Why oh why is there so much tansy ragwort this summer?

large tansy ragwort plants with person in the background
Large tansy ragwort plants seem to dwarf this King County noxious weed specialist. Photo by Avery Bowron.

As if 2020 wasn’t hard enough, tansy ragwort is exploding this year! Noxious weed specialists who have worked on it for a decade or more think it is one of the worst years they’ve seen. Is it just the bad luck of 2020, or is there something more predictable going on? According to noxious weed expert Sam Leininger from Clackamas County Oregon, a lot of it is due to bugs and weather. Specifically, it’s what’s happening with the usually helpful tansy ragwort flea beetle combined with our rather pleasant spring weather this year. I’ll let Sam explain the situation in his own words.

“We are certainly seeing a massive surge in tansy ragwort here in Clackamas County. Like you I am receiving a flood of calls and emails about the issue. The reports are usually from one neighbor complaining about a robust crop on neighboring property.

field of tansy ragwort and a hay field separated by a fence
Contrast between an unmanaged explosion of tansy ragwort and a well managed but threatened hay field. Photo by Sam Leininer, Clackamas County, Oregon.

“In terms of the cause, I have had numerous conversations regarding the efficacy of the biocontrols over the years. Based on the mild wet spring/early summer we have had, I knew we were going to be inundated with tansy ragwort.

“As noted by Judith Myers [in an earlier email*], the flea beetle is the superior of the biocontrols for tansy. This underappreciated beetle certainly loses out in recognition compared to the flamboyant cinnabar moth. The damage that the flea beetle inflicts is on the root system of the tansy over winter. In a typical year, we tend to see a warm up in mid-May to early June which shocks many of the tansy plants at a time when their root system is weakened. This can result in significant mortality. This season, with the regular rainfall we had every 5-7 days from mid-May through June (https://or.water.usgs.gov/non-usgs/bes/harney_pump.rain), the tansy ragwort plants were able to persist through the damage of the flea beetles. So the surge in tansy we are seeing is the result of the flea beetle damage being undermined by the abnormal and regular rainfall. We saw a similar pattern back in 2011 with the wet spring as well (https://today.oregonstate.edu/archives/2011/aug/after-three-decades-control-invasive-tansy-ragwort-once-again-threatening-oregon). So as you drive around seeing all the fields of yellow, take a minute to thank a flea beetle for all the years that you don’t see fields of yellow.

“This isn’t a good situation, but all we can do is focus on outreach and education to promote action by affected landowners.”


—Samuel Leininger, WeedWise Manager, Clackamas Soil & Water Conservation District, Beavercreek, OR 97004, https://weedwise.conservationdistrict.org/

* In response to someone suggesting that the tansy ragwort explosion might be due to a reduced presence of the cinnabar moth caterpillars this year, Dr. Myers from UBC explained: “It is the flea beetle that is the more effective control agent. They are more difficult to see as they damage the root crown. It is a good year for all flowering plants. The beetles will be back but it may take awhile for populations to build.” Judith Myers, Professor Emerita, Dept. Zoology, University of British Columbia, email, 7/29/2020.

tansy ragwort flea beetle on leaf
Tansy ragwort flea beetle on tansy ragwort. Photo by Jennifer Andreas.
tansy ragwort plant with flea beetle damage
Flea beetle damage to tansy ragwort. Photo by Maria Winkler.
cinnabar caterpillars on a tansy ragwort plant
Cinnabar caterpillars eating the leaves of tansy ragwort. Photo by Maria Winkler.

For those of you here in King County struggling to keep up with your tansy ragwort, take heart that you are not alone! If you can at least cut it back before it goes to seed you can keep it from spreading as far. It’s better to bag up all the flowerheads and dispose of them as trash, but if that’s not possible, it’s better to cut it down than leave it to spread seeds to your neighbors.

For large infestations, we highly recommend coming up with a plan for this fall or next spring. If you would like help figuring out what to do, send us an email or check out our materials online as a starting place. If you have tons of bags or truckloads of tansy ragwort to dispose of (and you live in the service area of King County Solid Waste), send us your address so we can mail you a voucher to pay for noxious weed disposal at a King County transfer station.

If your neighbors or nearby public lands have large amounts of uncontrolled tansy ragwort, report it to us and we will do our best to get it under control, or at least queue it up for work in the fall or next year.

Every little bit of control that is done now will go a long way to reducing future work. Remember, tansy ragwort seeds can persist in the soil for over 10 years, and they are easily spread by wind, animals and other carriers to infest neighboring areas. And whatever you, please keep all horses, cows and other animals from eating tansy ragwort, even after it’s been cut. It is very toxic and can be deadly if there is enough exposure.

tansy ragwort in flower in a field with person cutting flowers off
Preventing tansy ragwort from going to seed by cutting and bagging flower heads.
tansy ragwort with flowers and seeds shown with a garbage bag
There may still be time to bag up tansy ragwort this month but be careful not to release any seeds.

tansy ragwort bags

More information about tansy ragwort:

Tansy Ragwort – July 2017 Weed of the Month

County’s program steps in when people need a helping hand to battle tansy ragwort

King County Tansy Ragwort Information

Washington State Tansy Ragwort Information



    • Thanks Jason! The State Weed Board might still have some in their storage. A bunch of us from WA and OR did a custom bulk order in 2016 with a company called Poly Bag LLC. We provided the text, size and thickness we wanted and they produced them. If you email me, I’ll send you the contact info I have, although that was a bunch of years ago now.

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