Tom's Blog

Friday, June 3, 2011

Purple milkweed observations

The purple milkweed (Asclepias purpurascens), an endangered species in Wisconsin, does well in the savannas at Pleasant Valley Conservancy. It is one of our "signature" species.

I have been following native populations since 1999. Each new population has been given a permanent marker, and is then followed in subsequent years. In addition, Kathie and I have been raising plants from seed and transplanting them into likely locations. Some of these transplants now are also permanently marked. At present there are 27 known populations at PVC, 16 of which were spontaneous and the rest from transplants.

I published a paper on the purple milkweed work in the September 2009 issue of Ecological Restoration, and a PDF can be found on the PVC web site.

This is the time of year when purple milkweeds are first making an appearance. Although not flowering yet, most of my permanent stands are tall enough to find, and are generally in flower bud.

Yesterday I surveyed all my marked populations. Of the 27 populations, all except 7 had stems. It isn't uncommon for a population to skip a year (absent, and then returns the following year), and in two cases I have had a population thrive for a few years and then permanently disappear. Also, some years a stand has lots more shoots than other years.

This year, one of my spontaneous populations (photo above) has 47 stems, all with flower buds, whereas the most it ever had before was 9. The photo to the left shows how dense this stand is. If all these stems flower, this should be a specatacular sight!

It's not clear why purple milkweed is so uncommon, since it is quite easy to raise from seed (see my paper). The most likely conclusion from my work is that pollination is quite variable. Despite having dozens of flowering umbels, seed set is often quite uncommon. Last year, for instance, only a "single" seed pod was formed from the large number of possible flowering stems.


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