Planting Perennials in the Fall
New perennials planted in the spring must survive summer heat and stress with very young root systems while still producing enough nutrients to support months of growth and flowering. By planting perennials in the fall, roots have a chance to grow all autumn, and most of the winter as well, without having to supply nourishment for the busy growing season. This applies to shrubs and trees as well. So fall is a great time to plant!
While you enjoy your perennials for their fantastic foliage or beautiful blooms, it is really the roots you're buying -- because the roots allow the plants to come back every year.
Container-grown perennials are easy to plant. Start by digging a hole that's a little wider but no deeper than the growing pot of your new perennial. Loosen the roots by hand and spread them out prior to placing in the whole. Add some potting soil or planting mix and firm the soil in around the roots. Top all new plant material with 2-3 inches of mulch for winter protection and water well.