There is no better time to plant trees and shrubs than fall. They won't make much visible growth in the cool weather and shortening days, but below ground their roots will be growing and establishing themselves in preparation for a strong season of top growth next spring. In contrast, planting in spring or summer can lead to voluminous top growth before a healthy root system is developed, causing withering in summer's heat when the insufficient root system can't pull in enough water.
Planting shrubs and trees is far from rocket science, but there are steps that can help ensure success.
Dig a Worthy Hole
Dig your hole to the same depth as the height of the root ball and 2 to 3 times wider than the root ball.
TIP: In poorly drained soils, construct a raised bed first (or build up a low, broad mound of soil) and then dig your hole in the center of the raised area.
After the hole is dug, break up any clods in the soil that was removed. This is easiest to do in a wheelbarrow. If you will be adding amendments it is best to only do so for the top third of soil going back into the hole.
The most current research recommends a light hand when modifying the soil, as too much of a “texture" difference between the soil in the planting hole and the surrounding soil hinders the movement of water. It's similar to a potted tree with a saucer beneath it, sitting in a garden. This would leave you with a root ball that is easily water-logged when you use the hose, because it can't drain and dry when there's rain, because it can't pull moisture from the surrounding soil. A landscape tree or shrub needs to be able to grow roots that extend well beyond what a “pot" of amended soil would accommodate.
The roots of the tree or shrub need to be dislodged from the circular shape they're forced to grow in by the pot—otherwise they may continue to grow in a circular shape, rather than spread outwards. If the roots are not very crowded on the edge of the root ball, simply scuff the soil slightly with your fingers to dislodge any small roots that are there. If the roots are extremely 'root-bound', it may be necessary to pry them loose with the tip of a trowel.
Place the root ball in the center of the hole and check to see that the top of the roots are even with the top of the surrounding soil. Add the prepped soil back into the hole around the roots, compressing it slightly with your hands as you go to eliminate any air pockets. Once the hole is filled, spread any remaining soil in a thin layer over the area surrounding the plant.
TIP: Stand back and look at the plant in the hole before adding the soil. Adjust it so it is standing up straight. If one side looks nicer than the other, face the most attractive part in the direction where it will most often be viewed from.
Care for the New Planting
The planting process is not complete without giving your new shrub or tree a deep soaking from the garden hose. When doing a fall planting in an area where the soil freezes over winter, you may not need to irrigate again until spring, as long as the soil stays moist until it freezes. Plan on continuing to water through dry spells, checking soil moisture every few days for the first month and then every other week for the first 2 years of growth. Check the soil moisture several inches down, within the root ball area of your tree, to determine when water is needed. Aim for the soil to be moist, but not soggy. Don't rely on lawn irrigation as the turf quickly takes up that water for itself with its very shallow roots.
If you're planting a large tree (over 6 feet), in a windy or sandy location, this is the time to install stakes to prevent it from toppling over. Staking is only recommended for windy or very sandy locations. Allow for a bit of movement, as staking the tree too rigidly can inhibit the root development that naturally comes in response to trunk movement. Be sure to cushion any rope or wire where it comes in contact with the trunk to avoid damage to the bark. A staking arrangement should not be left in place for more than one year and should be checked frequently to assure it is in good condition and not damaging the tree.
Finally, spread a layer of mulch over the planting area to conserve moisture and keep the weeds down. Do not place the mulch right up to the trunk, as it will slowly cause the bark to rot, eventually killing the tree.