One of the problems with pots is they heave out of the ground making them more susceptible to freeze thaw. I haven't overwintered many shrubs or plants successfully but I would imagine the more leaves the better.

SCG

Thanks SCG,
This is my first year over wintering potted trees.
My inclination was to bury them fully with all my leaves...

-Pros:
- large critters cant eat
-moisture retention & insulation
- not having to re-pot in spring

Seeking advice on cons...
- will there be rot or disease problems in a compost pile environment for the winter?
Will there be better survival rate if taken out of pots (even if frozen now). and placed in ground.

Like I said I am far from an expert here. I would imagine if you could prevent freeze thaw and your seedlings are hardy you will have nothing to worry about.

Yes, people report issues with increased canker and the like from winter protection. Since you don't have your zone or where you are from in your profile I can't come moment farther.

What kinda of trees are they?

In MN , zone 4 to 4.5. Gets about -20 to -25 at worst or best, depending on your outlook.

I have the volunteers: maples of diff. varieties, black walnut, sumac, dogwood, poplar, juniper, pear and apple. Probably pretty hardy since they sprouted up on their own here. But I did pull them up this summer, not dig.

The hostas and lillies in pots are not much concern to me as I left many fully exposed last winter with next to no mortality.

The seedlings are:
in order of most concern for survival:

heptacodium miconioides - seven sons flowering tree from seed - zone 5

Bristlecone pine from bare root - zone 4

Eastern white pine from bare root - zone 3

Niobe weeping willow from cuttings. -zone 3

As I said before I have no experience with these plants so take anything I say with a grain of salt.

I would imagine that the seedlings that are hardy to your zone you will have zero problems with if you can keep them from freeze/thawing. The Seven Sons may require as much winter protection was you can provide. I am curious to your plans to protect this vigorous bush when it gets bigger, or are you just going to expect winter die back each year?

SCG

I bought 2, #10 of the seven sons on impulse last year for 75% off
...hoping for the best, they survived last winter - 22 and an abnormal 47 days of below 0 temps.

I collected the seeds from them and surprisingly no critters ate the seedlings this summer. (all of my little oaks were devoured in a day or two of going outside this spring!)

You bring up a good point with die back. In the pic you can see a hard trim followed by 2 years growth about 1.5' from the tops.
I just assumed the nursery was controlling size. But maybe they did experience some die back.

Maybe you will have just enough die back to contain this to a small shrub, if that is what you want.

I resisted the major sale this fall and instead ordered more roses for spring. LOL. I am already over winter and looking forward to spring...

SCG

I'm in zone 3. My pots are packed close together as they will fit, and placed on tarps. In September I reduce the water, only watering when very dry, then after leaf drop, give them one last heavy watering. I keep 20,000 trees this way. I lose 1-2% per year, but more from mice and deer than from cold.

Everyone says this shouldn't work.