This is as much a reminder note to myself as a blog post. This plant is a native California milkweed species. Looks a lot different than the ubiquitous (at the arboretum) A. curassavica, which has the orange-red flowers, and which the monarch caterpillars normally prefer.
Last week when I was distributing the school's excess caterpillars throughout the arboretum, I stuck a couple on this plant. I've only seen a monarch caterpillar on it once before, but it was in the summer, and it was a fat, healthy looking one.