I'll give my opinion and maybe others that are more local will have better facts. But I think you are gonna have at least another month of warmth. I think I would put them in the ground and let them grow as much as possible then dig and pot or store for winter. There is no way those plants are gonna be cold hardy this year. I know this basjoo is supposed to be the most cold tolerant but those little fellows are not gonna be. But whatever you do at least go ahead and repot so they can be growing. Next spring early as ground is warm plant them out.
James Maloy

James, thanks for the reply. The plants have been outside the whole summer. Do you think that would help with the cold hardy issue? Or is it a matter of their small size?