Tropical Milkweed – Asclepias Curassavica – bad for western monarchs

Are we doing more harm than good by providing this non-native milkweed to the western US monarch population?

We here at Monarch Milkweed Project strongly discourage planting Tropical Milkweed.

Because this variety of milkweed doesn’t die off in the winter but survives the mild winter climate of the West, its year round availability encourages the butterflies to adjust or delay their migration pattern.

Also, because of the plant’s hardiness, a dangerous spore-driven disease Ophryocystis Elektroscirrha (“OE”) continues to multiply over time, causing severe deformities and death in monarchs who are heavily infected. Those who are lightly infected are too weak to make the migration and instead continue to shed the protozoan and infect others.

While there are varying opinions and conclusions in the scientific community about the risks of tropical milkweed, there are some recommended actions: 

• Cultivate milkweed that is native to the local area. Here in Benicia, that is Asclepias Fascicularis (“Narrowleaf Milkweed”) and Asclepias Speciosa (“Showy Milkweed”). Begin to replace non-native varieties with these as they become available.
• Remove seed pods from Tropical Milkweed before they ripen and have a chance to open and scatter.
• Cut back Tropical Milkweed to the ground in the fall and winter months. This will prevent the parasites from surviving the season and also discourage monarchs from laying eggs out of season and failing to migrate to the coastal areas. The plant will grow back in the spring.

We ask you to spread the word to friends, nurseries, growers, and neighbors!

Additional resources:

• Xerces Society: Tropical Milkweed—a No-Grow
• Monarch Joint Venture: Potential risks of growing exotic (non-native) milkweeds for monarchs