Say you live in Zone 5 and experience cold winters. Shouldn’t affect your red oak, right? I mean, that’s a species that is cold tolerant down to Zone 3! But if it’s in a container, the otherwise-hardy tree is automatically more at risk in winter. That goes for shrubs and perennials, too, which need insulation for their roots. Fortunately, there are several solutions.
Here are several potted oaks that I am overwintering. I dug them into the ground and placed them--pot and all--in the hole, then backfilled and covered with shredded leaves. This is the best protection you can get, but it takes the labor of digging the hole. Be sure to protect the tender bark from rabbits and field mice (note white bark guard in foreground).
Lazy man's protection: I put several potted trees in a small cove and filled the gaps with bagged leaves I rescued from the curb. Pack the leaves so there are no air pockets; remove in spring and add to your compost pile (or use as a mulch for flowerbeds).
If you don't have an existing cove, make your own. I stacked some blocks near a chainlink fence, then filled the space with potted trees. After that, I dumped bags and bags of leaves around the potted trees, compacting them so they formed a tight mulch around the pots. Again, these leaves can be removed in spring and added to the garden as a mulch or compost ingredient.
Smaller trees can be stored in an unheated basement or attached garage (an attached garage probably won't go below 20 degrees in winter...temps of 15 and lower can damage or kill potted tree roots)..