A List B perennial member of the sunflower family (Asteraceae). Originally from Europe and Asia, it is an escaped ornamental. It has been known in Colorado since 1890 and the US since the 1600s. Also known as bitter buttons, cow bitter, or golden buttons.
Common tansy forms thick patches. It reproduces by seed and rhizomes and grows to 1.5-6 feet tall. Stems grow from the root crown and are persistent. The stems branch near top and are sometimes purplish.
Leaves are alternate, pinnately compound, deeply divided, 1-6 in wide and 2-12 in long. They have a strong smell when crushed.
Common tansy has rayless button shaped flowers. Each flower is ¼ – ½ inch wide and are in groups at the end of the branches. July-Sept.
Common tansy repels insects and is sometimes used as a companion plant. It is poisonous to livestock.
It has been used as an herbal remedy but is toxic at high levels. It contains thujone, a neurotoxin. Do not confuse it with Blue Tansy (Tanacetum annuum).
Control includes removal of new populations. For established populations, mow monthly and treat with herbicide in the fall.
CWMA Nox Weeds of Colorado
All images by JCISM
Weed of the Month is a joint effort between Jefferson County Invasive Species Management, CWMA, and Archuleta County.