I've been starting about 30 different kinds of seeds (flowers, fruits & veggies) this spring. Some of them have given me a LOT of trouble, or just take forever to sprout, so it's nice when run into a plant that's easy to grow. Tropical milkweed germinates in about a week or so and isn't too, too picky about humidity.
Because the monarch population declined by almost 60% last year and warmer temperatures are affecting monarch migration patterns, I think it's very important to have a variety native milkweeds in your flower garden.
Right now I have five tropical milkweeds that I'm growing for myself and I've already given away two sprouts to fellow gardeners with grow lights. My goal is to have ten milkweed plants (two tuberosas, two incarnatas, and five or six currasavicas) in the garden by late May so that I can apply for Monarch Waystation Certification. I'll also be starting large flats of perennial milkweeds this summer, as part of a future giveaway project!
--After further reading, I've discovered that Tropical Milkweed (Asclepias currasavica) is controversial as a Monarch host plant. The Native Plant Society of Texas has quite a bit to say about its pros and cons. New York City winters provide little opportunity for A. currasavica to become an invasive species, and for me it's a good "filler" plant while my other milkweeds mature, but you should decide for yourself whether it's right for your butterfly garden.