I live in New Zealand, and am hopeful that I can learn from your experiences in teh Northern Hemisphere with regards Asclepias sp.
Firstly, a bit of history. The Monarch came to NZ under its own steam (they think). Presumably blown on a storm, teh Monarch was first recorded in the mid 1800s. It wasn't until the early 1900s that Asclepias were introduced. We have two relatively common species: A. physocarpa and A. fruticosa, and there are two other species meant to be available in NZ but I haven't found a nursery stocking them as yet.
I would like to ask a question about the plant, especially where it grows naturally.
I notice that where there were once prolific stands of Asclepias, a few years later the plants have almost completely died out in that place. Of course their seed has now been spread far and wide, so there are plants nearby. But I was wondering if Asclepias actually changes the make-up of the soil - I know some other plants change a soil from acid to alkali or vice versa.
My friendsa and I are currently working on restoring what was once an overwintering site, and I have been told that Swan Plant was prolific there in the 1960s but there isn't much there now - and neither are there many butterflies overwintering in the trees there (it's midwinter here in NZ).
Hope someone can advise!