Life History

Common tansy is a short to long-lived perennial forb that reproduces vegetatively by rhizomes and sexually by seed. Aging by counting annual growth rings on rhizomes (herbchronology) from five populations in Gallatin County, showed the average age of rhizomes was 5.4 years (n=14, sd=2.5). Longevity varied depending on site. Dryer sites tended to support more long-lived plants, up to ten years old. Live and dead rhizomes from a moist stream bank site showed rhizomes survived about three to four years.

The seed biology of common tansy has not been studied, but the potential seed production is large. Stem densities in Montana averaged 100 stems/m2 and each stem had one inflorescence at the apex. The inflorescence can have many flowerheads each with many flowers. In Gallatin County, Montana, a survey of five populations indicated the average number of flowerheads per inflorescence was 67 (n=12, sd=38.7). The mean number of flowers per flowerhead was 144 (n=10, sd=44), and the estimated number of flowers per stem was 9,966 (n=10, sd=4,642). However, a large number of the achenes appeared unfilled. The estimated number of filled achenes per plant was 2,553 (n=10, sd=1978) and estimated achenes per square meter of area infested was 198,625 (n=10, sd=209,280). No information was found on the viability, germination, seedling growth, or rhizome development. However, about 75 percent of seeds collected in October germinated after one month of cold stratification indicating high viability and the ability to germinate upon seed set. The small size of the achene suggests that seedlings are small and not competitive. Common tansy most likely needs moist, disturbed soil for seedling establishment.

Information on the timing of the vegetative and reproductive life history stages and their transitions is observational. Stem development from rhizome buds was observed in November, however, shoots usually emerge in spring after many perennial grass species have emerged. Leaf expansion was observed in mid-May and by mid-June; plants can reach heights of three or more feet (1 meter). Flower bud formation begins in June, the late-bud stage was observed in late-June, and plants bloom throughout most of August. Flowering, but not seed formation was observed in late October and early November along roadsides. Information on seed-set was not found. Flowers are reported to be pollinated by insects; however, common tansy has also been reported to deter pollinating insects in gardens. Stems and leaves die when soil moisture is depleted as early as August, but leaves will remain green through October and into November where moisture is available. Flowerheads remain intact and hold seeds through the fall unless mechanical action dislodges seeds from dried flowerheads. There are no obvious physical or morphological mechanisms of seed dispersal aside from the leverage action of tall, stiff, dried stems.

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Last Modified: 02/27/2008