Root pruning

Just a small question about root pruning here.
If I have a collected tree with lets say, 5 big roots coming from the trunk, then going out to small feeder roots. And I take it out of the box, once each year, and cut the too big roots back hard, one by one for each year, how big is the chance that the cut-back root dies, instead of giving small feeder roots? I was planning to do this slowly with my blackthorn, since the pot I have for it now, isnt deep enough to get it into it. So I have to remove the root height one by one.
Im thinking of building a wooden box fvor it too this year, so it doesnt have to be in a unfitting see-through plastic box..

Hi Storm,
On my end i usually cut all the till i want how my tree would stand in the finish pot,i usually don't encounter problem of die back on the root,i also apply rooting powder,but it's not a guarantee,some don't believe in it,but it works for me.I live in a tropical place,it might not work in your area but put it in a very humid place to encourage rooting also,this are all base tru my own experience.
Happy hunting
Alex

Hi Storm

Why not just ground layer it a touch above all five roots - five birds with one stone, and in only one season?

Stickman Brent