I do not recommend planting tropical milkweed, as it does not lose its leaves in the winter. By keeping its leaves, it can lead to higher levels of infection by a protozoan parasite (referred to often by its acronym, OE). Some monarchs may twice in places like Florida or tropical islands year-round and this is not much of a problem. They are tropical butterflies that evolved migration to northern latitudes due to competition in the tropics. Tropical milkweed can encourage them to stay, which can be a problem but is second to that of the OE issue. 

It is best to cut back tropical milkweed to avoid harboring OE, or to avoid planting it at all, as it can escape gardens into natural areas. Try native milkweed, like swamp milkweed or butterfly milkweed. 

Good luck!