Root pruning can be carried out several ways.see ........
http://www.rhs.org.uk/advice/profile...ot_pruning.asp

I would use a spade around the edge of your borders, then if you hit large
roots then dig and cut.
I wouldn't do the recommended 4 ft from the trunk with a tree of that size,
also if you find a lot of large roots then do some each year, over about 3
yrs or so.


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