As far as my hornbeam is concerned when should I start working on that tree? It is in my garage now. If I read right somewhere, I can trim some of the unwanted branches now. Is that right? Also when would be a good time to remove from nursery pot for a root pruning? And lastly, if I can do that now is it to early to re pot?
I don't own one of these so I will speak in generalities. Most deciduous trees should not be re-potted this early in a climate such as yours. I suppose that it would be possible if you can keep it frost free after the re-potting but it sounds like that is a problem for you. Also by root pruning you, by definition, damage roots so there is a definite advantage to doing this just as they are coming into growth and will begin to heal. If you work too early the roots are more vulnerable.
Next, at this time of year all of the trees reserves are still in the roots from last summer/fall. If you root-prune too early you cut away all of those reserves. If you wait to root-prune until the new buds are swollen, or perhaps just breaking, the majority of those reserves have already been transfered above ground.
I know it is hard to wait in spring and I have begun as early as March before but I have learned that it is usually better to wait and not try to force the issue. On a similar note, don't be too anxious to bring your tree outside on the first nice day. Bonsai pots will warm much quicker than a tree in the ground and by warming the tree you are encouraging the it to leaf out early. If you should get a late frost you will find yourself in the position of needing to move the tree in and out, which is not ideal for you or the tree. Better to keep it inside the cold garage as late as possible.