Tansy ragwort has long tormented rural landowners who graze livestock. Horses and cows are especially susceptible to this poisonous weed.
Tansy Ragwort Flowers and Cinnabar Moth Caterpillar
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.
Tansy ragwort has a long history in Clackamas County. It was one of the few plants regulated under the former county noxious weed control district that was formed in 1949. At that time, landowners in Clackamas County could be cited for having flowering plants on their property. Back then, neighbors would come together for community tansy pulls to keep their horses and livestock safe and to avoid the dreaded visit from the weed inspector.
In the 1960s, several insects were introduced as biological controls to reduce the abundance of tansy ragwort. These insects feed on the plants and weaken or kill the tansy. The most recognizable of these is the crimson-red Cinnabar moth. The caterpillar for the moth feeds on the flowering plant during the summer months.
County Agent, Ron Davis, looks at a field of tansy, Clackamas County, ca. 1946
With the introduction of a flea beetle in 1971, we had the one-two punch needed to reduce the tansy ragwort problem to relatively low levels. Following the reduction of tansy ragwort, Clackamas County dissolved the Clackamas Noxious Weed Control Board on August 3rd, 1989 citing the effectiveness of the biological controls as well as budget constraints as the chief reasons for its dissolution.
In the years since the weed board was dissolved, tansy ragwort has continued to persist in Clackamas County, but typically at much lower levels than those encountered in previous generations. The biological controls introduced in the 1960s and 1970s are still working on our behalf!
The effectiveness of the biological controls can vary from season to season. The relationship between the biological controls and the tansy follows a typical predator-prey cycle. As the biocontrols become more abundant, the amount of tansy ragwort observed decreases. As the tansy ragwort decreases, the biocontrol numbers decrease leading to cycles of boom and bust.
In years with especially mild and wet springs, we see a strong revival of tansy ragwort seedlings. In these years, the effects of the flea beetle are dramatically reduced and we see tansy ragwort soaring to the top of Clackamas County’s least-wanted list of weeds.
This year, folks throughout Clackamas County are once again seeing the yellow flowers blooming in their fields. They are calling into the Clackamas Soil and Water Conservation District’s WeedWise Program asking for assistance in controlling this weed. But by the time tansy flowers appear, the best management of this weed is a good pair of leather gloves, a strong back, and a healthy dose of perspiration from pulling mature plants. Mowing and cutting plants can prevent seed production, but can also spread the poisonous vegetation around, making it more difficult for livestock to avoid. Tansy ragwort is normally a biennial plant, but mowing can cause it to behave like a perennial, meaning it will tend to come back year after year.
Some of our residents remember the neighborhood tansy ragwort pulls that were common many years ago. Now they seem to be a largely thing of the past. Tansy still poisons livestock but also affects the relationships of once cordial neighbors. Each year the Clackamas SWCD receives a large number of calls from neighbors complaining about their neighbor’s tansy plants.
There are no longer weed inspectors in Clackamas County, so our best recommendation is to work with your neighbors to control tansy ragwort. We have developed a Tansy Ragwort Best Management Practices document to help folks develop a management plan for their property.
Tansy Ragwort Management
Despite the onslaught of tansy ragwort, remember that all is not lost! Tansy ragwort is manageable. If you have a lot of tansy plants and are feeling overwhelmed, focus first on areas where grazing animals are present or along fence lines to help with your neighborly relations. Pull flowering plants and dispose of them as trash or pile them up away from grazing animals and then burn them when allowable.
If plants are already going to seed, cut off the seed heads and dispose of them as trash, or cover them and compost them on-site. Try to avoid spreading seeds further. Follow up the next spring by pulling emergent rosettes when the ground is still wet or use an approved herbicide recommended by the Pacific Northwest Weed Management Handbook that is appropriate for your property. Be sure to always read and follow the label directions on any herbicides you purchase. Work to keep a good vegetated cover on your ground. Avoid overgrazing, reseed good pasture grasses as needed, and implement rotational grazing practices when possible to rest vegetation over time.
Don’t let Tansy Ragwort poison your livestock, or your relationship with your neighbors: do the neighborly thing and control your tansy ragwort. Happy Pulling!
