Also Known As: garden tansy, tansy, buttons, bitter buttons, golden buttons, mugwort.
Common tansy is a Class C weed. The Class C status allows counties to enforce control locally if desired. Other counties may choose to provide education and technical consultation.
Common tansy (Tanecetum vulgare L.), a pungent smelling perennial herb, was brought to the United States from Eurasia for horticultural and medicinal purposes. The plant reproduces by both seed and creeping roots. Stems are erect, brown or purplish-red in color, and dotted with glands. Mature plants grow from 2 to 5 feet tall. Fern-like leaves are alternate and deeply divided into numerous, individual leaflets with toothed margins. Flowering occurs from July to the fall, varying by location. Flowers are yellow and button-like without petal-like ray florets. They appear in dense, flat- topped clusters at tops of the plant. Seeds are yellowish-brown, 5-angled achenes, without 
pappus. Common tansy is distinguishable from tansy ragwort (Senecio jacobaea), which is non-aromatic and
has ray florets and seeds with pappus. Common tansy plants prefer full sun and good subsoil moisture, and commonly invade disturbed areas, pastures, ditch banks, and riparian areas. This prolific plant contains alkaloids and is potentially toxic to both humans and animals, although cattle and horses seldom graze the bitter plant once it matures. This aversion allows common tansy to out compete desirable pasture plants.
Control Methods
As with other rhizomatous perennials, common tansy is best controlled using a combination of control methods (e.g., mowing + herbicides + maintenance of desirable plant communities).
Physical/Mechanical: Small infestations can be hand-dug, but the entire root system must be removed since plants can resprout from severed roots. Gloves and other protective clothing should be worn to prevent absorption of toxins through the skin. Although tillage can produce root pieces that result in new plants, common tansy does not persist in regularly cultivated lands. Repetitive mowing before flowering can prevent seed production, but other measures (such as chemicals) are often also used to achieve control.
Chemicals: Common tansy can be managed with a number of herbicides, usually recommended to be applied between bud and bloom stages. The most effective choices contain metsulfuron (Escort®, Cimarron®), chlorsulfuron (Telar®), or a combination of the two. A non-ionic surfactant or methylated seed oil (MSO) should be added to the spray solution to help with herbicide uptake. These herbicides should not be applied directly to water, and herbicide labels should always be followed. Other effective herbicides include those containing glyphosate or dicamba.
More information can be found in the PNW Weed Management Handbook
USE PESTICIDES WITH CARE. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.
Biological: No insect biological control agents are currently available. Sheep and goats can be used to graze common tansy, but some animals may find it unpalatable.
Questions: contact Steve Van Vleet or phone (509) 397 – 6290
Photo credits included in PDF