because you are changing ie reversing the conditions which control the plants responses ie photo period-length of night is growing longer where you are, avg temps are also falling not rising ,those trigger enzymes to begin dormancy, starch storage and such..you might be best off starting them indoors under artificial lights 6500K color temp CFLs 3-4 100w replacements so 3 or 4 23w bulbs kept 6in max above the plants,so you can control those factors..i have Milkweed plants at various stages of growth indoors now here in Ohio since december(Winter)..1st break dormancy via the best methods for your type mw, usually cool/moist strat. place under the lights after strat. once u have movable size plants u can move them inside and outside a little longer outside each day until they become hardend or climatized..your outdoor winter light levels may cause problems flowering.Brassinosteriods hormones mediate the effects of light,stimulating growth or dormancy, kinda like the plant see's how much and what spectrum the available light is, and im not familiar with if you have pollenators around in your area in winter??
theres a short vid of Asclepias sullivantii in hydro on my you tube.just search "sullivantii rescue".