A questionable milkweed called Gomphocarpus physocarpus

[James Price]

I'm interested in knowing whether anyone has any experience, either casual or scientific, with this tropical milkweed: Gomphocarpus physocarpus (Syn. Asclepias physocarpa), common name Swan Plant. It's sold in the floral trade for its unusual seed pods, which are used in arrangements.

I found some of these plants in a greenhouse, and they were covered with monarch caterpillars and eggs. I took them home, and they certainly do atttract laying monarchs. However, I have noticed that survival at all early stages seems to be much lower on this plant than any other I've seen, either domestic or tropical. Many eggs don't hatch, of those that do, many larvae die in their first molt.

It could be that this plant's toxicity is simply too high even for monarchs, or that the plant also attracts predators. I would get rid of it but I'm curious about it. Oddly enough, I found a Web site for educational monarch rearing kits, and this plant was sold with the kit. So somebody thinks it's actually good for them. Any thoughts?

[milkweedman]

Dude, this is the most common type of milkweed available in garden shops here in New Zealand. Ive grown it so many times myself because its so easy to grow here and always gets big and sets seed pods for next year. Sometimes it also over-winters so dosent need re-planting the following year.
Milkweed here doesnt grow in the wild so the monarch butterfly relies on the general public buying Swan, Tropical and Swamp milkweed from garden centres each summer. Im experimenting myself with Common, Showy, Desert, Prarie, Whorled and Glacous milkweed to see which ones are best suited for New Zealand gardening conditions and are not too invasive and will assist the monarchs numbers to get bigger here as their population is permanently small with no migration like you have in the Northern hemishere.