Milkweed is in demand, and that demand has been filled in recent years by tropical milkweed, a non-native species. But is planting tropical milkweed potentially doing more harm than good?
Tropical milkweed (Asclepias curassavica) is a non-native milkweed that has exploded in popularity in response to the demand for milkweed. It is simple to propagate, allowing growers to rapidly produce the plant for quick sale. The plant is also attractive, both to humans and monarchs, providing flowers and lush green foliage throughout the growing season – and that’s a problem.
Tropical milkweed becomes a problem when planted in temperate areas where it does not die back in winter. A protozoan parasite of monarch butterflies, Ophryocystis elektroscirrha or OE for short, can travel with monarchs visiting the plants and become deposited on leaves. When caterpillars hatch and start eating the plant, they ingest the OE. High OE levels in adult monarchs have been linked to lower migration success in the eastern monarch population, as well as reductions in body mass, lifespan, mating success, and flight ability.
When native milkweeds die back after blooming, the parasite dies along with them so that each summer’s monarch population feeds on fresh, parasite-free foliage. In contrast, tropical milkweed that remains evergreen through winter allows for OE levels to build up on the plant over time, meaning successive generations of monarch caterpillars feeding on the plant can be exposed to dangerous levels of OE.
Tropical milkweed can also interfere with monarch migration and reproduction. When grown in northern areas, where it can grow later in the year than native species, the presence of tropical milkweed may confuse monarchs into breeding at a time when they should be migrating. In California, where this milkweed is widely planted, it can be growing near overwintering sites along the coast and may spur monarchs to breed when they should be overwintering.
With tropical milkweed so readily available, what’s a gardener to do? Some advice has suggested plants can be cut back to the ground twice during the growing season to limit the spread of disease, and that plants should be removed late in summer so as not to interfere with migration. In practice however, we’ve found it’s been a hard-sell to get anyone to cut back plants that are actively supporting monarch eggs or caterpillars, or remove lush plants in full flower.
Climate Change Could Make Matters Worse
In addition to the concerns over OE and disruption of migration behavior, emerging research suggests that tropical milkweed may actually become toxic to monarch caterpillars when the plants are exposed to the warmer temperatures associated with climate change. Under these conditions, tropical milkweed produces higher cardenolide concentrations. Monarch caterpillars are tolerant of these chemicals—in fact, cardenolides are the very compound that protects the monarch from predation. But when the cardenolide concentrations are high enough, not even monarch caterpillars can withstand them. In contrast, native swamp milkweed (A. incarnata) has naturally lower cardenolide levels, and when used as a control in the study mentioned above, it did not exhibit the same radical changes in toxicity as tropical milkweed.
Another climate change consideration is that tropical milkweed is often sold as an annual in places where it is expected to die back in winter. With climate change warming our winters, tropical milkweed may begin sailing through winter in places it wouldn’t have in years past, carrying OE along with it.
While milkweed is needed in large numbers to support and expand the monarch butterfly population, we do not recommend planting tropical milkweed, and further suggest milkweed of any species not be planted within 5–10 miles of monarch overwintering sites in California.
How You Can Help
- Use our regional milkweed guides to identify milkweed native to your area, or if you live in the west, visit the Western Monarch Milkweed Mapper to identify the milkweed that’s best for the west.
- Use our milkweed seed finder to locate plants and seed, and visit our project milkweed page to learn more about growing milkweed.
- As an alternative to tropical milkweed, consider planting orange butterflyweed (Asclepias tuberosa) if it is native to your area. It has a similar habit as tropical milkweed and the same bright orange colors.
- Share this fact sheet from Monarch Joint Venture with your garden club, local nursery, friends, and neighbors to spread the word about the dangers of planting tropical milkweed.
- While several varieties of milkweed have become more available to the nursery trade in recent years, native milkweed can still be hard to find. Share this fact sheet with local growers, farmers, and nurseries to encourage them to grow and sell native milkweed.
Q&A about tropical milkweed – From Monarch Joint Venture.
Tropical Milkweed Fact Sheet – From Monarch Joint Venture – this is a great resource to share with nurseries and gardening friends that covers the issues of planting tropical milkweed.
Growing and Selling Native Milkweed Fact Sheet – Also from Monarch Joint Venture, this approachable fact sheet that introduces the reasons and resources for growing native milkweed.
Milkweed: A Conservation Practitioner’s Guide – A comprehensive guide to farming milkweed. Contains extensive information about propagation, pests, diseas control, and other considerations.
Regional Milkweed Guides – A series of regional guides to the native milkweeds of North America, developed in cooperation with the USDA Natural Resources Conservation Service.
Western Monarch Milkweed Mapper, milkweed directory – A community science effort tracking monarch breeding habitat across the west, the site has a comprehensive list of milkweeds found in western states.
Harvesting Milkweed Seed: a Pod and a Plan – Information about harvesting your own milkweed seed.
The 2017 Perennial Plant of the Year, and Other Milkweeds You Should Know – get to know orange butterflyweed, and other milkweeds commonly available.
Loss of migratory behaviour increases infection risk for a butterfly host (Satterfield et al., 2015).