Most gardeners get the urge to plant ornamental trees and shrubs during the warm weather of spring and summer. However, the cooler temperatures of autumn are the preferred planting conditions for woody plants.
Fall weather conditions are preferable to spring and summer because a plant's root system isn't as affected by water scarcity as it would be during a hot summer season.
In the fall, demands of tree branches and leaves lessen as they go into dormancy. Soil temperatures don't cool down as fast as air temperatures, so tree and shrub roots can continue to grow past dormancy. When temperatures warm back up the following spring, fall-planted ornamentals should have a well-developed root system ready to soak up water and nutrients.
One of the most common errors I encounter on my landscape troubleshooting visits is improper planting, in particular planting too deeply. It may seem trivial, but I have seen many trees and shrubs killed from being planted just 2 or 3 inches too deep.
Tree roots are designed to grow in the top several inches of soil. The upper layer of soil contains the correct amounts of oxygen needed for proper root growth. Planting too deeply amounts to piling several more inches on top of the roots, which disrupts oxygen levels and may also cause fungal rot on the base of the tree trunk.
Always dig your planting holes at the same height as the root ball or container soil. To be on the safe side, planting 1/2-inch high may help correct for any soil settling later on.
Another common mistake is the size of the planting hole. Most people will dig a hole just large enough for the root ball of the tree or shrub to slide in. For best results, planting holes need to be at least three times the size of the root ball. This is extremely important if you are dealing with heavy clay soils.
Digging out the extra soil around the root ball adds air pores to the soil when you replace the dirt in the planting hole. When new roots begin to grow, they will easily expand through the freshly-dug soil, ensuring a solid start for the new plant. With only a small hole dug, new roots may struggle to expand through the tough native soils. This can lead to root girdling and early plant death.
There is still debate over whether or not adding soil amendments to planting holes of woody ornamentals is beneficial or not. Much of the research I have seen tends to show there isn't a need for much amending, especially for trees.
Tree root systems will quickly expand several feet in every direction, so they won't see much benefit from soil conditioners near the trunk. In fact, it may actually cause roots to not expand as far because there isn't as much need to seek out water and nutrients.
Fertilizers should be added to new tree and shrub plantings only if recommended by a soil test. Slow-release fertilizers are best used on woody ornamentals. They can be spread over the top of the root zone, or incorporated into the expanded planting hole during transplant.
A 2- to 4-inch layer of mulch should be added after planting to aid in moisture retention and to discourage weed competition. Mulch should always be pulled back a few inches away from the tree trunk to prevent fungal disease issues.