Fall is the best time to plant hardy perennials, with many benefits over spring planting. Plants set out during the fall do not have to deal with summer’s peak heat, yet still benefit from warm soil temperatures and the probability of deep soaking winter rains. One of the greatest benefits of fall planting is the opportunity for plants to develop a deep, extensive root system before the arrival of summer. Days are shorter and cooler, and plants transpire far less. Soils remain warm during the fall, encouraging rapid root growth, and the plants have less tendency to wilt. Plants can be watered less often, aiding in the development of a deep rooted plant that can withstand heat stress.
Many perennials can be planted almost year round successfully. With care, evergreen perennials such as rosemary, sage, lavenders and penstemon can be planted even in the winter. Herbaceous perennials, those that die back to the ground in the winter, should be planted by mid October. This includes lemon verbena, ornamental sages, coneflowers, brown eyed susans and catmint. Plant scented geraniums in the ground by early October, unless you plan to bring them inside for use as houseplants. This will assure they will grow an adequate root system before they go dormant.
A final note about fall planting. Often plants purchased in the fall are root bound. Gently pull plants from their containers and check for vigor. Roots should not be discolored, mushy, slimy, or smell odd. A root bound plant with healthy white roots can be pruned by making 2 cuts opposite each other about 1/2 inch deep the height of the root ball. Gently tease the roots apart before planting.