Tropical milkweed – not for us!

Several years ago, I grew a native milkweed, Asclepias speciosa (showy milkweed), at the LEAF Nursery and donated the plants to the “Fremont monarch lady” to be distributed in the Tri-City area. However, there had been conflicting information about whether growing milkweed close to monarch butterfly overwintering sites like the one at Ardenwood Historic Farm was beneficial for the monarch population, so we put the project on hold. 

This year we tried a different way of propagating seeds by distributing them throughout a tray instead of in individual cells. We used four species of Asclepias milkweed: one native, A. speciosa (showy milkweed) and four non-native – A. curassavica (tropical or sunset milkweed), A. tuberosa (butterfly weed) and A. incarnata (swamp milkweed). An illustrated flyer was crafted and distributed, and within days the native showy milkweed sold out. Sales for the others were good, too.

Then , it came to our attention that the tropical or sunset milkweed (A. curassavica) has some detrimental effects for the monarchs when planted in our region. We read a blog post  from the Xerces Society, an invertebrate conservation non-profit, on their concerns about tropical milkweed. Here is an excerpt: 

Xerces concerns about Tropical Milkweed

“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.

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.”

Here’s one to avoid: Tropical or sunset milkweed, Asclepias curassavica. (© 2016 Jee & Rani Nature Photography, License: CC BY-SA 4.0)

What gardeners can do to benefit the monarch butterfly

  • Plant native milkweeds like A. speciosa (showy), A. fascicularis (narrow leaved), A. californica (California) and A. cordifolia (heartleaf) that die back after blooming.
  • Plant non-native butterfly weed instead of the tropical milkweed, since it has naturally lower levels of cardenolides, plant toxins that are taken up by monarchs for use in self-defense.
  • If you already have tropical milkweed growing, cut back plants to the ground twice during the growing season to limit the spread of disease (BTW, they grow back quickly.)
  • If you have tropical milkweed, remove plants late in summer so as not to interfere with monarch migration.

What LEAF Nursery is doing to benefit the monarch butterfly

  • Tropical milkweed was pulled from our inventory and will not be sold to the public.
  • People who purchased the tropical milkweed before we pulled it from our seedling sale were sent the above info on how to reduce the harmful effects to monarchs.
  • People who had already ordered tropical milkweed were also sent this information and given the option to cancel their orders or replace them with other milkweeds.

Would you like to do even more for milkweed and monarch conservation?

The Xerces Society and the Idaho and Washington departments of fish & game have a citizen science project called Western Monarch Milkweed Mapper that allows the general public to contribute sightings of both butterflies and milkweeds. You can find more information here.


Top photo: A monarch caterpillar on milkweed (Douglas Mills/flickr, CC BY-NC-ND 2.0.)

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