If time allows root pruning / root preparation can provide huge benefits when transplanting trees. If however specimens are being transplanted with sufficient rootball the benefits are negligible.
Roots are pruned at a diameter just inside the rootball size the specimen will be lifted with. This diameter has to be sufficient for the specimen, not what you can move!
The reaction of the specimen is to stimulate root growth primarily along the length of the root, increasing the percentage of root mass lifted.
In addition by spreading the severance of roots over 2+ events, makes it less stressful for the plant and less energy is consumed repairing the cuts.
When root pruning, it is vital to provide a clean cut, a jagged or frayed root is much harder for the plant to repair and provides a much bigger potential area for infection. Do not use a minidigger! This can tear the root at any point upto the trunk and will create a frayed break.
Ideally root pruning should be undertaken during the period November to mid-March. When planning root pruning the maximum time should be allowed between pruning and transplanting within the above timescale. Root pruning stimulates a reaction, so a minimum of 3 months should be allowed. If an extended timescale is available, the optimum will be Nov, Jan, Nov and move early Feb.