Too cold to plant a tree?

My apple and my damson trees have arrived and we have this second cold snap. The apple is quite small - are they usually only about four feet high when they arrive? Compared to the damson which is about six feet tall and has branches about 3/4" thick in most part.

The Apple is so slender overall it looks more like a shrub. It is a four variety family tree.

My main concern is if I plant them in their permanent positions on the allotment I can't keep them monitored in this weather all the time, whereas if I pot them into large pots I can keep an eye on them in the home yard and even bring them into the garage to thaw if I feel they need more water to the roots.

I know they are dormant at the moment, but if the ground freezes up, does this mean there will be no water to the roots and a risk they will die through lack of water?

I am surprised the damson looks so much more stocky than the apple. I had imagined they would be similar or the apple would be bigger.

Thank you. This is what I thought.

Any comment on the huge size difference between the Apple and the Damson? The Damson is huge compared to the Apple, which looks like a shrub in comparison.

Thanks Mum. The rootstock is M26 first year (dwarfing), whereas I remember now ordering a 2nd year tree for the Damson - makes sense it should be bigger   

Yes that was my concern is the frozen soil may leave air pockets around the roots and kill them off. I have potted them into large tubs ready to plant them out with the compost they are in if the weather improves or very early spring now the roots are in moist compost.

They are in the unheated garage off the floor on pallets so they will be too cold to germinate but warm enough not to freeze.