Flowers of Asclepias curassavica cultivated in New York
Asclepias curassavica has many common names including tropical milkweed, bloodflower, Mexican butterfly weed and wild ipecacuanha. The genus name, Asclepias, is derived from the name of the Greek god of healing, Asklepios, as tropical milkweed has many traditional medicinal uses despite its mild toxicity to vertebrates.
Tropical milkweed is a tender perennial subshrub, typically 1m (3 ft.) tall. The leaves are lanceolate, oppositely arranged, and 6 to 15 cm (2-6 in.) long. Like other milkweeds, Asclepias curassavica produces a milky white latex sap that exudes from damaged parts of the plant. According to Plant Physiological Ecology by Hans Lambers, F. Stuart Chapin III and Thijs L. Pons (Springer Science+Business Media, LLC, 2008), this sap contains the heart-affecting cardiac glycosides calotropine and calactine.
The bright scarlet to orange flowers are borne in groups of up to twelve in umbels from terminal or axillary peduncles. Each individual flower is approximately 12.5 mm (0.5 in.) across, with five sepals and five linear, reflexed petals that are fused at the base. The yellow corona is composed of five hood-shaped structures, each containing a single horn. The blossoms are pollinated by wasps and butterflies; however, some Australian and Costa Rican populations have been reported to be self-pollinated. Tropical milkweed blooms throughout the warm season wherever it grows. The paired seed pods are elongate and split open along a vertical line of dehiscence. When dry, 70 to 80 dark brown ovate seeds are released. Each seed is well-equipped for wind dispersal, possessing a silky coma at one end and a thin papery wing around its entire edge.
Originally from somewhere in the Americas (see previous BPotD entry for discussion: Asclepias curassavica, Asclepias curassavica has been introduced to Asia, North America, Australia, Africa, and Europe. It has naturalized and become invasive in many tropical or subtropical regions of the world. The species is successful in a wide variety of environments, including wetlands, grasslands and disturbed areas. In parts of Cambodia and Papua-New Guinea, tropical milkweed has been known to invade coconut (Cocos nucifera) plantations.
Home gardeners in the United States have sometimes been encouraged to plant tropical milkweed as a food source for monarch butterflies (Danaus plexippus). Unfortunately, new research from the University of Georgia (see: Satterfield, D. A. et al. (2015). Loss of migratory behaviour increases infection risk for a butterfly host. Proceedings of the Royal Society B-Biological Sciences. doi: 10.1098/rspb.2014.1734 ) suggests that the widespread cultivation of tropical milkweed in the southern United States may be doing more harm than good. Like milkweed species that are endemic to the United States, tropical milkweed can play host to a protozoan parasite, Ophryocystis elektroscirrha (What is OE?). This parasite is ingested by monarch larvae, internally damaging the caterpillar. Upon emergence from the chrysalis, the adult butterfly is covered with dormant spores–and may be so deformed or weak that it cannot fly (in some cases, even too weak to emerge from the chrysalis).
Under normal circumstances, the long migration to Mexico would curb widespread transmission and infestation, as only healthy monarchs would survive the journey. However, unlike native milkweeds, tropical milkweed does not senesce in the winters in the southern United States. This means that monarch populations can continue to breed on tropical milkweed throughout the winter instead of migrating to Mexico. Infected butterflies can then pass the spores on to their offspring, and potentially to healthy butterflies returning from Mexico. As a result of this, gardeners in the southern USA are encouraged to plant the winter-dormant native milkweeds instead of tropical milkweed.
Thank you for this timely entry. The gorgeous Asclepias tuberosa in my garden is just coming into bloom and I will look at its features with more clarity now. I planted it in my front garden many years ago, hoping to attract more butterflies and I’ve grown to admire the plant for its somewhat unique colour and form (not to mention those groovy seedpods). Interestingly, the butterflies still seem to prefer the more mundane plant variety in my backyard, but the bees just love the Asclepias in my frontyard.
Thanks for this valuable information. Fortunately, this tropical Asclepias will not grow for me. I will be sticking with growing the native milkweeds instead from now on.
I just learned of the issue with A. curavassica and Monarch butterflies this week and was pleased to see that you discussed it in this entry. I love the flowers of Tropical Milkweed but I will stick to the perennial A. tuberosa for orange in my Portland, Oregon garden from now on.
Same here tuberosa just bought it the other day, it will be planted along the other milkweed Asclepias syriaca or common milkweed . the flowers are beautiful and scented .Amazing plant
Would this plant survive if planted at UBC garden?
Nadia, I just quickly looked it up in Dave’s Garden, but according to that web site A. curassavica is hardy to zone 8a. It doesn’t have a chance of overwintering here in southern Maine (Zone 5), so after some agonizing a few years ago when I first heard of this problem, I do plant it as an annual, since there is no chance it will keep butterflies in Maine too long. If I’m mistaken about this, I would appreciate being corrected.
As for the comparisons among the milkweeds, besides A. curassavica I grow a large patch of the common A. syriaca and several smaller patches of A. incarnata, Swamp milkweed, and A. tuberosa, Orange milkweed. This is far from scientific too, but it’s clear the Monarchs (the few we get this far north) as well as Fritillaries and many other native pollinators greatly prefer A. incarnata over the others; there are also far more Monarch larvae on the Swamp milkweed. There is a white form, but most pollinators seem to ignore it.
I first saw the flowering plant in the Warsaw Botanical Garden. Thank you for the interesting write up!
The name A. curassavica refers to Curacao, does it not ? I wonder whether it originated there ?
Several years ago in a friend’s waterside garden, I saw common milkweed growing next to swamp milkweed (A. incarnata). A half-dozen monarch caterpillars were riddling the leaves of swamp milkweed, but the common milkweed went untouched. Just an anecdotal observation.
This is really interesting! I have both the tropical and the native milkweeds in my garden for the butterflies. I think next year I’ll just grow the native ones (from seeds I found in a field nearby). They are not as brightly colored as this one, but still lovely and with a heavenly smell.
Thanks.
I am quite curious about the name “wild ipecacuanha” as the true ipecac syrup is derived from the tropical plant Carapichea ipecacuanha in Rubiaceae – has this been used as a substitute for ipecac at some point?
It seems like it will also induce vomiting, yes. I don’t know about the side-effects, though…