I am often asked when to plant trees purchased from the nursery. The question has both a long and a short answer, and I'll begin with the short version. Note that this article specifically addresses when to plant trees in areas with cold winters; if you live in a warm climate, then essentially any time of year except summer is a good time for planting (the summer heat presents too great a threat to young trees).
When to Plant Trees: The Short Version
In general, the best time for planting trees is late winter or early spring. If that doesn't fit your schedule, then aim for autumn. Summer's a bad choice, because the weather's too hot and the actively growing plants 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've had the foresight to do all your digging ahead of time (before the ground freezes), it's not impossible to plant trees in winter. But unless you can water them sufficiently, early-to-mid winter is not the best planting time, either.
When to Plant Trees: The Long Version
For the long answer to the question of when to plant trees, we'll remember what's been said above -- but then just expand on it a bit. To begin, I need to introduce some vocabulary items:
Planting trees when they're dormant is advisable, since that's when handling them is least disruptive to them. When do they go dormant? 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. Thus the short answer given above to the question of when to plant trees.
However, in the longer version, we need to be more precise by breaking the original question down into two parts: one for deciduous types, another for evergreens. For examples of both, click "More Images" under the photo (above right) to access the mini-photo gallery.
When to Plant Trees That Are Deciduous
In a way, the best time to install deciduous types is more obvious. The dropping of their leaves in autumn signals that they are entering dormancy. The unfurling of buds in spring signals that they are leaving dormancy.
When to Plant Trees That Are Evergreen
You have more leeway in the matter of when to plant trees that are evergreen. You can undertake the operation earlier in the fall and later in the spring than their deciduous counterparts. However, you'll still want to avoid planting them when it's hot. If it's still hot in your region in late September, hold off till later in the fall. Likewise, if early June brings hot weather to your neck of the woods, plant those evergreens earlier in the spring!
When to Plant Trees: A Follow-Up Consideration
Intense heat is a major enemy to newly planted saplings. But lack of water for their root systems is another. And although you may not think of winter as a dry time -- what with all the snow -- 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's why watering trees properly in fall is important -- regardless of whether you've opted for planting in autumn or in late winter / early spring.
Now that you know when to plant trees, what about the how? Consult my tips for planting and transplanting.