would like to know if I put a wire cage over my new young trees and fill with hay then I have a large thick about 1" thick, 5 ' high, plastic barrel they use to bury nuclear waste in below ground to cover over that and seal the edges with dirt would that be ok or is the barrel a no no because it is black and generates heat. All trees are on south facing side of my property and get a lot of sun in west Tn zone 7 but can get pretty cold here . Last year a few nights at 0 for a few hrs. but not typical for here, average 10-20 degrees is the normal lows in Jan an Feb. at night and 20-30 day time I'd say. Barrels are so thick I don't think they would heat up all that much though I could be wrong but the trees would be in total darkness in there for sure.

You can paint the barrels white, or shade them with something else.

I am also in zone 7, and I am going to use a similar approach to what you described. However, I would not seal the edges of the barrel with dirt. I tried that last year and the trees seemed to dry-out very badly under the barrels. This year after wrapping and mulching the trees, I am going to set the cover barrels on 2x4s or bricks to allow them to breathe just a little.

The way I figure it the goal is to keep them from hard freeze, not any freeze at all. Or at least that is what my goal is considering the varieties I am growing in ground.