Depends on what you mean by "medium" and how likely it is for your soil to freeze where you live. If your soil freezes only slightly (a couple inches, give or take) and the soil mass surrounding the roots is sufficiently large, you should be OK.

Roots are the most vulnerable aspect of the plant regarding cold - if they are small trees in small pots and could freeze in total, then you will need to protect them. Here, I grow a lot of Japanese maples in pots, from 6" on up, but as our winters are quite mild with virtually no soil freezing, the maples are fine left in situ with no additional protection.

Root damage/death at soil temps at/below 14F. Either heavily mulching the pots or placing them in a protected place like a shed or garage is probably the best way to go. Its ok for the soil to freeze and stay frozen for weeks on end. It's how cold the soil is that determines whether the roots are damaged. My potted trees spend the winter in my unnattached garage, mulched with woodchips and frozen solid...air temps in garage below 10F last year...all do fine.

Dave

Death of rootage occurs in stages & there is no way to put a "lowest temperature threshold" on a plant. Even if the hardiness of a plant that was vegetatively propagated is well known, there is still wide variation in that plant as cultural conditions change. E.g., a drought stressed Acer is likely to be able to withstand root temperatures as much as 10* F. lower than a fully hydrated plant. Add to that the fact that finer rootage succumbs to low temperatures many degrees before the more woody roots do. Then, we know that there is tremendous difference in cold-hardiness within species, genetics within species being of greater importance in the area of cold-hardiness than the species itself.

Plants that see temperatures fluctuate into the 40's (like in an unheated garage) will exhibit a lower degree of hardiness o/a than a plant growing in situ, where soil temperatures fluctuate less. If I had to pick a minimum actual root temperature, below which I would say is approaching "very risky" for most Acers, I would put it at about 20 - 22* F. Hair roots will be killed at about 25*, the finer, suberized, but not yet lignified roots will be lost in 22 - 25* range, and there is danger of the entire plant being lost at actual root temperatures under 20*. Of course, the trunk, branches & other dormant tissues above ground are hardy to much lower temperatures than roots.

If you've ever had a containerized plant that leafs out very late when compared to other Acers, suspect winter root damage. The plant will direct energy to regeneration of roots so it can support the canopy before it puts on a spring flush.

Al

You must be on Long Island. I'm in Larchmont, near the Sound, and lost a tree a few years ago that was the one I left out. It was in the largest container, a hardy Shirasawanum, and it was not as cold as last winter. It was a hard lesson. Now, whenever I get to 15 degrees, the trees go into the garage. Better to be safe here. Wind is as much a killer as the base temperature when it gets that cold.