You'll find tropical milkweed in many nurseries across the U.S., but it's not the best for your garden. Pollinator enthusiast Melissa Strauss explains why tropical milkweed can be harmful and offers alternatives to consider.
Plants in the milkweed genus are the sole larval food of the Monarch butterfly. They’re wonderful nectar plants for most pollinators, and with many native species, an overall great garden plant. The exception to this, sadly, is the tropical milkweed species, which you shouldn’t grow in your North American garden.
Since the 1980s, the Monarch butterfly population has declined by more than 95%. In the 1990s, millions of these beautiful insects migrated from Canada to Mexico. While it’s difficult to give an exact number of monarchs still making the trip, it’s estimated that fewer than 330,000 are still migrating as of 2024.
We attribute this decline to several factors. Pesticide use is a significant problem, and climate change plays a substantial role. The loss of their breeding habitats and the decrease in milkweed supply are also contributing factors. It seems natural that gardeners would want to plant as much milkweed as possible.
Sadly, many retailers carry only one species of milkweed, Asclepias curassavica. This species is native to Central and South America, but not North America. The good news is that there are more than 60 species that are native to North America. It’s more challenging to find these native species, but many smaller nurseries are making an effort. If you look in the right places or grow it from seed, it’s not difficult to incorporate native species into your garden.
There are several ecological and conservation-related reasons why you shouldn’t grow tropical milkweed. Let’s talk about why it’s better to seek out native species than to plant this one. It may be easier to find, but it’s worth searching for a better alternative, like one of these native species:
Potentially Invasive
The most obvious reason you shouldn’t grow tropical milkweed is its potential to become invasive. This species is popular in gardens for its vibrant flowers and its ability to attract monarch butterflies. However, it has a significant invasive potential in the United States.
Unlike native species that will die back in winter, tropical milkweed can stick around year-round in warm climates. This is particularly an issue in the southern United States. Tropical milkweed is not officially classified as invasive in most parts of the U.S. However, it’s listed as potentially invasive and ecologically disruptive.
Asclepias curassavica is an aggressive spreader. It can potentially outcompete native species, which is, again, never a good thing. It’s a crucial reason you shouldn’t plant tropical milkweed.
Disrupts Monarch Migration
The monarch butterfly migration is a remarkable natural phenomenon in the insect world. Every year, monarchs travel thousands of miles from their breeding grounds in Canada and the United States. They migrate from here to their overwintering sites in central Mexico, and then back north in the spring.
This round-trip migration is multi-generational. The butterflies that begin the process are not the same ones that will return. In late summer to early fall, a super generation emerges. These butterflies are larger and longer-lived than their parents.
This super generation has to make the 3,000-mile trek from specific locations in the U.S. to the forests of the transvolcanic mountains of Mexico. Here they cluster together in the fir trees in a cool, high-altitude microclimate. Their metabolism slows, which conserves energy.
In the spring, this super generation mates and lays eggs on milkweed plants before dying. Their offspring begin the migration northward. So, how does this relate to the idea that you shouldn’t plant tropical milkweed in the United States?
Tropical milkweed is what they eat in the spring. Eaten at the wrong time, it has the potential to disrupt their natural migratory cues. They may breed out of season instead of migrating. This can result in their overwintering in unsuitable environments, where they are unable to survive.
Undermines Conservation Efforts
In recent years, scientists, government agencies, non-profits, gardeners, and farmers have been working together on conservation efforts. The goal is to protect monarchs, one of North America’s most iconic butterflies. The goal of these efforts is to increase the availability of native milkweed and nectar plants.
By planting and sustaining these plants along migratory routes and in breeding grounds, they hope to increase the population. It involves restoring grasslands and prairie ecosystems. Part of the process is removing invasive species.
Efforts are made to distribute native milkweed seeds and create monarch waystations. Here, monarchs have habitats that support both feeding and reproduction. Also involved is the removal of tropical milkweed in warm climates.
Organizations continue to track migratory patterns and population sizes. Other priorities include disease monitoring and reducing the use of neonics and other pesticides. They’re also under review for protection under the U.S. Endangered Species Act.
Canada and Mexico join forces with the United States in conservation efforts. The cooperative trilateral efforts include the protection of breeding grounds. Additionally, efforts include migrating and overwintering sites.
With all of these essential works in progress, you shouldn’t plant tropical milkweed. Planting tropical milkweed is counterproductive to conservation efforts. While it may bring monarchs to the garden, it ultimately undermines these efforts. Over time, it contributes to their overall decline.
Spreads OE Disease
Finally, there is a strong link between this plant and the spread of OE disease (short for Ophryocystis elektroscirrha). This is a harmful parasite that affects monarch butterflies. It’s a significant reason you shouldn’t grow tropical milkweed in North America.
OE is a microscopic protozoan parasite that infects butterflies that feed on infected plants. This includes monarchs, as well as other species that use it as a nectar source.
Native species die back in the winter, which breaks the parasite’s lifecycle. In warmer climates, tropical milkweed doesn’t die back. Unless pruned, the OE spores continue to accumulate. Since California and Florida are integral to their migration, this is particularly harmful.
Remember when I mentioned that this plant can interrupt the migratory behaviors of these butterflies? They sometimes stick around because it continues to bloom late in the fall. The result is that more breeding generations are exposed to OE-contaminated plants.
Butterflies become infected and shed spores onto other plants with which they come into contact. Since milkweed is their larval host, the caterpillars then feed on the infected leaves. They ingest the spores, becoming infected in the process.
One solution to this is to cut back your plants in the fall. Then, clean up any leaf litter and stems to reduce further contamination. However, we really shouldn’t plant tropical milkweed to begin with. It’s best to pull it up before it goes to seed.
Appropriate Alternatives
As I mentioned earlier, there are a significant number of milkweed species that are native to the U.S. I find many of these to be just as lovely as the tropical species, some even more so. By planting these milkweed seeds, you contribute to the conservation of these important butterflies.
Butterfly weed (Asclepias tuberosa) is a visually striking species that closely resembles A. curassavica. Its tall stems culminate in umbels of bright orange flowers. This species is native to all of the eastern and central United States, as well as California.
Swamp milkweed (Asclepias incarnata) is my personal favorite. It’s native to most of the United States except for the West Coast and Louisiana. It’s a lush plant that’s tolerant of a wide range of environments. The blooms are small and pink, and plentiful.
Whorled milkweed (Asclepias verticillata) is tall with finely textured foliage. It has a delicate and airy feeling that looks beautiful in the garden. This is native to the eastern and central United States, as well as New Mexico and Arizona.
Common milkweed (Asclepias syriaca) is a striking, broadleafed species. It’s large and produces significant clusters of gorgeous pink blooms. The flowers on this species aren’t terminal; they bloom along the entire stem. It’s also native to the eastern and central United States, as well as Oregon and Utah.
There are more than 60 species that are native to the United States, and they’re all lovely. They also all serve as food for monarchs and their larvae. If you want to add milkweed to your garden and help with conservation efforts, choose a native species. It may be easier to find, but you shouldn’t plant tropical milkweed.
