Author: Melissa Fery, Small Farms Program, Oregon State University Extension Service
Publish Date: Spring 2018
Tansy ragwort, Senecio jacobaea, is a noxious weed that causes alarm for most cattle producers and horse owners. The poisonous alkaloids in this plant causes irreversible liver damage to animals (and humans) if consumed. All of its parts are toxic, with the highest amount of alkaloids in flowers then leaves, roots and stems and the plant remains toxic when dried in hay.
In the summertime, with showy yellow flowers standing tall, tansy ragwort is easy to identify and seeing it prompts people into action. The problem is that in July and August, management options are few and landowners are often discouraged by the recommendation given to manually dig and bag up the plants to take to the landfill.
In the spring however, there are several effective management options. Right now is time of year to walk through your fields and identify tansy ragwort. If tansy was a problem on your property last year, you will likely find young plants. Right now, plants are actively growing at the rosette stage with ruffled dark green leaves that may have a reddish tinge.
Biological control is working right now too. The commonly known cinnabar moth will be seen later in the year, but right now the ragwort flea beetle, Longitarsus jacobaeae is out in force and devouring tansy plants. The adults feed on the leaves and the larvae damage the roots. Look for these golden to light brown colored beetles on and under the leaves of the plants. If you find them on your site, consider leaving some tansy ragwort as a food source, especially in areas that may not impact your livestock.
Sheep can also help manage tansy ragwort, as they are known to tolerate the toxic alkaloids. However, they may choose to graze other desirable plant species before consuming large amounts of tansy. Nevertheless, grazing sheep on your property will help with long-term weed management.
Since the ground is moist and the plants relatively easy to pull, right now is the time to manually remove tansy. Tansy spreads vegetatively, so be sure to remove the fleshy taproot otherwise, the plant will regrow. Right now, these young plants could be added to a hot compost system, buried or added to a burn pile. Mowing is not a suggested management practice as it stimulates more vegetative growth.
If you have a large infestation, you may choose to apply an herbicide. All of the broadleaf herbicides labeled for tansy ragwort are most effective on young, actively growing plants. If you would like to spray to help manage tansy you need to do it right now. Oregon spring weather is unpredictable; plan ahead and watch for a window of calm, dry weather to spray. The Pacific Northwest Weed Management Handbook includes a list of labeled herbicides for tansy ragwort: https://pnwhandbooks.org/weed.
Finally, the truth of the matter, which is tansy ragwort infestations are often the worst in overgrazed pastures with bare or compacted soil. Along with managing weeds this spring, also consider ways you can manage pastures that will enhance forage growth for the long haul. Information about pasture and grazing management is available at: http://smallfarms.oregonstate.edu/pastures. Start planning, right now, if you are considering replanting your pasture in the fall.