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Tansy ragwort

There is more tansy ragwort than usual this year. There are several possible reasons. First, a cold, wet April and May slowed and probably hurt the emergence of the cinnabar moth which consumes tansy. This is discussed below. Second, I see more over-grazed pastures. The best defense against tansy ragwort is a strong stand of grass. Third, fewer people are spraying tansy before it goes to seed. Perhaps it is a lack of knowledge of tansy ragwort’s poisonous character.  At the end of this article, I go into more detail about control methods.

Tansy ragwort is a biennial plant, which means that seeds germinate in the spring/early summer to produce “rosettes” which winter over. The plant then grows the following spring and early next summer, sends up a stalk with yellow flowers, goes to seed, and dies. The seeds are dispersed by wind though a recent report says the seeds generally go less than 10 feet.

Tansy ragwort appears to have been introduced into Oregon from offloaded ballast of a French ship docked in Tillamook Bay in the late 1920s. Tansy ragwort (Senecio jacobea) is native to the Mediterranean region of France and Spain and found our mild wet winter/dry summer climate much to its liking. Over the next thirty years, it spread up and down the coast, north into British Columbia and south to the San Francisco Bay region. By the 1960s, much of western Oregon was covered with tansy’s bright yellow blooms in mid to late summer.

As the invasion progressed, it became apparent that tansy ragwort didn’t just reduce pasture production but was a seriously poisonous plant. European research had shown tansy contained alkaloids that had a profound impact on liver function of susceptible livestock. Horses, cattle and hogs were the most afflicted. Further research in Oregon showed the cumulative pattern of liver damage. Susceptible animals have to eat a certain percentage of their body weight in tansy ragwort to cause enough liver damage to create serious health problems. For cattle and horses, this is about 4% or 7% respectively of their bodyweight in dry weight of tansy over their lifetime. For a 1200 pound animal at 7%, it would be 84 pounds of dry weight of tansy which equals 560 pounds fresh weight (moisture content of fresh tansy is about 85%) over their lifetimes.

That seems like a high amount of tansy considering they avoid it when it is in bloom unless there is nothing else green to eat. However, horses and cattle eat most of their tansy when spring pastures are lush and when it is hard to separate a mouthful of grass from the tansy mixed in. When tansy covered the fields in the 1960s and early 70s, older cows and horses were the ones most often killed by tansy. Veterinarians routinely offered liver function tests to at-risk livestock. There is no cure once the liver has been damaged enough. With the decrease in tansy ragwort, tansy caused deaths are far less common now.

Additional Oregon research showed why sheep and goats were largely unaffected by tansy alkaloids. It turns out that the plant alkaloid is bound with a sugar and that it takes certain enzymes to cleave the bond and unleash the toxic alkaloid form. Goats and sheep don’t have the enzyme system and so the alkaloid passes through them. It can be in the milk, however, and pose a risk to humans (we are susceptible to tansy poisoning as well) consuming milk from goats feeding in tansy rich pastures.

As tansy spread and its impacts were documented, scientists explored the idea of importing insects that fed on tansy to slow the invasion. Field trips to France (oh, to have been one of those scientists!) found several promising insects that fed exclusively on Senecio species. They were brought to the United States and held in secure facilities while their food preferences were carefully examined. Several were eventually released and two prospered: the cinnabar moth and the tansy flea beetle.

The cinnabar moth is a red to purple moth with a black background that flies in the late spring through early summer. Its larva are orange and black (Go Beavs!) striped caterpillars that feed on tansy flowers and foliage. When caterpillar numbers are high, fields of tansy can be completely stripped of leaves. The flea beetle adult feeds slightly on tansy foliage but its larva feed directly on tansy root systems. Again, as flea beetle numbers rise, whole fields of tansy can disappear from larval root feeding. The introduction of these insects has dramatically reduced the amount of tansy ragwort in western Oregon. What we see this year is nothing compared to what the hillsides looked like in the 1960s and 70s.

No bio-control agent will ever eliminate a plant from the landscape since the insect numbers will drop as their feed source drops. Once the plants recover, insects increase slowly over several years. Blooming tansy ragwort was more prevalent this past summer than it has been for a long time. I think there are several reasons for this. First, tansy populations have been building over about ten years. Second, a wet spring does damage to cinnabar moth survival and we had one heck of a wet spring. There is little doubt that both insect populations will recover and ultimately knock tansy ragwort back.

In the meantime, pasture owners should consider several steps to reduce tansy in their fields. First, maintain a vigorous pasture. Consider fertilizing in both fall and spring to invigorate the grass. Don’t graze fields November through March if at all possible to give grass some grazing recovery time.

Fall is an excellent time to spray tansy rosettes that will bloom next year. We have much better spraying weather then than we have in the spring. In the spring, the window for tansy spraying ends, depending on location, between early to mid-May. After that, chemical control drops off quickly. Once the floral stalk is evident, brush hogging the plants into a million pieces is the best (temporary) solution. But it is possible that more tansy will be accidently consumed once it is in small pieces.

Another option is to clip the flowers and burn them. This is less time consuming than pulling whole plants and there is evidence that pulling plants leaves root pieces and may set their clock back a year. Thus, they may grow and bloom again next year. Clipping the heads can lead to a secondary bloom this year but old research at OSU indicated that the late summer blooms often didn’t produce fertile seed.

Any of the common pasture herbicides will do a fine job on tansy in the early fall (or spring) after the first good rain.  Some products include 2,4-D, dicamba, mixes of the two like Weedmaster™, Crossbow™, Escort™, Curtail,™ and a number of others. Some of these damage clovers, other do not. Call me if you have questions.

Most pasture herbicides have relatively modest grazing restrictions. For example, 2,4-D requires that you not pasture milking dairy animals within seven days after spraying and that you remove meat animals from a field that is being sprayed if you intend to slaughter them within three days of the spraying. The label also notes that you shouldn’t cut hay within 30 days of application. You probably don’t need the reminder, but you are legally responsible to use herbicides in the manner specified on that label.

For more complete information, go to the small pastures section of the PNW Weed Control Handbook at https://pnwhandbooks.org/weed/pasture-rangeland/small-pastures

There is one issue that is a little more complicated. If you have poisonous weeds in your pasture, an herbicide can increase their palatability to grazing livestock. Tansy ragwort is the most widely recognized problem in this regard but there can be others. Some farmers remove their animals for 2-4 weeks after spraying if they have a lot of tansy to avoid animals selectively eating tansy.

Biological control of Tansy Ragwort



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OSU Columbia County Extension

505 N. Columbia River Hwy

St. Helens, OR 97051

PH: 503-397-3462

FAX: 503-397-3467

For information on public transportation, visit the Columbia County Rider website.


We offer classes, workshops, opportunities for youth and adult volunteer activities:

Volunteer as a Master Gardener, Master Woodland Manager, or with the 4-H youth program

Nutrition education events

Home Garden educational events


Columbia County General State/County Resources

It’s the “Strange Case of Dr. Jekyll and Mr. Hyde” in a nursery setting
This "power food" may help counter effects of fatty, sugary diet
Wet landscapes are getting wetter and dry landscapes are getting drier
Oregon's Agricultural Progress
The research magazine for Oregon State University Agricultural Experiment Station
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OSU Extension Catalog
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