Common Tansy (Tanacetum vulgare)
Common tansy is an herbaceous perennial that typically grows 3–5 feet tall, especially in shaded or moist areas. Plants form dense clumps and spread through a network of creeping lateral roots.
Stems are erect and somewhat woody at the base but die back each year. Toward the top, stems branch extensively into short stalks that form flat-topped clusters of numerous button-like flower heads.
Leaves are alternately arranged along the stem and are pinnately divided with toothed margins, giving them a fern-like appearance. Leaves become progressively smaller toward the top of the plant. New plants or shoots arising from rhizomes may produce only low-growing leaves without fully developed stems.
Flowering occurs from early summer through fall. Flowers are arranged in corymbs (flat-topped clusters) at the ends of stems. Each flower head is small (¼–½ inch wide) and composed of dense clusters of bright yellow disk flowers, lacking the showy ray petals typical of many daisies.
Common tansy has a dense, creeping root system that allows it to spread aggressively. Seeds are small (1–2 mm), ribbed, and dotted, and are topped with a small pappus that allows limited wind dispersal.
This species produces the monoterpene ketone thujone, a toxic compound that can be harmful to humans, livestock, and wildlife. Although animals generally avoid it, ingestion can cause convulsions and spontaneous abortion in humans, and plant contact may cause skin irritation. Care should be taken when handling common tansy.
Quick ID Guide
Life Cycle:
• Perennial
• Spreads by seeds and creeping roots
Height:
• 3–5 feet tall
Stems:
• Erect
• Woody at base
• Form dense clumps
• Highly branched at the top
Leaves:
• Alternate
• Fern-like
• Pinnately divided
• Toothed margins
Flowers (Early Summer–Fall):
• Bright yellow
• Button-like
• Flat-topped clusters
• No ray petals
Roots:
• Dense, creeping lateral roots
Seeds:
• Small (1–2 mm)
• Ribbed and dotted
• Limited wind dispersal
Management Options
Prevention and cultural control strategies should be emphasized whenever possible.
There are currently no approved biological control agents for common tansy.
Hand pulling or digging can effectively eliminate small patches if all lateral roots are completely removed. Extreme care must be taken to extract the entire root system to prevent regrowth.
Mowing before flowering can prevent seed production and may improve herbicide coverage by opening the canopy. Do not mow once seeds are present. Tillage is not recommended, as it can fragment and spread reproductive roots.
Controlled grazing with sheep or goats may help reduce seed production and deplete root reserves. Both species can consume common tansy in relatively large amounts without toxic effects.
Large infestations or areas where manual removal is impractical can be treated with herbicides. Applications are most effective when plants are in the bud stage. Fall treatments may improve control by targeting the root system. Always read and follow herbicide label instructions before application. Contact Sublette County Weed & Pest for assistance with product selection, rates, and timing.
