Tom's Blog

Wednesday, July 2, 2008

Butterfly milkweed at its peak

Butterfly milkweed (Asclepias tuberosa), often called butterflyweed, is one of those unbelievably handsome plants that inhabits dry prairies. The population at Black Earth Rettenmund Prairie is outstanding, and from my long work as a volunteer there I knew I wanted some at Pleasant Valley Conservancy. The year we planted the Valley Prairie we had a good seed source, and now have some nice plants.

Last year I counted 25 separate plants, most of which were growing in the upper (drier) part of the prairie. I haven't counted yet this year, but there are obviously quite a few plants.

This is the only milkweed without milky sap. Also, it has alternate rather than opposite leaves. Finally, there aren't many forbs with orange-colored flowers, making this species unique. Since it grows well in gardens, it can be found in many backyards.

There is considerable color variation within this species. Some plants have almost red flower, most are a deep orange, but some are almost yellow in color. The color does seem to be inherited. At Black Earth Rettenmund we have almost the whole range of colors.

The late botanist Robert Woodson, who was the world expert on milkweeds, did quite a bit of work on this species because of its peculiarities (Annals of the Missouri Botanical Garden, Vol. 34, No. 4 (Nov., 1947), pp. 353-432). A fully developed specimen has numerous (as many as a hundred) flowering stems arising from a large woody tap root. Plants flower within two to three years after germination of the seed, and may persist for as long as twenty years or more. However, the do not send out rhizomes, each plant remaining as an isolated unit.

Woodson was interested in the color variation and at one time did a road trip from his home base in St. Louis (he was at the Missouri Botanical Garden) to the east coast, stopping whenever he saw a butterflyweed plant. (In those days, it was very common along roadsides.) He also did many transplant experiments, digging up plants and moving them into his extensive home garden. Also, he collected seed from plants of a variety of color phases and raised them to flowering. In all cases, the flower color bred true. However, there was a strong correlation of flower color with geography. In the central U.S., orange and red-orange predominated, but as one moved away from this locus, yellower specimens predominated. (Evolution, Vol. 18, No. 2 (Jun., 1964), pp. 143-163)


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