Life History

Tansy ragwort’s life history is typical of biennial plant species: it reproduces for the most part by seed; it overwinters in the seed or rosette stage; and it passes through one growing season in the vegetative rosette stage before becoming reproductively mature. However, because both the root and the caudex of this species have the capacity to form perennating buds under environmental or mechanical stress, tansy ragwort has the vegetative regenerative capacity of a perennial. A study of 179 plants in Australia found 2% were annuals, 45 percent (%) biennials, and 39% were perennials. A separate study in England found 8% of the study plants were annuals, 39% were biennials, and 53% were perennials. Tansy ragwort has been observed as a true perennial by continuing to grow after flowering, particularly after disturbance. An Oregon study reported 20% of the tansy ragwort plants in a population were true perennials.

Tansy ragwort has four distinct life history stages: seeds, seedlings, rosettes, and flowering plants. The viability of a tansy ragwort seed crop is about 80%. The seeds have no mechanisms of innate dormancy other than the relatively thick pericarp on ray floret achenes which is believed to delay germination by a few days. The maximum potential for germination is reached when achenes leave the flowerhead: however, experimentation showed this potential did not significantly decrease after three years of storage under field conditions. Most seeds germinate in the late summer or early autumn, but seeds lying dormant throughout the winter will germinate at the beginning of the growing season in the spring. Ideal temperatures for germination range between 41-86^o Fahrenheit (5-30^o Celsius). Soil moisture influences germination: at any given temperature an increase in soil moisture increases germination and the optimum temperature for germination changes as soil moisture changes (maximum germination is reached at a lower temperature when soil moisture is high). Vegetative cover may inhibit germination and frost, drought, or burial may induce dormancy. Seeds buried in soil deeper than 0.75 inch (2 centimeters) remain dormant. Light is required for germination. Studies showed 24% of seeds buried for six years maintained their viability. Also, seeds buried up to 2 centimeters deep had a higher germination rate than seeds left on the soil surface, most likely because moisture is consistently greater below the soil surface than on the surface. Seeds subjected to drought or cold temperature shock had delayed germination; the longer the shock, the longer the delay.

Seedling establishment and survival are variable, the largest determinant of survival being the amount of vegetative cover. In experimental garden plots two months after seeding tansy ragwort, no seedlings survived in plots with long grass or in plots with short but continuous turf, whereas seedling survival was greater in cleared or woodland areas than grassy areas. Grazed pastures on Prince Edward Island, Canada, had about eight times more tansy ragwort plants than ungrazed pastures.

Once established, seedlings form flattened rosettes that are effective competitors. The large rosette leaves are able to cover neighboring plants, thereby suppressing their growth. When the rosette leaves die, sites are opened for tansy ragwort seed germination. One study found seedling establishment was over four times higher on sites opened by dying rosettes than on sites in surrounding vegetation. Alkaloids produced by rosettes may have the allelopathic effect of suppressing other plants. Rosette leaves die when plants flower.

Plants typically flower during the second year of growth. Flowering has been reported to begin as early as June and to last as late as mid-November. It is believed rosettes must achieve some minimum size before flowering can begin, and the probability of flowering increases as rosette

size increases. Floral expansion is rapid and florets are receptive to pollination as soon as the floret is fully expanded. Hymenopteran (bee/wasp) and dipteran (fly) insect species are the primary pollinators of tansy ragwort. Seeds disperse throughout the fall. Rosettes that do not flower continue to grow into the fall accumulating storage carbohydrate important for winter survival. Carbohydrate content of the plant is highest going into winter.

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Last Modified: 06/18/2009