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!
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.