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Tansy ragwort can kill horses and cattle. Goats are sometimes affected by it, but seldom sheep. The plants contain several different alkaloids that are concentrated mostly in the flowers, followed by leaves, roots, and stems. The plant itself isn't toxic until it is digested by the animal's system. Alkaloids in all parts of the plant, when eaten by livestock, are broken down by the animal's liver enzymes into toxic compounds which may kill the animal, especially if repeatedly consumed. If you raise horses or cattle, you must know this weed.
Tansy ragwort colonizes places where the soil has been disturbed or moved: roadside edges, pastures, and cleared land resulting from timber harvest. The stems grow 1 to 4 feet high with clusters of flower heads. Each flower head is composed of bright yellow, daisy-like flowers. Around the center of the flower are 13 petals, called ray flowers. Identification of this noxious weed is easy. If you see a cluster of small daisy-like yellow flowers, count the yellow individual petals around the center of one flower; 13 in number gives you identification of the tansy ragwort.
The King County web site at www.metrokc.gov has a good color picture of tansy ragwort.
Tansy ragwort plants spread by seed. They are biennial, meaning that the first year plant produces a rosette of leaves that clings close to the ground. The second year plant sends up a spike of stems and flowers, and then goes to seed. Seeds have a feathery white attachment resembling the parachute on a dandelion, and they can be spread a long way by wind. One plant alone can make between 500 and 200,000 seeds. Those seeds can sprout even after 15 years of dormancy in the ground.
Obviously it's important to identify this weed and keep it from going to seed. After seeding, the plant generally dies. However, a well-established plant can develop new rosettes and new plants from the roots. If the stalk is chopped or cut or trampled during flowering, the plant will develop new rosettes. This gives the tansy ragwort the unfortunate characteristic of being able to grow from both seeds and roots.
Mowing the plant is NOT recommended. It may destroy the seed stalks but will cause the plant to turn perennial and grow hundreds of rosettes from the roots.
If the infestation is small, hand pulling the plants before they seed can help with control. Water the ground before pulling, because it's necessary to get all the root fragments out. The plant will regrow from root fragments. After hand pulling on a small site, mulch the ground with about 3-4 inches of an organic mulch to keep tansy ragwort seeds from germinating.
Several herbicides are effective against tansy ragwort. During the flowering stage, when the stalk is tall, use dicamba. Be very careful with this pesticide. It can damage shrubs and trees if misused. Read the label carefully. If controlling only the short, first-year rosette stage, use 2,4-D. Again, these herbicides must be used with caution.
For more information about noxious weeds, call the Noxious Weed Board in your county. In Pierce County (253.872.2972), Snohomish County (Sunny Gorman, message number 360.422.6083) and Skagit County (360.336.9430). King County's number is 206.296.0290. For more information, contact local Noxious Weed Boards in those counties.
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