Whether your garden is in Georgia or Montana, it needs your help to prepare for winter's cold and drying winds. The only plants that don't need a little extra attention in late fall are native plants--provided that the summer has been absolutely perfect in terms of temperature and rain for your location. Even plants in "xeriscaped" gardens that have been planted to take best advantage of available precipitation need a little "tucking in" to look and grow their best next spring.
Beginning in August, keep plants neat and trim foliage as it begins to shrivel and die. Stop fertilizing plants in late August or early September; they are trying to ease into dormancy and force-feeding nutrients to them will compel new growth. Sudden cold snaps and frosty nights can kill a plant full of tender growth.
Cut back peonies, daylilies and other soft-tissue perennials to about 2 inches tall to discourage rodents from taking shelter in the foliage as it shrinks and lies on the ground. Trim fans of iris across so they stand from 2 to 4 inches tall; new leaves will begin rising in very early spring.
Keep watering plants until the ground begins to freeze. Plants need to fill up with water to fight off the desiccating effects of cold and wind while they are sleeping. Water at the normal rate, an inch a week, until late October or November. The University of Vermont extension's Green Mountain Gardener suggests that fertilizer or lime applied after plants enter dormancy in October, however, will not be used until the following spring. Gardeners from areas further south should adjust this "winter feeding" accordingly.
Remove summer mulch from native plants in late fall. Replace it with an inch or two of fresh mulch composed of equal parts of compost and manure or humus; the University of Illinois Extension suggests waiting until late November to add winter mulches. The purpose of mulch is to cover roots and keep the ground around the plant from drying out when there is no snow to cover the ground.
Tender plants like roses may need more mulch. "Bury" crowns and lower branches of tender plants with a mixture of compost and well-rotted leaves or grass clippings.
Wrap young saplings with paper and tender shrubs with burlap to protect them from hungry critters. Fruit trees, maples and lilacs are attractive to rabbits. Plant spring bulbs like tulips in open-ended cans to keep rodents away. The University of Illinois Extension suggests poultry wire or hardware cloth "fences" to protect shrubs and young trees from rabbits and deer without covering them completely.
Trim any broken branches or branches that have grown so long that they will surely break during winter from wind, snow or grazers. Save the pruning, whether it's for shape on a fruit tree or the hard pruning on a Knockout rose, for late winter when wounds will not attract pests or disease.