See also: Herbaceous Perennials
Asclepias curassavica, commonly known as tropical milkweed or blood flower, is not reliably winter hardy in gardens north of zone 8. Native to South America, tropical milkweed has showy scarlet and yellow flowers. Asclepias curassavica is longer flowering than the much beloved native Asclepias tuberosa (butterfly weed).
Tropical milkweed is attractive to monarch butterflies and easy to grow in a garden setting. It prefers full to partial sun and can tolerate dry conditions once established in spring. Tropical milkweed is also at the center of ecological research at my alma mater, the University of Georgia.
Tropical Milkweed and Monarch Butterfly Populations
There is widespread agreement in ecological research that monarch butterflies are under pressure. Corn genetically modified to be immune to Roundup may be endangering Asclepias tuberosa (butterfly weed), an important native plant crucial to monarch butterflies.
Monica Maeckle at the Texas Butterfly Ranch wrote a fascinating article focusing on research into the effects overwintering of tropical milkweed could have on monarch butterfly populations. Maeckle provides an overview of research being done at the University of Georgia by Ph.D. candidate Dara Satterfield, who works with noted researcher Dr. Sonia Altizer. Learn about Dr. Altizer's 'monarch butterfly gym' at the University of Georgia.
The focus of the research is whether tropical milkweed encourages butterflies to forego migration in areas where tropical milkweed persists through winter. Tropical milkweed is an evergreen plant in its native habitat. There is the potential that the butterflies that overwinter may be more likely to harbor and pass on butterfly parasite (Ophryocystis elektroscirrha) to migrating populations of monarch butterflies. More info about Ophyrocystis electroscirrha at UGA.
Should you plant tropical milkweed in your garden?
If you live in zones where freezing temperatures prevent monarch butterflies or milkweed's foliage to persist through winter, based on current research, you are safe to plant tropical milkweed, and the monarch butterflies will benefit. In areas where tropical milkweed can over-winter (Florida, Texas, for example), cutting back foliage in fall is recommended.
Research is still ongoing on this issue, but in the end I agree with researcher Dara Satterfield, who promotes native plants whenever possible. If one had the choice (and one does), perhaps the wisest path is to plant Asclepias tuberosa. I'm all for living life to its fullest, but in ecological concerns, if we don't know, why take the risk?
Scientific name: Asclepias curassavica
Common name(s): tropical milkweed, scarlet milkweed, blood flower
Plant type: herbaceous perennial* - not reliably winter hardy in zone 7 and colder. Grow as an annual.
Native status: native to South America.
Winter hardiness: 8-9
Of note: Longer bloom period than the native milkweed (Asclepias tuberosa). Research is ongoing into monarch butterfly populations that suggests cutting back Asclepias curassavica in winter will help prevent monarch butterfly parasite (Ophryocystis elektroscirrha) from infecting migrating monarch butterflies the following spring.