Tom's Blog

Monday, August 1, 2011

Woodland thistle: a savanna native

For some years we have been trying to get woodland thistle (Cirsium altissimum) established in our savannas, and we are finally having some success. This biennial is the tallest native thistle (hence its Latin name), its height often exceeding 8 feet.

From seed the first year it grows as a vigorous rosette. After overwintering, the next year it forms the tall flowering plant, which produces seeds. Occasionally it may not flower the second year, but continue as a rosette for several years before finally flowering. Such plant species are called monocarpic, which means "flowering once". (Hill's thistle can also behave as a monocarpic species.)

We started by throwing seeds out in some of our savanna areas, but never saw any plants. We then turned to greenhouse-grown plants which we transplanted. It grows vigorously in the greenhouse and transplants well. Most of the plants we set out grew as rosettes the first year and then flowered and set seed the next year. However, since they aren't perennial, once they flower they are gone. To get an established population, one needs continuous production of viable seed.

It seems now that we are finally getting this species established, as we are seeing more plants scattered throughout, even in areas where we had not set out transplants.

Although this species is found throughout eastern United States, in Wisconsin it seems to be found only in the southern part of the state (per UW Herbarium records). It has a C value of 6.


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