Can confirm they need to be stratified and they take at least two years to flower and produce pods. Mine took three, but I'm not sure the first batch of plants over-wintered successfully. As I planted seeds that fall also and carved out more space for them from the goldenrod next to it.
This has been a five year experiment for me and I had my first flowers and pods this year. Year 1 and year 2 I didn't stratify and were total busts. Year 3 I had ~3 baby plants. I didn't intentionally stratify or even know of the term at that point, I just happened to scatter seeds in the fall instead of spring. Last year I had several large plants, but no flowers. This year my patch went bananas, I had several 6' tall plants and got probably 20 or 30 pods at one point. This is just a small flower bed maybe 5 x 5.
I had to cut most of it down in August before the pods matured because it's right next to my mailbox and there were so many yellowjackets the mailman said I need to get rid of it or no mail. I was able to reserve a few plants farther away from the mailbox. There were no flowers at this point, the wasps appeared to be feeding on the milk.
Also milkweed beetles, something I'd never seen. I generally hate bugs especially in quantity, these things freaked me the F out. They were there from tiny little bugs in the spring, regathered after I cut it down, and are still there on the remaining plants. All year they lived in this orgy-mass thing, never separating.
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Had about 5 of these dudes this year but I think the birds got them? They were just there one day, then whoosh, all gone. Look for chrysalles but couldn't see any. Kids loves out little pets while they were there though.
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Come to think of it I'm not sure stratifying in a fridge alone would work because I keep all my pods in the garage so year 1 and 2 would have been exposed to very cold temps out there before being planted in the spring. I don't really know my horticulture lingo and proceses though, I don't have a green thumb.
I've enjoyed figuring it out though and hunting uses for the pods aside it's a beautful and fascinating plant. It is incredible how much insect life is drawn to it.