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).
Backfill With the Dirt You Took Out of the Hole
For years, experts recommended adding compost, peat moss or fertilizer to the planting hole. However, most now agree that you shouldn't backfill with anything other than the original soil from the planting hole (despite what the plant tag says). Soil amendments in the planting hole can discourage the tree roots from spreading into the surrounding soil and can cause poor water drainage. Also, in some instances, fertilizers can kill young roots.
Use a spade to backfill around the tree with the dirt you excavated when you dug the hole. Be sure to keep the tree properly in place (right depth, straight up and down) and shovel in the soil evenly around the roots as you backfill. Once the hole is about half filled in, run water around the roots to eliminate air pockets in the soil and continue filling with soil. The tree will move easily until the hole is completely filled.