Toxic tansy ragwort is having a boom year

Toxic to animals with livers, tansy ragwort fills a gap in the seasonal need for nectar for some butterflies
and other insects. Above, an Oregon branded skipper rests on tansy ragwort flowers. (Ann Potter photo)

By Alex Meacham

— Pernicious, invasive and even sometimes deadly for livestock, tansy ragwort has enjoyed a booming bloom this summer in Whatcom, Skagit and San Juan counties and across the rest of western Washington and Oregon. Local weed boards and landowners seeking to eradicate the weed say they are finding more sites than ever this year.

Tansy ragwort contains pyrrolizidine alkaloids, chemicals which can build up in an animal’s liver and eventually prove fatal. At least one farm animal in the area has died from tansy ragwort poisoning this year, according to Joseph Shea, coordinator of the Skagit County Weed Board.

Some members of the Cascadia Prairie-Oak Partnership (CPOP) and other individuals around the region have said they suspect that the plant’s rapid growth this year across the region may have been due to the effects of climate change on the timing of the winter season and a cooler and rainier May and June. CPOP is a consortium of people and organizations promoting species recovery across the West Coast.

Tansy ragwort is classified as a Class B weed — non-native weeds designated for mandatory control in some regions — by the Washington State Noxious Weed Control Board and has limited-to-widespread distribution in the state. Even where there are very few plants present, the board directs that they should be removed.

Late-summer boon for pollinators

Yet while tansy ragwort — Jacobaea vulgaris, syn. Senecio jacobaea, a tall, yellow-petaled stalky plant — is a threat to the region’s pastured animals, it is a valuable source of sustenance for some imperiled butterflies at a time when there are few or no other food sources.

Ann Potter, a conservation biologist and insect specialist, has for many years been studying the Oregon branded skipper (OBS), Hesperia colorado oregonia, which relies heavily on the plant for its lifecycle. The OBS leaves the caterpillar and pupae stage (“flies” in lepidopterist parlance) and turns into a butterfly in the very late summer. This timing means that there are very few native plants that they can feed on. Tansy ragwort fills that hole. 

Dozens of other insects like honeybees and bumblebees, pollinators of our ecosystem, also feed on tansy ragwort in late summer. 

Since tansy ragwort is nearly impossible to eradicate, Potter said that efforts should strive to control it, recognizing the important role it plays for late-season pollinators. 

Potter said that the OBS isn’t listed as endangered because it takes a lot of work to petition for listing and insects aren’t especially iconic. There are lots of insects that listing could protect, she said, but there aren’t many people working on butterflies in Washington state.

Vigorous and prolific

Invasive, adaptable, pernicious tansy ragwort has been
identified in at least 645 sites in Whatcom County this year.
(Courtesy Whatcom County Noxious Weed Control Board)

The Whatcom County Noxious Weed Control Board was formed in 1982 after many animals died from tansy ragwort. 

According to Laurel Baldwin, the board’s program coordinator, tansy ragwort is especially threatening to livestock that eat hay that was grown, cut and dried alongside tansy ragwort and thus contaminated by the weed. It is also a threat in the spring if mixed in with forage grass.  

Tansy ragwort is especially hard to get rid of because it can regrow from its root base.  If the plants are mown, they come back the next year — and can grow several inches per night, according to Baldwin. She has found plants in Whatcom County up to about 7 feet tall. 

Each plant has hundreds of thousands of dandelion-like seeds that can float on air and water currents, and the plant can go to seed even after being cut. 

There are currently 645 sites in Whatcom County, 65 of which are new this year. According to Baldwin, that’s probably an underestimate. There are only two people on her team going out to find the plants — along with around 150 other plants that the staff of the weed board are searching for. 

Bag and uproot it, or get Nature to help

Todd Beld’s family runs Meadow Fed Farms in Everson, raising pasture-fed livestock. This year, the tansy ragwort on his farm has doubled or tripled compared to years past, according to Beld. The plant grows in bare spots where the plants cattle graze on don’t grow, so Beld can control the tansy ragwort by spraying in spots rather than fighting a whole field at once. 

Landowners trying to eradicate this plant should snip the flowers into a plastic bag and pull the plants up from the root, leaving nothing behind and minimizing the spread of seeds, local officials advise. 

“It’s a very adaptable plant, which is one of the traits of a good noxious weed,” said Baldwin. “It can occupy all of the ground in a pasture pretty easily.”

In San Juan County, the Noxious Weed Control Board has provided free disposal of tansy ragwort. But due to the rapid spread this year, a recent press release announced they’re having to curtail the program. Residents will be able to dispose of only one 32-gallon bag of tansy ragwort for free. The program had a $7,500 planned budget, but has come close to $15,000 in noxious weed disposal fees, according to Jason Ontjes, Noxious Weed Program Coordinator for San Juan County. (See a video on how to control tansy on the board’s website.)

Another option for controlling tansy ragwort also provides food for some creatures. The cinnabar caterpillar (a tiger-striped caterpillar that becomes a red and black moth) and the flea beetle (a small, black beetle that puts holes in the leaves of tansy ragwort) both feed off of the plants, making it harder for them to survive and spread. 

According to Baldwin, this threatening plant isn’t going away any time soon, so people should be aware of it and report potential sites to the weed board.

For a look at how tansy ragwort is impacting another local in the region, see “Rampant tansy causing a weed ‘pandemic’” in the Kitsap Sun (paywall).

Alex Meacham is a recent Western Washington University graduate with a degree in environmental journalism. He spends his free time playing with his dog and enjoys craft beer, cooking and videography.