Besides the cold temperatures, it is very hard to kill milkweed. I had milkweed that was eaten away from caterpillars, and the stem was rotted and paper thin from no water in 2 weeks, during the summer. I threw the plant away in the garbage. It rained that night, and I checked on the plant that morning, and there were little green buds all over! That was 2 years ago, and I still have the plant to this day, its spreading it's seeds!