Ray wrote:

I have rooted a cutting from a golden bark willow, and it's thriving in a
still a little too small to put in the ground. But winter is coming, and
I don’t know whether I should leave the cutting outside in its pot, or
bring it inside.

I won't ask why you don't want to plant it permanently... now is the
perfect time. I'd say it's best to temporarilly plant it in the
ground outdoors, called "heeling in".
http://www.gardeningknowhow.com/gard...-in-plants.htm