How much protection a containerized plant needs depends on how hardy it is. Your user info gives no clue to where you live, and your post no clue to what you're growing.

Very generally, plants hardy to 2 zones colder than yours need no protection (other than maybe being sited out of the wind and watered (or throw some show on the soil from time to time) so the soil doesn't dry completely. Plants to 1 zone colder need some protection (but we have no idea what options are open to you). And plants hardy to your zone or warmer need to be dug into the soil, overwintered in a structure that traps heat from an extraneous source, or otherwise kept warm enough that roots never see killing low temps.

Al

Thank you for responding, Al! I was just checking out your Trees in Containers thread.

Sorry; my zone info is 6, NY and the containers will be placed against a wall protecting them from any South-West wind while still receiving sunlight from the South-East.

I have baby Royal Star Magnolia, Ann Magnolia, chocolate mimosa, regular Albizia Julibrissin, Furman's Red Salvia, and black and blue Salvia. The trees are in 22" wooden planters. The salvias are in smaller insulated--2" thick plastic pots.

Would encircling a layer of cardboard around all the containers pressed together be enough? Maybe a blue tarp or black plastic garbage bags wrapped around with the tops uncovered for ventilation too? By the way, is more sun a threat or benefit in winter?

I'm also wondering, with the eventual need for plant lifting/root pruning anyway, perhaps wintering in the ground is better? The only issue then is a drainage problem in the minimal available ground I have.

Sorry for the deluge of questions.. I should have prepared for this ahead of time. With winter approaching, I'm really starting to become concerned. Any additional advice from you is most welcome. Thanks again.