For many butterfly gardeners, tropical milkweed (Asclepias curassavica) is a go-to plant to attract Monarchs: The plants are annuals here in the mid-Atlantic, and provide nectar for adult Monarchs and food for caterpillars. Common milkweed (A. syriaca) and swamp milkweed (A. incarnata) are perennials that frequently grace butterfly gardens.
But researchers at the University of Michigan warn that, if you grow tropical milkweed for monarchs, don’t treat it too well: Fertilizing the plants (with nitrogen, anyway) increases the levels of toxic cardenolides that stunt or kill Monarch caterpillars. This didn’t seem to be the case with the two native perennial milkweeds.
Read the paper here: 2014 Tao et al milkweeds