In most parts of North America, fall signals the time when plants stop actively growing. As the temperatures drop, perennials go dormant and annuals die with the frost. Deciduous trees and shrubs lose their leaves in preparation for winter. Even evergreens change by sloughing off old needles. Gardeners can do their part with simple methods to help plants survive the winter.
Cleaning up garden debris helps prevent the spread of fungal diseases such as leaf spot as well as insect infestations that may overwinter on fallen leaves. Some clean-up chores are for aesthetic purposes, such as removing old flower stems from daylilies (Hemerocallis). Prune most plants in the spring. Head off next year's weeds by pulling them out now, but don't be too meticulous in the areas where plants such as foxgloves (Digitalis) have seeded. Allow disease-free leaves to remain in the garden as a winter mulch.
Perennials that have only been in the garden for one growing season should be mulched with straw, leaves or pine needles. Some shrubs such as hydrangeas, especially in exposed areas, can benefit from enclosure within a wide strip of burlap wrapped around stakes.Mulch roses with maple leaves at soil level to protect the graft area. If a plant may not be reliably cold-hardy, protect it with leaves, hay or burlap. Established plants don't usually need extra protection.
As a general rule of thumb, don't fertilize plants after the middle of August. Subsequent growth will be killed by the frost and may weaken the plant. Perennials, ornamental grasses, vines, trees and shrubs (evergreen or deciduous) utilize fertilizer much more efficiently in the spring. The exceptions are annuals that require fertilizer into September to keep them in flower.
An application of lime is fine for perennial beds, and around trees and shrubs that aren't acid-lovers.
New plants require more water than the established ones. If fall rains aren't sufficient, supplement with water up until the ground freezes. A dry growing season will require more water for established plants as well. Some plants, such as the viburnums, are always thirsty and need extra water to have a good flower show in the spring. If you follow the caveat to plant in the fall, sufficiently water any new plants, divisions or transplants. North Dakota State University recommends spraying anti-desiccant on the foliage of tender evergreens such as azaleas.
Some perennials, such as hostas or daylilies, can be divided in the fall. Any divisions should be replanted almost immediately. If you haven't decided where they should go, plant them in pots and place in a protected area. Annuals such as cosmos or self-seeding perennials should remain undisturbed for this process to occur. Collect seeds and scatter those that don't need to be chilled and refrigerate those that need a cooling period. Lift dahlias and other tender bulbs to store indoors in a dry place for the winter.
In many ways, nature can adequately provide fall care. Fallen leaves covering the perennial beds and blowing up against shrubs, the autumn rains to snow cover in late fall are all ways nature takes care of your garden.