"Fall is for Planting". You probably though it's just a saying to get you to help your local nursery clear out those end of season perennials, trees and shrubs. While we certainly love to see the nursery stock go out the door and into a good home, it's not a marketing gimmick at all. Many factors make fall an excellent time to get those plants into the ground.
A tree, shrub or perennial transplanted in the spring has some big jobs ahead of it: grow new leaves, grow more roots, grow additional length on branches, put out flowers, and maybe set some seeds or fruit. These all take a lot of energy for a plant. Often the plant has to cope with heat and water stress at the same time. Phew! Did you realize what you were asking that poor little azalea to do?
In fall, leaves have gone dormant or will drop. For most, seasonal growth, flowering and fruiting is completed. Root growth, however, continues for most of the fall and winter since soil temperature drops much more slowly than air temperature. In a mild season root growth may continue unimpeded throughout the winter. Once spring arrives, those established roots can better support strong plant growth and improved flowering. Disease and insect resistance is also increased. Well established roots also give plants superior drought resistance-and we all know how important that can be come summer.
Environmentally, fall is a time of reduced insect populations as they are killed by frost or go into hibernation. Diseases that flourish in high temperatures and humidity are largely inactive and cooler air temperatures and higher rainfall provide for improved plant development.
There are also practical advantages on the human side of the fall planting equation. Since you're not trying to get post winter cleanup completed you probably have more time to plant. Your garden is "fleshed out" and you can see where a new tree or shrub would fit. Milder fall temperatures make outdoor gardening more pleasant. Keeping up with irrigation of new plantings is much easier in fall.
Hopefully the advantages of fall planting are apparent. And now you know why, hopefully you will take advantage of the fact the "Fall IS for Planting"!