September is planting time, be it spring bulbs, shrubs, trees or perennials. Perennials? Who ever said to plant perennials in the fall?

We do. It's been a trade secret among veteran perennial gardeners for decades, but it seems to have taken forever for the practice to work its way through the ranks of novice flower garden growers.

If you can spend one day setting out established perennials over the next couple of weekends, you will be miles ahead of your neighbors for your spring flower show.

Consider the assets of planting in the early days of autumn:

Selection. Since May and June, perennials haven't been selling well, so the selection hasn't changed since late May. While annuals sold, perennials did not. Today, you still have an array of perennials from which to choose.

Condition. Perennials never had it so good. Over the past four months they were subjected to summer stress, disease and insect attacks, even viral maladies, yet today's plants at the garden shop are much healthier than perennials at home.

Price. Bargains are for the asking. Discounts were hard to come by last May when demand for plants was peaking, but discounts are now the norm, not the exception. In most cases, you can inspect perennials in quart- to gallon-size plastic containers.

October planting. Despite declining temperatures, perennials planted in early October perform better than perennials set out in the spring. The major difference is soil temperature. In the spring, air temperatures warm up quickly, but soil temperatures do not. Except for the Eastern Shore, soil temperatures do not warm up adequately until later than the second week in May, even though air temperatures are well into the 80s.

In contrast, October air temperatures generally hover in the 60s while soil temperatures remain mild. In such an environment, newly planted perennials develop substantial root systems in the fall and therefore bloom promptly next year. They also become sturdy plants capable of surviving a summer drought. Shop now and target your planting before the weekend of Oct. 13.

Thankfully, perennial gardening is not a science. Planting spring tulips in a rainbow of colors is not suggested, but never with perennials whose colors harmonize like instruments in a symphony orchestra. Some garden shops offer booklets pinpointing perennials for all garden environments, so ask the manager for printed instructions.

Not only will veteran perennial gardeners be planting now, but October is a suitable time to divide spring-flowering perennials. Such plants should be divided and reset every three to five years, so you may want to proceed with this work while the weather is mild. Candidates for dividing now include primrose, peony, anchusa, astilbe, day lily, false indigo, acanthus, lamb's ear, marsh marigold and columbine.

Soak the garden in the evening the night before you plan to begin. Spade an entire clump from the soil, examining the clump to identify areas where roots have rotted or died.

Divide the clump without cutting through the cluster with a knife. The foolproof way of dividing is to insert two spades back to back into the clump and draw the handles toward each other. Dust each part with powdered sulfur before planting, working compost or leaf mold into the hole. Don't use fertilizer until late April.

Other priorities as you begin fall gardening:

Nothing is to be gained by keeping houseplants outdoors, so transfer them inside the house or apartment this weekend.

Before the move, make your third and final spray of Safer's insecticidal soap to tops and bottoms of leaves, stems and stalks.

To prevent fungus gnats from getting indoors, mix liquid Diazinon (two teaspoons to a gallon of water) and apply to the soil of each plant, enough so the fluid drains out the base of the pot. Do this early in the day so the soil insects are dead before the move.

Indoors, move plants to a room with east, south or western exposure, returning to the spring watering schedule. Once indoors, plants won't need watering anywhere as often as they did outdoors. The only plants belonging in direct sun are gardenia, hibiscus, lantana, weeping fig and African violets, which should have been moved to sunny windows in early September.

Indoors, prune leggy branches from plants and side shoots with sparse foliage. Such pruning lessens energy demands on the plant, thereby preventing yellowing and dropping of energy-starved leaves.

If you went overboard buying annuals last April for the flower garden, now is when you recoup some of your dollars. Spend time this weekend spading geranium, impatiens, marigold and such from the garden to overwinter plants. Processed properly, the plants will survive the winter in good health.

Your best choice is winter dormancy. Soak the soil the night before to simplify digging the next day. Spade each plant carefully from the soil to save as many roots as possible.

Place plants on the lawn or driveway, spraying them promptly with the garden hose to wash soil away from the roots. Prune all plants back to within five inches, discarding stems and foliage in the process. Find a large galvanized or plastic tub, and add to it moistened sphagnum peat moss.

Fold plant roots into a small ball, then stuff salvaged plants into the moist soil; keep geraniums in one container, impatiens in another, for identification next spring. Place containers on the basement floor in a cool, dark corner.

Write the dates every 18 days from the time the plants are moved to the basement. Pin the reminder sheet to the side wall of the basement stairs. Every 18 days, sprinkle a half-gallon of lukewarm water to the peat moss to keep plant roots alive. Geraniums will be brought upstairs and potted in mid-March, impatiens after Easter next year.

If you choose to keep annuals going, bring up as much garden soil with the plant as possible, even mixing a half-teaspoon of acrylic copolymers with the soil in an effort to increase moisture retention. Move plants into full sunlight and water them almost daily to keep them going.

You're on deadline for applying grass seed. If you do not seed this weekend, you will have to postpone matters until January when "latecomer seeding" opens up. In this case, you will attack the weeds in November, then de-thatch mechanically or by hand the first weekend of December to prepare the lawn surface. Meanwhile, bag all clippings as you mow to capture seeds from fall weeds soon to appear on the lawn.

Meanwhile, don't waste your time going after wild onions (full pulp in stalk), wild garlic (cavity in the center of the stalk) or annual bluegrass. Wild onion and wild garlic have already set offshoots in the soil that represent next spring's crop of weeds. So attacking the weeds now does no good. Keep these weeds cut with the weed trimmer and they won't be that noticeable.

As for annual bluegrass, the time to have prevented the weed was in mid-July when granular Balan could have been applied where to lawn borders where the bluegrass had grown last March. The weed killer Gallery suppresses bluegrass, but doesn't control it entirely.

Gloxinia has been semidormant since midsummer, but now should be forced to sleep for the winter. In late July, you were to have moved your potted gloxinia to a cool, dark corner of the basement, letting the soil dry between waterings.

This weekend, stop all watering. Carefully remove the tuber from the old soil, doing your best to keep the roots intact. The roots are money in the bank because they supply the energy for the first flowers next spring.

Massage the old, dry soil from the tuber. Next, fill a bucket with dry sphagnum peat moss, then set the tuber entirely in the soil. Leave the bucket in a cool, dark corner of the basement.

Apply no water or no fertilizer. The tuber will send out the first leaf buds next January, after which you will pot the tuber and start the process of forcing the flowers. The best time to buy gloxinia tubers is late January or the first days of February because flowering follows soon thereafter.

Jack Eden is the host of "Over the Garden Fence" Sundays from 10 a.m. to 1 p.m. on WTOP Radio (1500 AM).