The searing heat of summer not only makes gardening less than pleasant, but it can be damaging to newly planted or otherwise delicate plants. The early fall cooling temperatures make this an ideal time for most gardening tasks. The spring soil is usually quite wet from the winter moisture and spring showers. Planting in the wet spring can cause plant rot, and tramping around in the wet soil is unpleasant and can create undesirable compaction of soil.
Besides the drier soil in the fall compared to spring, fall is optimal for installing most plants because the cooler nights and good sunlight allow new plantings to grow easily without the stress of summer heat. Many plants use their energy to send out roots through the fall while the soil is still warm. Planting in the fall will give them more time to become well established before facing the cold, wet winter and then hot, dry summer.
Timing. Plant trees and shrubs 6 to 8 weeks before the ground begins to freeze; perennials, 4 to 6 weeks before frost. Plant bulbs as long as you can work the ground.
Water. New plants must be kept well-watered going into the colder winter months. The cooler weather though will make your watering chores more manageable than during summer heat. In general, plants installed now will need about an inch of water per week until the ground freezes (which is later in the year than the air freezes). Evergreen trees and shrubs need extra watering in the fall to hold them through winter months when watering may not be feasible. Deciduous plants do okay without supplemental water once they have dropped their foliage in the fall.
Unfortunately, garden center plant stock may not be optimal in the fall because nurseries prefer to sell as much plant stock as possible before winter. However, discounts can often be found at that time. Therefore, do not delay your plans. Shop around for plant material early.