In Good Thyme - by Mariana Graham

Appeared in WNT, April 18, 2001

A dangerous weed is proliferating in Island County, a weed that is deadly to livestock, potentially harmful to humans, and a threat to the economic well being of our state. Tansy ragwort is a pretty but poisonous plant lethal to horses, cattle and other grazing animals. In the long term, its toxic properties are also perilous to humans via the food chain.

According to Judy Feldman, program coordinator of the Island County Noxious Weed Control Board, the control of tansy ragwort is a high priority in Island County, and she needs your help. Even if you live on a suburban cul de sac, the presence of tansy ragwort on your property may have far-reaching effects. Each plant is estimated to produce 5000 to 200,000 seeds which can be transported by birds, wind, and unwitting humans.

All parts of the plant are poisonous, but the flowers are the most toxic. Tansy ragwort contains alkaloids which cause chronic, cumulative poisoning and irreversible liver damage. While horses and range animals are its primary victims, scientific studies done in the late 1980s cite tansy ragwort as potentially carcinogenic and a possible contaminant to the human food chain via flour, milk, honey and herbal remedies.
First, let's distinguish it from another wild plant named tansy. The innocent common tansy, Tanacetum vulgare, is a perennial with ferny foliage and flat clusters of button-like, bright yellow flowers. It can easily be identified by its strong medicinal odor, which is repellent to humans and animals alike. In fact, our ancestors called it "fly flower", and hung it in windows and doors to ward off flies.

Toxic tansy ragwort (Senecio jacobaea) has a much different appearance. This biennial (it lives two years, often longer) spends the first year of its life as a ruffled rosette of deeply lobed dark green leaves growing close to the ground. The undersides of the leaves are pale, almost white, and covered with fine hairs. In its second year, it puts out one or more flowering stems from one to four feet high, crowned with flat-topped clusters of yellow daisy-like flowers. Each flower head is composed of several yellow discs surrounded by 13 yellow rays. Tansy ragwort has a woody rootstock anchored by a taproot.

Native to Europe and western Asia, tansy ragwort arrived in North America via sailing ships during the 19th century. It was first noted on Vancouver Island and infiltrated from British Columbia south to northern California, from the coast to east of the mountains, where its economic impact is potentially devastating to agriculture.

It usually establishes itself in locations that humans have disturbed, such as roadsides and recently logged areas. Often found in pastures where it can be consumed by animals, it is sometimes inadvertently baled with hay and straw which may be sold, spreading the menace. Although it's found throughout Whidbey Island, Ms. Feldman says that Holmes Harbor and Ledgewood are particular hot spots for tansy ragwort.
Combating this invasive, aggressive weed is rather like trying to eliminate a monster alien. Mowing just encourages growth. Burning is not recommended because the smoke is toxic. Cutting the stems reduces flowers, but stimulates the growth of side shoots. So how do you get rid of it?

If you've read this column before, you know that we Master Gardeners discourage the use of garden chemicals if at all possible. Although biological controls are available in the form of three tansy-destroying insects, several years are required to establish an insect population that will have an impact on tansy ragwort. While biological controls may be the long-term solution, the tansy ragwort problem is immediate and rampant. Correctly using specific herbicides may be the best option for eliminating large stands of this weed.

Spring is the time to use the herbicide 2,4-D, dicamba, or a combination of the two as recommended by the 2001 Pacific Northwest Weed Control Handbook. The earlier the application the better the control. Respraying for more than one year is necessary to control late-germinating seeds. Dicamba can be effective used in the flowering stage and in the fall. Always read and follow label instructions. Of course, if there are only a few plants, pulling or digging them out is your best option. Do this in the spring, when the ground is moist. When the ground is hard, plants may break off at the ground level. Ms. Feldman recommends wearing protective gloves at your pulling party. Always place plants in closed plastic sacks and put them in the garbage. If bags are marked "noxious weeds," transfer stations will take them at no charge. Don't allow animals or children near sprayed or pulled tansy. Fill in spaces left by the weeds with desirable plants.

"Tansy ragwort affects all of us, whether or not we own livestock," says Judy. "We're all neighbors on this Island. If we work together we can make an impact." For more detailed information on the control of tansy ragwort, see www.wa.gov/agr/weedboard or contact Mrs. Horton at 679-7327 (Susan Horton replaced Judy Feldman as Island County Noxious Weed Coordinator in 2003).