Dig a Saucer-Shaped Hole
Dig a saucer-shaped hole three to five times the diameter of the root-ball (or the spread of the roots for a bare-root tree). This allows the roots to easily penetrate the softened backfill and properly anchor the tree. If you're planting in clay or wet soil, use a garden fork or your spade to roughen the bottom and sides of the planting hole to avoid glazing. Glazing happens when the sides and bottom of a hole become so smooth and compacted that water can't pass easily through the soil. In extreme situations, it could block roots from penetrating the sides of the planting hole.
Don't Plant Too Deep
Plant the tree so its root collar—the trunk flare right above the root system—is about 1 in. above the soil level. Take the tree out of the container (slitting the container sides) or cut away the wire cage and burlap. Then measure the distance from the root collar to the bottom of the root-ball and dig the hole to that depth. Don't rely on the container size, the wires or the wrapping around the roots as an indication of the depth you want to plant your tree. If the tree is planted too shallow, the roots could be exposed above the ground, especially as the tree grows. But don't plant it too deep either (a common mistake!). New roots can girdle the trunk and they need oxygen to get established.
If you're planting a bare-root tree, leave a cone of soil at the bottom of the planting hole and set the root system on top. Place the handle of your shovel flat across the hole from one side to the other to make sure the crown is level with the surrounding soil.You should be able to partially see the root collar, or trunk flare, after the tree is planted.
Set the Roots Free
Before placing the tree in the hole, break up the tightly wound root-ball and carefully fan out the roots. Don't pull too hard or the roots will break. It's OK if some of the soil in the root-ball crumbles and falls off. It'll help free the roots. Pulling apart the root-ball encourages the roots to expand into the surrounding soil. If the roots circle the root-ball, but none are thicker than a pencil, use your fingers to tease the root-ball apart.
If the tree is severely root-bound and has circling roots larger than a pencil in diameter, box-cut the root-ball using a pruning saw to shave off all four sides, creating a square root-ball. Once the roots are free, you'll have to be careful when you handle the tree, or what's left of the root-ball will fall apart and you could tear the smaller roots.
Never pick up the tree by its trunk. Instead, support the tree from under or from the side of the root-ball. Set the tree in the center of the hole. Again, keep the root collar about 1 in. above ground level. If it's too high, remove the tree and dig the hole a little deeper. If the trunk flare is too low, add soil under the roots. Cut away all rope, twine, wire, staples and burlap before backfilling (you can leave natural burlap underneath the root-ball if you can't cut it all away).