When I first got the idea to write this article, I was going to title it “Bad Tansy, Good Tansy.” However, once I studied up on tansy ragwort (Senecio jacobaea) and common tansy (Tanacetum vulgare), I realized the title should be more like: “Bad Tansy and Not Quite as Bad Tansy.”
Every year around late summer, I have noticed in local ditches and in open fields, clumps of tall plants with pretty yellow flowers growing in profusion. I gathered from overheard conversations that these flowers were either “tansy,” “tansy ragwort,” or “common tansy” and that they were either the same plant (or not) or that one was good (or not) and one was bad (or not). So basically, the only thing I knew was that they were yellow. This year I decided once and for all to determine what was what…
Two Plants — Both on the Washington State Noxious Weed List
What I discovered was that there are two plants growing in Washington that include “tansy” in their common name. Common names can be confusing because they are not the “official” name of a plant, thus two very different plants can have the same common name. Nevertheless, there is general agreement in regards to tansy.
The noxious weed, Tanacetum vulgare, a class C noxious weed in Washington State, is called “common tansy”, or sometimes simply “tansy.” Senecio jacobaea, a Washington State class B noxious weed, is called “tansy ragwort” or just “ragwort.” Unfortunately, sometimes it is also called “tansy.”
As a class B weed in Washington, tansy ragwort must be controlled by landowners so that it does not spread. Because common tansy is a class C weed in Washington, the state recommends that it be controlled, but control is not required.
From a distance, it is very hard (at least for me) to distinguish between common tansy and tansy ragwort. Although references state that both plants can reach 5 feet tall, most of the plants I observed averaged between 3-4 feet in height and the color of their flowers were identical (yellow!).
However, once you get up close and personal with these two weeds, they are very easy to distinguish. The key is the shape of their flowers—common tansy has small button-like flowers (actually made up of many small flowers, the botanists tell me). Tansy ragwort has flower heads that appear to have small petals (the “petals” themselves are flowers, the botanists tell me).
The leaves of the two plants are dissimilar as well, but the difference is not that striking. Both have leaves that are deep cut and remind me a little of yarrow (Achillea millefolium). However, tansy ragwort’s leaves are a lighter in color and fleshier than those of common tansy.
So, why are these plants categorized as noxious weeds? Like all noxious weeds, these plants are not native to Washington State. This is not to say that all non-native plants are noxious weeds.
Tansy ragwort came in from Europe in ballast water and traveled here as a contaminant in the seed of crops such as alfalfa (Medicago lupulina). Common tansy was imported from Europe intentionally, as a medicinal herb and an ornamental plant.
The Washington State Noxious Weed Board notes that tansy ragwort is by far the worse plant of the two. All parts of this species are toxic to cattle and horses when they ingest it while grazing or in hay or silage. The toxins build up slowly, damaging the animals’ livers and eventually causing death. Unfortunately, the build-up of toxins is cumulative and sometimes difficult to spot right away—it can take place over months and even years. Sheep are immune to tansy ragwort toxins and can graze it effectively enough to control it.
King County recommends an Integrated Pest Management (IPM) approach to controlling the spread of tansy ragwort. This includes pulling it and the judicious use of herbicides. Mowing tansy ragwort is not recommended because it re-sprouts from the roots left in the ground.
Although common tansy has been used as a medicinal herb for centuries, there is now little evidence that it has any curative or palliative capabilities. The U.S. Forest Service cites sources telling of multiple medicinal and household uses. For example, common tansy was put in shoes to relieve fevers, was wrapped with corpses to aid embalming, and the tea was drunk to treat ulcers, constipation, hysteria, intestinal worms, rheumatism, jaundice, digestive problems, and to both induce abortion and prevent miscarriage. Now, however, it is known to be toxic or lethal to people who take large doses or take any amount for a long time.
Common tansy has also been prized for its beauty in the garden. However, while it can add color and architecture to an individual garden, it spreads rapidly, like most noxious weeds. Its short rhizomes and seeds can be spread by wind and water and are hard to contain.
Common tansy crowds out native plants in open areas, thus decreasing food and habitat for wildlife. Horses and cattle will not eat common tansy. However, as in the case of tansy ragwort, sheep will eat this weed and can provide a means of controlling its spread. If you find common tansy on your property, you can best serve the environment by eradicating it—cutting it before it goes to seed, pulling it, and so on.
As I get older, I need more aids for remembering stuff, so here is how I now distinguish tansy ragwort (bad) from common tansy (not as bad):
- Petals are poison
- Buttons are beautiful
OK, I know it’s not great, but it’s the best I could come up with. At least it may help you tell the bad from the could-be-worse!
For more information on tansy and other noxious weeds, see the fact sheets on the Washington State Noxious Weed Control Board’s web site:
John Neorr is a Washington Native Plant Steward and serves on the board of the South Sound Chapter of the Washington Native Plant Society.
Editor’s Note: When I worked at the University of Washington Herbarium, I had a long and very confused conversation with a graduate student about these two different tansies. I was talking about common tansy (Tanacetum vulgare) and she was talking about tansy ragwort (Senecio jacobaea), and, well, we got quite bewildered and miffed with each other until we figured out we were talking about two different plants!—S.G.