Gardeners should start now to move indoors those houseplants that have spent the summer on the patio or balcony or under the shade of trees or garden shrubbery. You should move them by the weekend of Sept. 29 to Oct. 1 at the latest.

Freezing night-time temperatures can come unexpectedly any day. Most species grown for their beauty as houseplants cannot stand even a little frost. In addition, they need time to adjust to indoor conditions of less light, furnace heat and dry air. It is important to make the move indoors before the heat is turned on and windows have to be closed.

Always inspect plants before bringing them in. Examine them to be sure that they are not harboring any mealy bugs, aphids, scales or other pests. Some people would rather discard a plant than bring in any that show signs of bugs.

Wash the plants with a brisk-hose spray. If washing has to be done indoors, use the kitchen sink spray or bathroom shower, but first cover the pot and soil with a plastic bag. As an alternative, a spray of mild soap suds, followed by a clear water rinse, is effective for removing pests. If you intend to use a houseplant insecticide, work outdoors. Read the label to be sure that the product is safe for your plants. Use such sprays with caution because they are poisonous.

Soaking plants in their pots in a tub of water for 10 or 15 minutes will bring out any insects that may have entered the soil, and that short time in the water won't harm the plants.

Pots that have been sunk in the soil should be examined for earthworms, which are fine in the garden but not in pots. Knock the plants from the pots to examine the rootball for the worms, then return plants to pots, replacing any soil lost in the process.

Clean the outside of pots that have been on or in the soil. Use a stiff brush, or household convenience such as Golden Fleece, to remove soil that might harbor insects or eggs. Remove dead leaves, flowers or other debris from soil surface.

Some plants that have grown rapidly during the summer may need to be repotted, especially if roots can be seen growing out of the drainage hole. Ideally, this reporting should have been done a few weeks earlier as suggested in a recent column. After repotting, keep the plant in its accustomed place outdoors as long as temperature does not go below 40 degrees at night. Be sure to inspect it again for insects before bringing it indoors.

Plants that have been badly damaged by wind, pollution or insects may no longer look their best. They may just as well be discarded as they cannot be counted on to improve in appearance indoors. Start new plants with cuttings from the old.

Moving plants inside can be accomplished over a period of several days to two weeks - as long as the night temperatures remain above 40 degrees. Plants that have been in the garden or on the patio can be moved to a sheltered area, such as a screened porch, for gradual adjustment before going indoors. Apartment dwellers should move plants from balconies well before the heat is turned on. Give the plants a cool, light and airy spot indoors while they adjust to the change. Open doors and windows when you can to provide as much good light, fresh air and natural humidity as possible.

Plants that have spent the summer indoors may as well have some special attention now, too. Wash the foliage. Water is the natural and best cleanser. Prune, trim and neaten them. Plants that have been in the same pot for a year surely need new, fresh soil. Either repot or remove a layer of soil from the surface and add fresh mix. You may need to be ruthless and throw a plant away if it doesn't measure up to what a good-looking, healthy plant should be. Your also can start new plants from cuttings. Time and space are too precious to be devoted to poor specimens unless a cherished or valuable plant must be coddled from year to year.

Very soon the nights will become so cool that for your own comfort you will need to close windows and turn on the heat. House plants that have been outdoors for the summer should have been inside long enough to have adjusted to the changed environment.

Plant Show and Sale

The Tysons Corner Town Hall will be the scene of the Second Annual Plant Show and Sale by the National Capital Area Chapter, American Gloxinia and Gesneriad Society on the last weekend in September.

The hours are Saturday, Sept. 30, from 2 p.m. to 9 p.m. and Sunday, Oct. 1 from noon to 5 p.m.

Admittance is free. African violets, gloxinias, streptocarpus, episcias and many more plants will be exhibited by the members. Choice, named varieties of gesneriads will be available for purchase.

Bonsai Symposium

The Potomac Bonsai Association will sponsor its Fourth Annual Bonsai Symposium Sept. 23 and Sept. 24 at the Sheraton Inn-Washington East, New Carrollton, Md. Registration for the two days of lectures begin at 8 a.m., Saturday, Sept. 23; the fee is $20. The program will conclude at 1 p.m. Sunday, Sept. 24. The exhibit and sales area is free and open to the public.

Indoor gardening questions may be sent to Jane Steffey at The Weekly. The Washington Post, 1150 15th St. NW, Washington D.C. 20071. Please include your address and telephone number.