Gardeners often debate whether spring or fall is the best time for planting perennials. The truth is that both seasons are fine. Spring and fall are usually cooler, with more precipitation, yet warm enough to provide the perfect growing environment to promote new root growth.
However, perennials can also be planted during the summer; just remember that they will need more frequent watering to help them get established. When you are planting perennials, make sure to loosen the roots of pot-bound plants before placing them in the ground. Mixing in some compost or sphagnum peat moss can help provide both moisture control and good soil aeration.