Koelreuteria paniculata (Goldenrain tree)

I have a question about the Goldenrain tree. I found some seedlings this year by the mother plant, I dug them up and potted them in 8 inch pots for the summer. They have done extremely well and I would like to know how to preserve them over winter here in Vancouver. I have to keep them in the pots as I don`t have the garden space to plant them. Does anyone have any suggestions for overwintering these seedlings.

 

Plunge the pots in woody material like bark or chips, or aerated mineral material like sand.

 

Thank you kind sir, I will proceed with that suggestion. My other question is this; should I leave these outside exposed to the elements or under a patio cover? (with natural light)