July Invasive Weed of the Month: Tansy Ragwort

Tansy Ragwort is manageable

Tansy Ragwort is manageable.

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 1900’s 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.

Look for the plant’s low-growing, ruffly rosette in the first year.

How Can I Identify Tansy Ragwort?

Tansy ragwort is a biennial plant. This means that it takes two years for it to complete its lifecycle. 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 which 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 can 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.

Don’t be fooled by its pretty yellow flowers. This weed is deadly to livestock.

Why Should I Care About Tansy Ragwort?

Tansy ragwort is a killer. This noxious weed is dangerous to humans and livestock due to a 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 feeding animals to avoid consumption.

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.

Cinnabar Moth is a bio control for tansy, but provides minimal control. (Photo: Eric Coombs, ODA)

Cinnabar Moth is a bio control for tansy, but provides minimal control. (Photo: Eric Coombs, ODA)

How Can I Control Tansy Ragwort?

Tansy ragwort can be controlled manually by digging or pulling in spring and summer before they flower. Rosettes should be dug up, removing as much as the root as possible. Because this weed is toxic, be sure to wear gloves and protective clothing when removing tansy. All pulled plants should be bagged and placed in the municipal waste. Once plants bloom, be sure that pulled plants and flower heads are bagged and placed in the municipal waste.

Mowing is not a good control for tansy ragwort. While it may prevent the plant from immediately producing seeds, it also stimulates additional vegetative growth. This leads to more plants and more stems per plant in the same season. Mowing is especially problematic in pastures. Mowing can spread the toxic leaves, making it harder for grazing animals to avoid.

In the 1960s, several insects were introduced as a biocontrol. These insects feed upon and weaken or kill the plant. Unfortunately, they are not enough to completely control an infestation. The most recognizable of these biocontrols is the Cinnabar moth. The bright yellow and black striped caterpillars of the moth feed on the flowering plant during the summer months.

Landowners may consider chemical controls when working in hay or pasture lands to prevent livestock poisoning. Want more information on how to control tansy ragwort with chemical controls? Please contact the WeedWise program at 503-210-6000 or review our Tansy Ragwort Best Management Practices.

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Clackamas SWCD