Fall is, objectively, the best time of year to plant trees and shrubs here in Virginia!
The combination of warm soil temperatures and cold air temperatures means deciduous trees and shrubs will be focused on growing their roots, while the above-ground portion of the plant goes dormant (i.e: it begins to lose its leaves). Since the plant is not putting any energy into growing leaves or flowering, it will be focusing all of its resources on growing strong roots.
It is very important to establish a robust root system before the heavy rains of spring and the extreme heat of summer set in the following year. Intense heat and drought are (typically) far more stressful to a tree than the cold and snow of winter. That means their best chance of surviving a long, hot summer is to have been planted in fall, giving them the longest amount of time to grow strong and healthy roots before they are introduced to heat stress.
When planting in spring, soil is still cold from winter and the plant will be attempting to leaf out, leaving its root system at a disadvantage as the air becomes rapidly hot and the plant has not focused much energy into establishing strong, deep roots.
Now is your time to rid trees and shrub of common garden pests and diseases! If your nursery stock comes with a few leaves left, be sure to pick all of them up after they drop so that any diseases they may have been carrying will not affect new growth in the spring.
The timeframe for planting different species in the fall varies somewhat. Always put evergreens in first, ideally around mid-September, as they need to establish an extensive root system before winter. Because they do not lose their foliage, they are subject to drying from cold winds and weather fluctuations. However, as long as you can break ground with a shovel it is safe to plant deciduous trees and shrubs.
Some tips for fall planting:
Prepare the site first by amending the soil - we like to bust up the clay here in Roanoke with a product called Turface, and then mix in plenty of compost for nutrition.
Water weekly until the ground freezes.
Use a thick layer of mulch around the base of the plant (but keep away from the stem!) to help insulate the roots and regulate moisture.
Avoid using a high nitrogen-containing fertilizer, as this will promote new growth above ground. Instead, opt for a high phosphorus fertilizer such as SUPERthrive that will stimulate root growth and reduce transplant shock.
If your evergreens went into the ground a little later than ideal, or your site is particularly open and unprotected from wind, products such as Wilt-Pruf can help prevent your evergreens from losing moisture through their leaves.