If you are going to move a 6ft holly tree then I would cut a trench about 3
ft from the trunk all round to keep as much root as possible.
If the tree was originally container grown then you will have more chance
than if it is a natural seedling which may well have a deep tap root.
I have moved holly up to 4ft with most surviving, but be ready for a lot of
leaf drop.

--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk