Several years ago the nursery industry came up with a slogan, "Fall is for planting." The objectives were to help educate gardeners and to extend the selling season. The advice is sound and for several reasons.
Consider the experience my teenage son had in a job this past summer working in a rather large garden. His employer's home sits on a multiacre piece of property with a spectacular view. During the hot, dry months of July and August, he helped her plant scores of shrubs, perennials and annuals.
Unfortunately, because they didn't realize how much water new plantings require, especially during weeks of hot, dry weather, a good percentage of these plants didn't survive, or were so stressed that their chances of making it through the winter are greatly diminished.