The Tropical Storm Surrounding
Tropical Milkweed(Asclepias curassavica) is a non-native milkweed that has exploded in popularity over the past decade with both North American butterfly gardeners and the objects of their desire…monarch butterflies!
Why has this non-native become a staple in so many North American butterfly gardens?
- Popular host plant for monarch eggs
- Popular nectar plant for butterflies…especially around the fall migration!
- Leaves and flowers always ready to support monarchs
- No taproot so it won’t spread invasively underground
- Very little reseeding in regions that freeze during winter
- One of the most attractive milkweed varieties
- Grows backs quickly after being devoured
- Easy to propagate
Unfortunately, it didn’t take long for native purists to magnify potential problems for adding this new milkweed to our garden landscapes. While there are relevant issues that deserve further research, this milkweed has endured an unrelenting smear attack over the past few years.
Somewhere along the way, non-native naysayers stopped acknowledging any benefits to growing Asclepias curassavica. They even started trying to position it as monarch enemy #1a, right along side the ultra-controversial buddleia davidii (butterfly bush)…but that’s a post for another day.
Monarch expert Karen Oberhauser from the University of Minnesota recently did a Q & A for Journey North and this is what she had to say about Asclepias curassavica:
“When tropical milkweed is planted in the coastal southern U.S. and California, these plants continue to flower and produce new leaves throughout the fall and winter, except during rare freeze events.
Potential negative effects on monarchs include 1) continuous breeding on the same plants, which can lead to a build-up of Ophryocystis elektroscirrha (OE) infection, and 2) availability of milkweed during a time that it is not naturally available, and so potential consequent impacts on monarch breeding during the fall migration.”
While these issues are still being studied, some are calling for a moratorium on tropical milkweed and lesser known non-natives that could potentially pose the same threat.
But, with the monarch population in peril, is this a good time for eradicating some of their favorite milkweed species from our garden landscapes? Are there solutions that would allow the monarchs to “have their tropical milkweeds…and leave them too?!”
How To Grow Tropical Milkweed Without (Allegedly) Hurting Monarchs
1. OE Disease Spores Build Up on Overused Asclepias curassavica plants
This is a potential problem for those in US coastal regions including Florida, Texas, and Southern California. We still need conclusive data on this issue to understand how the reuse of tropical milkweed is negatively impacting the monarch population.
Overuse isn’t an issue with native milkweeds is because the native leaves are only viable/desirable during the earlier parts of their growth cycle. The leaves of non-natives like tropical milkweed and balloon plant (Gomphocarpus physocarpus) stay viable from first leaf until first frost.
Texas Butterfly Ranch reported that a tropical milkweed research patch left overwinter in San Antonio had an occurrence of 15% OE…however, other monitoring sites observed by the Monarch Larva Monitoring Project had 47%! This is 6 times the OE levels of the monarch population returning from Mexico.
A Tropical Solution: Gardeners in tropical milkweed perennial regions (USDA plant hardiness zones 8-11) can cut it back to the ground a couple times each season. This will allow fresh, healthy foliage to emerge for new generations of monarchs.
When to Cut Back Milkweed?: I get this question a lot, and I’m not sure there’s a perfect answer. You’re looking for a lull after a flurry of activity, but these times aren’t always obvious.
Try staggering your cuttings (cut back half now, the other half a few weeks later). This way, there’ll always be some milkweed available for unexpected monarch visitors.
2. The Monarchs aren’t finishing their fall migration because viable milkweed is available year-round in the US
A Tropical Solution: Once again, cutting back milkweed plants (or potting them to bring indoors) can make a huge difference. If viable milkweed plants aren’t available to receive monarch eggs, this should encourage the monarch majority to finish their Mexican migration. However, it’s important to note there is currently no conclusive data telling us if/how much this is happening.
In 2016- with more gardeners planting tropical milkweed, the overwintering population in Mexico grew 3.5 times: from 57 million monarchs…to 200 million!
3. Non-native plants won’t support the ecosystem
If you believe this, then I invite you to plant tropical milkweed to see the truth with your own eyes.
In Minnesota I’ve seen monarchs, swallowtails, hummingbirds, honey bees, and other small pollinators on ours…it’s even supported tussock moth caterpillars, which I admittedly wasn’t so excited about. Further south, Asclepias curassavica attracts even more pollinators…
A Tropical Solution: I don’t see a problem here but non-native plants are best used for complementing your natives…not replacing them!
Another Tropical Benefit
Asclepias curassavica thrives as a potted plant. Portable tropical milkweed has several advantages:
1. Bring it indoors– once monarch season is over in your region (or should be) bring your pots inside so any late comers won’t be tempted to continue the season in (too) cold weather.
For the past 5 years growing tropical in Minnesota, late egg laying has never been an issue. It would seem the monarchs are taking other environmental cues to start their fall migration. Also, if tropical milkweed keeps monarchs from migrating, then why don’t the butterflies stay south of the border where they would have tropical milkweed growing year round?
2. Raise Monarchs on Milkweed Cuttings– raising monarch butterflies is an awe-inspiring experience, and a much simpler one using potted milkweed plants. By raising them indoors, you can potentially raise their survival rate from under 5%….to over 95%!
2016 UPDATE: we still plant tropical milkweed containers, but we only take stem cuttings for raising indoors because they are easier to clean and keep predator-free.
If you’d like to reach your raising potential, check out my updated monarch raising guide where I share personal raising tips and techniques that allow us to consistently raise monarchs with a 95% survival rate:
So, What’s Your Tropical Milkweed Verdict?
If you’re gardening in USDA hardiness zone 7 and below, it’s highly unlikely you’ll be dealing with overused milkweed or fall-lingering monarchs. But if you want to be 100% sure your tropical milkweed isn’t negatively affecting them, you can take the same cut-back precautions in your northern garden.
For those above zone 7, your answer isn’t a difficult one…are you willing to cut back your milkweed or bring it indoors to avoid overuse and migration tampering? If the answer is no, stick to natives and avoid contributing to these potential issues.
If you are willing to take simple precautions growing Asclepias curassavica, then it can be a valuable asset for attracting and supporting monarchs inside your butterfly garden.
By growing tropical milkweed responsibly, you’ll be helping more monarchs in a time when monarch support is crucial to the survival of their storied migration.