Fall planting means strong roots. 80% of a plant's root growth occurs in the late summer and fall months. While you won't see as much stem and leaf growth in the fall, the plants are busy growing new roots. With the arrival of spring, fall-planted perennials are ready to burst forth with vigorous new growth and a profusion of flowers.
In Fall: cooling air temperatures and still-warm soil temperatures are ideal for establishing new transplants.
In Late Fall and Winter: root growth will continue as long as the soil is not frozen. This continued root growth increases the number of nutrients stored in the plant over the winter. Perennials usually enter dormancy around late fall or summer, and resume active growth in spring - but different perennial species will have different seasonal timing.
In Early Spring: fall-planted perennials will have a more established root system, and they'll be ready for vigorous growth and earlier flowering. These established plants are not held back by transplant shock, as compared to new spring-planted perennials.
In Late Spring and Summer: your fall-planted perennials will display larger top growth, more abundant flowering, and more resilient plants. As well-established plants with strong roots, they will be more drought-tolerant, and better able to handle the harsh, drying winds of spring and the withering heat of summer.
Fall planting is also beneficial for gardens in dry climates or – any garden that is experiencing drought conditions.
As plants begin to go dormant in the fall, and their energy shifts from top growth to root growth, they use much less water. Plus, soil is cooler in the fall, and the weather tends to be less windy than spring. As a result, plants and soil are also less likely to dry out from heat and evaporation. Less frequent watering not only helps to conserve resources and makes caring for new transplants easier, too.
In spring, young plants are well-established with strong roots, so they will be more drought-tolerant by the time spring and summer weather arrives.