Our season is shorter that yours, anyway, I planted 3 BB's in spots that won't block sun from the rest of the plantings. I have 1 milkweed in the center already, it's just starting to come up and i've reserved space for more in the center. When it seeded last year, i put alot of seeds/pods under dirt...we'll see.They grow huge out here, no matter how heavily I hack them back. We have several 10-12 footers and one that is about 15 ft around. I have cut that bush down to the ground and it is back to full size in about 3 years.
If you want to help butterflies grow some milkweed.
Do your BB's attract buterflies?