Look for container-grown, or balled and burlapped, shrubs and trees. For all the season's benefits, fall is not a good time to dig trees out of a field where their roots have sprawled and would have to be trimmed back.
Fall transplanting means using less water. It's cooler, and the plants are less thirsty while they're going dormant. But those recently planted shrubs and perennials and trees do need a deep, thorough soaking at planting time; and when there's no snow cover, water every four weeks or so through the winter.
Here's the most important caution. You may want to give your new plants a little artificial assistance in the form of fertilizer. Don't give them a fertilizer that is high in nitrogen, which would encourage succulent, green, springlike growth at a time when you, and the plant, want dormancy.
For immediate cheer, remember that anywhere but in the most tropical parts of the country you can put in pansies. A newly bred super race of the velvety flower, with names like "Snowman" and "Icicle," has just hit the market. In horticulture labs, scientists have intensified pansies' natural cold tolerance. (They're related to wild violets, which developed a sort of antifreeze in order to survive the cold on the deeply shady forest floor.) These new pansies will bloom now, sit tight under the snow, and pop back to bloom again in spring. Not surprisingly, they hate heat, and you'll probably have to compost them next spring.
Even if you don't feel like buying anything new, fall is a good time for cathartic, satisfyingly destructive garden activities. Leave echinacea, sunflowers, and cosmos to go to seed and feed the birds, but tear up annuals like impatiens, marigolds, petunias, zinnias, and tomato plants. If you're not ready to start a compost pile, here's an alternative that requires high aerobic output (and saves on plastic garbage bags): Dig a trench in your vegetable plot. Pile in the tomato stalks and shriveled pepper plants and blackened basil plants, etc. Cover with earth, and next spring you will have Worm City and good soil.
Since we've gotten candidates from Hawaii and Alaska, I've been enjoying the sound of the phrase "the noncontiguous United States." It turns out that even in Alaska, fall is a good time to plant. It's true that by Halloween Anchorage usually has snow on the ground. But up until then, as long as the ground isn't frozen, Alaskans should be digging.
For residents of everywhere else: Even if you've never done a thing in your garden, plant some spring bulbs. You can do this late; it's actually best to plant bulbs after a killing frost. Their only demand is a sunny spot and well-drained soil.
One more note on composting: When wild tulips first came to Western Europe from Turkey in 1562, the burghers of Antwerp, who saw them as onionish, tried to eat them. Not surprisingly, they didn't like them and had their servants toss the extra bulbs onto a pile of organic refuse. The bulbs rooted themselves in the muck and flowered—another tribute to the generative powers of compost heaps. A few centuries later, the Dutch, including a young Audrey Hepburn, would eat them during the days of the Nazi occupation of the Netherlands.