If you have noticed clusters of yellow flowers in fields and roadsides this summer, it’s probably tansy ragwort, which is toxic to livestock. Benton SWCD has received numerous inquiries from landowners about the boom this year. Since the 1980s, tansy ragwort has been kept in check by three biological control agents.
- Cinnabar moth– this is the most well-known of the three biocontrols. Cinnabar moth larvae strip the plants of foliage and flowers from mid-spring through summer.
- Ragwort seed head fly– the larvae of this fly attack the flower receptacle and reduce seed production.
- Tansy ragwort flea beetle-the larvae of the flea beetle mine tansy ragwort roots. The adults feed on the rosettes.
So, with three great biocontrols, why are we seeing so much tansy ragwort? According to Sam Leininger of Clackamas County SWCD, “the warm and wet spring and the mild summer have completely ameliorated the impacts of the flea beetle. The root damage caused by the flea beetles weakens the root system, so if we get a strong dry out in spring they are very effective at controlling the tansy. The warm then wet weather cycle we saw all spring allowed plants to persist through this damage. This weather cycle also seems to have increased the inflorescence number and we are seeing about double the amount of flowering heads per plant compared to a standard year. We are seeing cinnabars, but they are not in high enough abundance to compensate for the deluge of tansy. Right now it’s a wait and see regarding whether the cinnabars will help us out enough to make a strong difference. Right now it isn’t looking good.”
In years like this, it is best to dig or pull out plants between May and June- after plants bolt but before they flower. If the plants are flowering, cut off seed heads, bag and seal and dispose of as trash. Then pull emergent rosettes the next spring. For more control information, view the informative references listed below.
- Tansy ragwort identification and control flyer created by King County Noxious Weed Control Program. December 2015.
- Tansy ragwort update (Biological Control & Herbicide Options) by Oregon Department of Agriculture and OSU Extension. August 2011.
- Best Management Practices for Tansy Ragwort created by Clackamas County SWCD. July 2016.
- Tansy Ragwort Poisons Livestock and Neighborly Relations! on Clackamas County SWCD website. July 2016.
- Biological Control of Invasive Plants in the United States. Editors Eric M. Coombs, et al. 2004. Western Society of Weed Science.