Inland Northwest, High Desert
Choose a Healthy Root System
When you've decided on a tree, slide the root ball out of the planter and inspect the roots. Have they grown in circles around the inside of the pot? If only a few have done so, you can gently separate and spread them outward. Masses of circling roots should be cut. I use a sharp knife to slice into the root ball every quarter turn. Make a big "X" on the bottom of the root ball with your knife, as well.
Take Care With B&B Trees
B&B, or balled and burlapped, is a treatment you might find, especially on larger trees. If the root ball is in a wire basket, cut down three or four sides of the wire to allow root expansion, or remove the upper half of the basket after the tree is set into its hole. Check for twine encircling the trunk and root ball. It's often hard to see when soiled, but if you leave it, it will choke your new tree to death in about three years. Cut it all off. Cut the burlap off at least half way down the root ball, too.
Pamper Bare-Root Plants
Be sure to keep the roots moist at all times. Don't let those bare roots sit out in the sun while you dig a hole. As with all trees, put the tree into the hole at the same level it grew in the nursery. Make sure the hole is wide enough to spread out the roots.
Don't Amend the Backfill
Don't add anything to the backfill. The tree needs to learn right off what it's getting into. Otherwise, it will enjoy the added goodies, reach the edges of the hole, and then treat it like a pot: The roots will turn and circle inside the original hole, eventually choking itself to death. Nobody said they were smart.
Make a Moat Around Newly Planted Trees
Use some of the backfill to form a moat around the planting hole. It will corral water and send it down where you need it - to the tree roots.