Tansy ragwort (Jacobaea vulgaris) has long tormented hay producers and rural landowners who graze livestock. Horses and cows are especially susceptible to this poisonous weed.

In open fields, grazing animals will generally avoid eating this invasive weed. In heavily infested pastures, however, they may have few other options. Contaminated hay is a particular problem because it becomes impossible for feeding animals to avoid consumption.

Noxious Weed Control District and Bio-Controls

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 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.

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, however, Clackamas County dissolved the Clackamas Noxious Weed Control Board on August 3rd, 1989. They cited 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. It is present in much lower levels than those encountered in previous generations. The biological controls introduced in the 1960s and 1970s are still working on our behalf!

Where Have All of the Caterpillars Gone – And Where Can I Get Some More?

Glenn Miller, the Northwest Oregon Invasive Weed Management Coordinator for the Oregon Department of Agriculture, shared information about the current status of the cinnabar moth. “We haven’t done a release of caterpillars in over 20 years, ” said Miller. He agreed that populations of the moth and its larvae are decreasing in Oregon. He noted that “Changes in climate and climate instability have had a negative effect on the survival of our moths and larvae. Tansy is blooming earlier in the season now and getting ahead of the existing larvae which just can’t keep up.”

Landowners often ask where they can obtain cinnabar moth larvae to repopulate their pastures. Miller says “There is no source for purchasing these caterpillars. The original releases came from caterpillars we provided which we hand collected from locations that had an overabundance. We don’t have any now to give, nor are we able to recommend any place where they can be obtained.”

Miller also said that the tansy ragwort flea beetle (Longitarsus jacobaea) is a much better bio-control for this invasive weed than the cinnabar moth. “People can’t see the flea beetle like they can the caterpillar or moth, so they don’t think anything is happening, but they’re there and very effective.” “Bottom line,” says Miller, “Folks need to get out there and pull their tansy to keep it from reseeding. We can’t just count on these insects to control it for us.”

The Tansy Seems Worse This Year. Is It?

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 the Clackamas Soil and Water Conservation District (SWCD) 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 only 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. This means that it will tend to come back year after year.

Education is Key to Controlling Tansy Ragwort

Some remember the neighborhood tansy ragwort pulls that were common many years ago…but now they seem to be a 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. The District has developed a Tansy Ragwort Best Management Practices document to help folks develop a management plan for their property.

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. Focus too along fence lines to help with your neighborly relations. Pull flowering plants and dispose of them as trash. You may also 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. Try to avoid spreading seed further. Follow up the next spring by pulling emergent rosettes when the ground is still wet. You may also 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 good vegetated cover on your ground. Avoid overgrazing and reseed good pasture grasses as needed. Rotational grazing practices should be used when possible to rest vegetation over time.

The article was contributed and reprinted with the permission of the Clackamas Soil & Water Conversation District and may contain minor editorial edits. View the original article by clicking here.