Wednesday, October 28, 2015

Tropical Milkweed--Plant or Not?

A gardening friend of mine gave me several milkweed plants she had acquired at a plant swap this spring. I already had one variety in my garden, a short, bushy yellow plant that seeds politely around my garden. (By politely, I mean that I discover one or two new plants a year.) The plants my friend gave me were of "grab bag" variety. She did not know what kind they were but I was hoping for a purple one. Two of the plants soon bloomed and they turned out to be the common tall, yellow plants. Although I planted these in late spring, they have produced two bloom cycles this year.

The third plant didn't grow very fast and I almost forgot I planted it. Last week, I noticed some red buds on it, which later opened to a yellow center. I was thrilled that it was so pretty!
Asclepias curassavica?
After researching it on the web, I believe that it is Asclepias curassavica, tropical milkweed. I also discovered that tropical milkweed is the source of hot debate among butterfly enthusiasts. On the one hand, gardeners, hearing about the decline of Monarchs, plant milkweed because of its critical role in in Monarch reproduction, and tropical milkweed is a beautiful, commonly-available variety. On the other hand, tropical milkweed is much criticized across the internet for contributing to the Monarch decline. What's a good-intentioned gardener to do?

My research on tropical milkweed revealed a complicated problem. The gist is that the decline of the Monarch is partly attributable a parasite infection, and the infection rate is much higher for Monarchs that breed on tropical milkweed and overwinter in the southern part of the United States compared with those that migrate to Mexico for the winter. According to the Monarch Joint Venture, a partnership of government and private organizations to preserve the Monarch, the problem lies in winter-breeding made possible by the tropical milkweed rather than in the plant itself. As long as tropical milkweed is not available for winter breeding (that is, it dies back in the fall), it should not pose an increased risk of parasite problems for Monarchs. The Joint Venture recommends that in those warm areas where tropical milkweed might not die back in the winter, it should be cut back to the ground in the fall to ensure that parasite-free foliage is available for migratory Monarchs. (Click here for a full discussion of these issues).

So it seems that in our zone 7/8 gardens in the Memphis area, tropical milkweed bears watching. Since this is my first experience with it, I intend to watch mine closely as cold weather arrives and I'll give it a helping hand if it resists dormancy.

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