July Invasive Weed of the Month: Tansy Ragwort

Tansy ragwort grows tall and flowers in the second year.

Tansy ragwort grows tall and flowers in the second year.

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 in native to Europe and Asia, but is now well established in Western Oregon.

Tansy ragwort 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, vacant land, waste places, horse trails, roadsides, rangeland, riparian areas, forested areas, and clear cuts.

This aggressive invasive weed can grow up to 6 feet in height at maturity. 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, and the leaves are dark green and ruffled.

Tansy ragwort is a biennial plant which 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. One adult tansy ragwort 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, either 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 deadly to livestock.

Tansy ragwort is a killer. This noxious weed is dangerous to humans and livestock due to a poisonous alkaloid in its tissue which causes liver damage when ingested. Horses and cows are especially susceptible to this poisonous weed with death occurring after consuming 3-8% of body weight. Poor control of this weed in our rural communities can lead to difficult relationships between neighbors.

Areas of greatest concern in Clackamas County are unmanaged pastures and disturbed areas where tansy ragwort competes with and displaces native vegetation. In open fields, grazing animals will generally avoid eating tansy ragwort, but in heavily infested pastures they may have few other options. Contaminated hay is particularly a problem because it becomes impossible for feeding animals to avoid consumption.

Humans can also be harmed from tansy ragwort by consuming the plant, consuming livestock suffering from liver damage from tansy ragwort, by consuming animal products such as milk (made from liver damaged cow), and honey (made with tansy ragwort nectar).

How Can I Control Tansy Ragwort?

Tansy ragwort forms a rosette in the first year.

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 tansy ragwort 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, where it can spread the toxic leaves, making it harder for grazing animals to avoid.

In the 1960s, several insects were introduced to help control tansy ragwort. These insects feed upon and weaken or kill the plant, but are not sufficient to completely control an infestation. The most recognizable of these is the Cinnabar moth. The bright yellow and black striped caterpillars of the moth feed on the flowering plant during the summer months.

Chemical control methods may also be considered when working in hay or pasture lands to prevent livestock poisoning. For 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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