Wild South Florida — Tropical Milkweed!  
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Tropical Milkweed
Asclepias curassavica
butterfly weed
Tropical milkweed, photographed at Green Cay Nature Center in Boynton Beach. Others taken at the Key Deer National Wildlife Refuge.
butterfly weed

To say tropical milkweed, Asclepias curassavica, is the subject of controversy is an understatement of nearly seismic proportions, at least within the world of butterflies and those who love and study them.

It's seen as both the solution to an existential threat and an existential threat in itself. We're talking about the iconic monarch butterfly and its reliance on milkweeds as host plants for its offspring. Butterflies lay their eggs on specific types of plants; once hatched, caterpillars munch away at the leaves and flowers, picking up whatever chemicals might be present.

In the case of the monarch, milkweeds are the plants of choice. They contain poisonous chemicals that provide monarchs with the toxins and bright orange color that are the butterfly's main line of defense.

Now, there's no shortage of mikweed species in the United States or Florida, for that matter. But there is a shortage of habitat for the plants. Milkweeds made up for the decline by popping up in farmers' fields, growing between rows of corn and other cash crops. The development of herbicide-resistant varieties of these crops have allowed farmers to hit their fields with killer chemicals that keep their fields clear of plant pests, as well as milkweed.

Which is where tropical milkweed comes into play. It s a native of the Caribbean, Mexico, Central America and South America that has been exported around the globe. It's easy to grow, continues to flower when other milkweed species are done for the season and monarchs love them. It's the perfect plant to fill the gap, and perfect for butterfly gardeners, seemingly.

butterfly weed in seed

But the virtues of tropical milkweed are also its vices. Because it continues to bloom well after other milkweed species are done, monarchs are more likely to hang around rather than continue on their epic migratory journey to Mexico. And they're more likely to become victims of a protozoan parasite known scientifically as Ophryocystis elektroscirrha, or simply OE, which leaves adult monarchs with deformed wings and too weak to complete their migration.

Now, monarchs were victims of OE before the introduction of tropical milkweed to the United States. But infected butterflies would continue the migration, and die off in the process, sort of purifying the species. Plants infected would die back in the winter and emerge clean in the spring. Instead, the concentration of monarchs has led to increased OE infection rates.

But others would argue that the corelation between the higher rates of OE, fewer migrating monarchs and the introduction of tropical milkweed isn't as strong as some suggest. And even if it is, there's a simple solution: cut back tropical milkweed so it doesn't attract monarchs past the normal season. Even the North American Butterfly Association advocates planting tropical milkweed.

Some tropical milkweed basics: It is a perennial shrub, about two or three feet tall, with long, narrow leaves and star-shaped flowers typical of milkweeds.

Photographs by David Sedore
tropical milkweed

The looks are somewhat similar to butterfly weed, a native milkweed found over most of the eastern United States, including Florida, but the flowers of butterfly weed are scarlet, while tropical milkweed's are multi-hued — various shades of red, yellow and orange. Butterfly weed also has a deep taproot; tropical milkweed doesn't. Because of that tropical milkweed is often potted and taken indoors during winter. That allows fully flowering plants to be placed outdoors in the spring. The fruit is a pod filled with whispy seeds that drift with the wind.

Tropical milkweed doesn't readily naturalize in most of the United States. According to the United States Department of Agriculture, Florida, Louisiana, Texas, Tennessee, California and Hawaii are the exceptions. Again, according to the USDA, it's mostly found in Florida's Peninsula, mostly along the Atlantic Coast, from Volusia County south into the Keys and also in a few Panhandle counties. Favored habits include coastal areas, disturbed areas and rockland hammocks. Some groups list it as invasive in Hawaii, California, Texas, Louisiana and Tennessee but to the best of our knowledge it's not listed officially by state authorities. Tropical milkweed is found throughout parts of Asia, including China, Cambodia, Japan, Malaysia and Thailand, Africa, including South Africa and Mozambique, Australia and the Pacific islands, where it is considered an invasive.

Other common names for Tropical milkweed include Mexican milkweed, scarlet milkweed, blood flower, swallow-wort and silkweed. It is a member of Apocynaceae, the family of periwinkles and oleander. The USDA puts it in Asclepiadaceae, the milkweed family.

Links for Tropical Milkweed
Institute for Regional Conservation Natives for Your Neighborhood   USDA PLANTS Database Center for Aquatic and Invasive Plants
Flora of North America   Atlas of Florida Vascular Plants   Lady Bird Johnson Wildflower Center
Unless otherwise stated, all photographs are property of the publishers and may not be used without their express permission.