Early fall, from early September through late October, is a great time to put plants in the garden. The shorter days and cooler air temperatures reduce the demand for water and keep plants from producing an abundance of new leaves, while the still-warm soil promotes root growth. Thus plants have plenty of time to build a healthy root system and get established before the onset of winter.
Late August through mid-September is also an ideal time to divide and replant those perennials that have become overgrown. Irises, Hostas, Daylilies, Coreopsis, Black-eyed Susan, summer Phlox, and Astilbe are just some of the perennials that can be divided at this time.