A container tree should only be planted once it develops a good root system. Once it does, plant a container tree at any time during the year, unless you live in a northern climate. If you live in a northern climate, you might not be able to dig the frozen ground. Further, depending on the type of container tree you have, you could shock the tree too much, causing it to die. The best time to plant container trees is in the fall.
Plant the container tree in the fall, so that the roots do not have to compete with the foliage for nutrients, while they are establishing. A container tree with an extensive root ball establishes itself in the ground faster than a tree with a poor root system or a bare root tree. When planted in the fall, a tree that loses its leaves in the winter can concentrate all its nutrient intake into the root system.
Dig a planting hole that is three times the width of the root ball and as deep as the root ball. Scarify the sides of the planting hole with a pitchfork. Making the planting hole larger than the root ball means that the soil around the root ball is not as compacted, which allows the roots to expand and grow outward more easily. The hole should not be deeper than the root ball. If you backfill over the top of the root ball where the trunk is exposed to air, it is possible to suffocate the tree, especially if the tree has a graft union near the top of the root ball. Graft unions must be kept open to the air.
Ensure that the soil is well-drained. If the area tends to hold excess water after heavy rains, provide drainage for the area by installing a drain pipe or a French drain. A French drain is a trench filled with gravel and sand, and topped off with sod. That allows excess water to be moved away from the area.
Remove any debris from the planting site, especially if you are planting near new construction. Debris includes stones, sticks, oil spills and construction debris. If the soil is compacted, loosen the soil, using a pitchfork around the perimeter of the planting hole. If the soil does not become loose enough by probing with a pitchfork, till at least 3 feet around the perimeter of the planting hole.
Test the soil with a test kit for pH and for nutrients. If the pH is incorrect for the type of container tree you are planting, amend the soil appropriately. If the soil is lacking one or more nutrients, till nutrient-specific fertilizer at least 6 inches into the ground.
Scarify the edges of the planting hole with the pitchfork. Fill the planting hole with water. Center the container tree in the planting hole, then backfill with soil. Lightly pack the soil around the root ball as you are backfilling. Water the tree with at least 1 inch of water. Mulch the tree with 3 inches of compost or pulverized bark.