Begin by digging a trench around the root ball. As long as your tree is dormant, cutting the roots will not damage your Japanese maple.
In fact, root pruning will force the tree to produce more fibrous roots at the cut. This will help your tree become established in its new home.
The rule of thumb is 12″ of root ball per 1″ of tree caliper. Tree caliper is the diameter of the trunk, measured six inches above the soil line. A tree with a 2″ caliper trunk would need a root ball 24″ in diameter.
Japanese maple roots tend to grow more outward than downward. You will want to dig wide– at least as wide as the tree’s crown, if not wider.
Once your trench is done, it is time to shape the root ball for burlaping. If you are transplanting your Japanese maple immediately there is no need to ball and burlap it.
If you plan to keep it above ground for any length of time, you should protect the roots by wrapping them in soil and burlap.
Season makes no difference when planting your Japanese maple. You can plant them at any time. Time is only a factor when digging them.