Anytime! The ideal time to start a new bed is the year before you plan to plant in order to build rich, healthy soil. Of course, you can plant anytime and almost anywhere as long as you know your soil conditions and pick the right plant for the right place.
Most of us are guilty of being enthusiastic springtime gardeners and end up nursing new plants through hot, dry summers. The old saying that 'fall is for planting' is a great reminder. When you plant in the fall, especially shrubs and trees, you take advantage of nursery sales and give your new plant a healthy start by utilizing the cooler, wetter fall climate.