The tropical milkweed problem
It is the most widely sold milkweed in America and, south of the frost line, the most damaging plant a monarch gardener can buy.
What tropical milkweed is
Asclepias curassavica is a tropical American species, native from Mexico through South America and the Caribbean, but not native to the United States. It is easy to spot: red-orange petals with a bright yellow crown, and near-continuous flowering.
Garden centers love it because it germinates without stratification, blooms in twelve weeks from seed, flowers all season, and looks spectacular in a pot. Monarchs love it because it is nutritious and always available. Both of those facts are the problem.
Two documented harms
It keeps monarchs breeding through winter. Native milkweeds die back with the first frost, and the disappearance of the host plant is one of the cues that pushes late-season monarchs into reproductive diapause and southward migration. In Florida, along the Gulf Coast, in southern California, Arizona and Hawaii, tropical milkweed never dies back. Monarchs encountering green, flowering milkweed in November behave as though it is still summer: they stay, and they breed.
It concentrates the OE parasite. Ophryocystis elektroscirrha is a protozoan that infects monarchs. Spores from an infected female's abdomen fall onto the leaves she lays on; the caterpillar eats them; the parasite develops inside it. Infected adults emerge weak, sometimes unable to expand their wings, with shorter lifespans and reduced flight ability.
Migration normally keeps OE in check. Heavily infected butterflies cannot complete a long flight, so they are removed from the population, and the plants they contaminated die back over winter. A perennial evergreen milkweed patch breaks both mechanisms. Spores accumulate on leaves that never die, and a resident non-migratory population passes them from generation to generation. Studies of resident populations on year-round tropical milkweed have found infection rates approaching 100 percent, against roughly 8 percent in the migratory eastern population.
There is also evidence that cardenolide concentrations in tropical milkweed rise under high temperatures to levels that can harm the larvae feeding on it, a concern that increases as summers get hotter.
Where it is genuinely a problem, and where it is not
Zones 9 to 11: Florida, the Gulf Coast, coastal and southern California, southern Arizona and Texas, Hawaii. This is where the harm is real and documented. Do not plant it.
Zones 8 and below, most of the country. Frost kills it every year, so it functions as an annual, dies back on schedule and cannot sustain winter breeding. The evidence of harm here is much weaker. It is still not a native plant, still supports fewer other insects than a native species, and there is still a better option for every situation. But a tropical milkweed in a Michigan garden is not causing a monarch crisis.
If you already have it
You do not have to rip it out today, but do one of these:
- Cut it back hard. In zone 9 and warmer, cut every stem to within six inches of the ground in October, and keep cutting any regrowth through the end of February. This mimics the winter dieback the plants would otherwise never have.
- Replace it gradually. Plant natives alongside it this season, and remove the tropical milkweed once they establish.
- Grow it in a pot and move it into a shed or garage for the winter.
What to plant instead
| If you garden in | Plant instead |
|---|---|
| Florida | Aquatic milkweed, pinewoods milkweed, butterfly weed |
| Gulf Coast & Deep South | Aquatic milkweed, swamp milkweed, green antelopehorn |
| Texas | Green antelopehorn, antelope horns, zizotes milkweed |
| Southern California | Narrowleaf milkweed, California milkweed, showy milkweed |
| Arizona & New Mexico | Rush milkweed, desert milkweed, pineneedle milkweed |
| Hawaii | No native milkweed exists; see the note below |
Hawaii is a special case. There is no native Hawaiian milkweed, and the resident monarch population there has never migrated. The standard advice does not translate directly, though OE remains a real concern and cutting plants back periodically still reduces spore loads.
The policy picture
Several California counties and municipalities have restricted the sale of tropical milkweed, and a growing number of native plant societies and retailers have stopped stocking it voluntarily. Legislation has been debated in Florida. Nothing is settled nationally, so for now the decision sits with individual gardeners and nurseries.
If you buy plants, ask what species they are. "Milkweed" on a label almost always means A. curassavica, and the red-and-yellow flowers give it away instantly.
Frequently asked questions
Is tropical milkweed bad for monarchs?
In frost-free regions, yes. Because it never dies back it encourages monarchs to breed through winter instead of migrating, and OE parasite spores accumulate on leaves that are never shed. In cold-winter regions where frost kills it annually, the evidence of harm is far weaker.
When should I cut back tropical milkweed?
Cut it to within six inches of the ground in October and keep it cut through February if you garden in USDA zone 9 or warmer. That reproduces the winter dieback of native milkweeds and reduces parasite build-up.
Is tropical milkweed illegal?
Not nationally. Some California counties and municipalities restrict its sale, and many native plant nurseries have stopped stocking it voluntarily, but there is no federal ban.
What is OE in monarchs?
Ophryocystis elektroscirrha, a protozoan parasite. Spores on milkweed leaves are eaten by caterpillars, and infected adults emerge weakened, often with deformed wings and shortened lives. Infection rates are far higher in non-migratory populations breeding on year-round milkweed.
References
- Satterfield, D. A., Maerz, J. C. and Altizer, S. (2015). Loss of migratory behavior increases infection risk for a butterfly host. Proceedings of the Royal Society B 282: 20141734. Year-round breeding on evergreen milkweed raises OE infection sharply. View
- Altizer, S. M., Oberhauser, K. S. and Brower, L. P. (2000). Associations between host migration and the prevalence of a protozoan parasite in natural populations of adult monarch butterflies. Ecological Entomology 25(2): 125–139. Establishes the link between migration and low OE prevalence. View
- Faldyn, M. J., Hunter, M. D. and Elderd, B. D. (2018). Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies. Ecology 99(5): 1031–1038. Cardenolide levels in tropical milkweed rise with temperature, harming larvae. View
- Agrawal, A. A. (2017). Monarchs and Milkweed: A Migrating Butterfly, a Poisonous Plant, and Their Remarkable Story of Coevolution. Princeton University Press. The standard general account of the milkweed and monarch relationship. View
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Milkweed USA. (2026). The tropical milkweed problem. Retrieved DATE, from https://milkweedusa.com/guides/tropical-milkweed/
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title = {The tropical milkweed problem},
author = {{Milkweed USA}},
year = {2026},
url = {https://milkweedusa.com/guides/tropical-milkweed/},
note = {Accessed DATE}
}