas, and clear cuts.

How Can I Identify Tansy Ragwort?
Tansy ragwort is a biennial plant. This means that it takes two years for it to complete its life cycle. It grows as a ground-hugging rosette in its first year. In its second year of growth, it transitions into its mature, tall, flowering form.

This aggressive weed can grow up to 6 feet in height at maturity but is usually 2-4 feet tall. It blooms in late spring to early summer with yellow flowers that form a flat cluster at the top. The stems of tansy ragwort are green, sometimes with a reddish tinge. The leaves are dark green and ruffled.
One adult plant can produce up to 200,000 seeds which remain viable in the soil for more than 10 years! If left to spread, it can form dense patches. Tansy spreads from seed or by vegetative reproduction when its roots or crown are injured, and new shoots develop.

Why Should I Care About Tansy Ragwort?
Tansy ragwort is a killer. This noxious weed is dangerous to humans and livestock due to the poisonous pyrrolizidine alkaloids in its tissue. This alkaloid causes liver damage when ingested. Horses and cows are especially susceptible to this poisonous weed. Death can occur after consuming 3-8% of body weight. Poor control of this weed in our rural communities can definitely lead to difficult relationships between neighbors.

Areas of greatest concern in Clackamas County are unmanaged pastures and disturbed areas. Tansy ragwort competes with and displaces native vegetation and forage. In open fields, grazing animals will generally avoid eating it. In heavily infested pastures, however, they may have few other options. Contaminated hay is a serious problem because it becomes impossible for animals to avoid eating the weed.
Tansy can cause serious health problems for humans. This can happen by eating meat from livestock that suffered liver damage from tansy ragwort. Harm can also occur by consuming animal products such as milk made from liver-damaged cows. Honey, made with tansy ragwort nectar, may also be harmful if eaten. Skin contact with the plant can also cause a rash.



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