Why is fall perennial planting ideal? The soil is perfect. Warm soil temperatures (unlike spring) enhance rooting. Also the soil is soft and workable -which is excellent for root growth.
Cool air temperature minimize wilting and transplant shock. The days are growing shorter. Thus the plants receive fewer hours of searing sun compared to mid-summer. With less heat, less water is lost through the leaves, thus plants don't suffer as much as in hotter months. Cooler temperatures also make garden chores more pleasant.
Moisture levels are generally higher - precipitation is often greater in the fall and thus soil moisture is more abundant and available to the plants.
Many insects are going dormant, thus plants suffer less from insect attack. Also, those pesky bugs will be less bothersome as you garden.
Fall planting allows you to get a jump start on the spring gardening chores. Spring is often a very busy time in many people's lives. There are numerous garden chores to accomplish. Planting hardy plants in the fall will lessen your spring work.
Fall planting enables you to plan for and plant a better display for the next year. Plants will generally be bigger and showier than those planted in the subsequent spring.
Your garden needs are fresh in your mind. You may not remember the problems, the changes, the editing that needs to be done several months from now. Now is a great time to evaluate your perennial plantings.
Does your garden lack color at times?
Are there holes or empty areas?
Are your plants overcrowded?
Do you have misplaced plants?
Tall in front of short, colors that clash, etc.
Fall is a great time to remedy these problems while they are very apparent and still fresh in your mind.