Like any other plant, trees experience stress when they're transplanted. That's why it's important to plant trees when the stress is lowest and the opportunity for growth is greatest.
In general, the best time for planting trees is late winter or early spring. In temperate areas, this period is typically followed by a period of moderate weather during which the new transplant will have time to become established. If that does not fit your schedule, then aim for autumn.
When to Plant Trees
Planting trees when they are dormant (or, at least, when they are not operating at their peak growth rate) is advisable since that is when handling them is least disruptive to them. In the Northern Hemisphere, they begin to enter dormancy at some point in the autumn and begin to leave it at some point in the spring.
When to Plant Deciduous (Leaf-Dropping) Trees
The best time to install deciduous types is more obvious. The dropping of leaves in autumn signals that they are entering dormancy. The unfurling of buds in spring signals that they are leaving dormancy. This is clear-cut and goes a long way toward answering the question of when to get your new specimen into the ground.
When to Plant Evergreen Trees
While they do not grow as vigorously in winter as in other seasons, evergreens do not undergo the kind of dormancy that deciduous plants do. Thankfully, evergreens tend to be tough customers, and this toughness gives you more leeway with them. You can undertake the operation earlier in the fall and later in the spring with evergreens than you can with their deciduous counterparts.
You will still want to avoid planting them when it is too hot (or too dry). If it is still hot in your region in late September, hold off till later in the fall. Likewise, if you know that early June tends to bring hot weather to your neck of the woods, you must plant those evergreens earlier in the spring!
Why It's Best to Avoid Summer and Winter Planting
Intense heat is a major enemy to newly planted saplings. In summer the weather is too hot and the actively growing plants are too susceptible to damage.
Weather also restricts your options in the winter (at least in the north), because the cold causes the ground to freeze. If you have had the foresight to do all of your digging ahead of time in fall (before the ground freezes), it is not impossible to plant trees in winter. Unless you can water them sufficiently, early-to-mid winter is not the best planting time, either. Although you may not think of winter as dry time, what with all the snow, you must remember, the moisture from the snow can't get to the roots until the snow melts and the ground thaws. Thus in cold climates, winter brings desert conditions of a sort. That is why watering trees properly in fall is important—regardless of whether you have opted for planting in autumn or late winter (or early spring).