By far, the best time to plant a tree is the dormant season, which is in the fall after leaves drop or in the spring before buds sprout. Weather conditions during these times are cool and help they tree establish roots in its new location.
Planting in the summer is not recommended, as the weather's too hot and makes your new tree susceptible to damage.
Evergreens give you more leeway in planting. You can plant earlier in the fall (before the leaves fall) or later in the spring (after budbreak). Heat is still an enemy of evergreens, so you'll still need to avoid summer planting.
Regardless of whether you plant in the fall or spring, proper handling is critical for ensuring the success of your tree.
Before digging the hole for your tree, be sure to locate all underground utilities. You utility companies can usually come out and mark the location of your buried cables at no charge.
The most critical factor in the success of your tree is establishing new root growth. This is the main reason for planting during dormant season. It puts the trees energy below ground, instead of growing more branches and leaves.
To help ensure the success of your tree, dig a shallow, wide hole. It should be about 3-times the diameter of the root ball. This large area breaks up the ground around the roots to encourage root growth. When you recover the root ball, make sure you leave the trunk flare exposed. Stake the tree if necessary. You might have to if you live in an area notorious for high winds, consistently windy days, or storms.
Lack of water is a new tree's biggest threat. Be sure to create a dirt dam around the perimeter of the hole and to cover the base of the tree with mulch. Both will help retain water for the new root system to flourish.