You could do it any time, since now is when Mother Nature is sowing her seeds. But, the longer they are out, the more likely they are to be carried off by rodents, birds, and water run-off. I would wait until we have a mid-winter melt, or even until spring. You could stratify them in a plastic bag in the fridge and then plant them when it's truly spring, and avoid most of the risk. Sorry I didn't give you a straight answer.

Martha

I grow milkweed. Martha's answer is really good, but if you don't have time to do the refrigerator stratification, I would say just plant them now. I have started putting my seeds out right before winter and have not had a problem with them being taken away by birds. They seem to grow earlier and extra strong after experiencing a true winter. Plant a lot more than you need; Survival of the fittest! http://bit.ly/1O3VLJK

Mostly I just take the open milkweed pods around my 5 acres and send them flying about. Sometimes I can find a bare patch of dirt that had a board or bucket or something on it to smother the weeds, then I press them into the dirt a bit. Very lackadaisical, and my milkweed crop has increased dramatically over the past 10 years. Mostly they probably plant themselves, but I try to help out by draping ripe seed pods in trees in other parts of the yard. Works fine. Mother Nature is the best gardener.