Start now to bring your houseplants indoors. By that I mean don't wait too long to get them ready for indoor life. The actual moving takes place later.

The first step is to decide which plants you wish to save or, more pratically, which you have room for. Then, prepare them for a changed environment. This may involve pruning, making cuttings, potting and repotting.

Some plants that have grown vigorously during the summer may need merely a light pruning to shape them or to encourage healthy growth during the remaining warm weather.

If plants have become too large for their allotted space in the house, take cutting now for grwoing indoors during the winter:

Coleus, creeping fig, wandering Jew, geranium, miniature rose, wax begonia, Swedish ivy, impatients, purple velvet plant, polka dot plant.

It is better to take cuttings of coleus, wax begonia, geranium and impatients, for instance, than to try to move a full grown plant from the garden outdoors to the garden indoors.

To prepare cuttings, snap off or cut off a branch or stem with a "heel" - the joint where the branch grows out of the main stem. Short pieces, three or four inches, are best. Cut off any leaves that would be submerged or buried when the cuttings is set in water or in a rooting medium.

Starting "slip" in water is a common practice. If you like to root your cuttings this way, transplant them to soil when roots are no more than one-quarter to one-half-inch long because roots at this stage can adjust to growing in soil. Roots formed in water are not structurally adapted to growing in soil.

The preferred method of rooting cuttings is to set the cutting in peat moss, perlite, vermiculite, sand, or in a combination of two or more of these ingredients.

The container for rooting should be deep enough to hold two or three inches of moistened rooting medium. Stick the cutting into the medium to a depth of about one inch and firm the medium around it with your finers. Provide a humid atmosphere by enclosing the container in a plastic food bag or by covering the cutting with an inverted plastic tumbler. Set it in bright light but not sunlight, where it will remain moderately warm.

Length of time required for rooting differs with different plants. Resistance to a gentle tug is indication that roots have formed. Some of the easy ones like coleus and impatients may root in a week. With a medium such as perlite or vermiculite, it is possible to lift the cutting to examine it. If it has not rooted, it can be replaced in the medium.

Methods of rooting cuttings vary among growers and gardeners. The basics are these: Select a stem from a vigorous plant, use a rooting medium that will retain moisture without being soggy, maintain a humid atmosphere and warmth in bright light.

Plant the newly rooted plants individually in small pots, using potting mix suited to their needs. Set each plant at the depth at which it grew in the rooting medium. Water it and place it in a bright indirect light for several days.

Besides pruning and making cuttings, repotting can be done now, too, so that plants will have recovered from the shock before it is time to move them indoors.

Some plants that are rampant growers outdoors in the summer are asparagus sprengeri, Christmas cactus, ginger, syngonium. Tahitian bridal veil and various begonias. Examine them to see if they have filled the pot with roots or if roots are growing out the drain holes. If this is the case, there may not be sufficient soil left in the pot to sustain the plant through the winter. Move the plant to the next size larger pot with fresh soil.

If you have planted any of your houseplants in the ground outdoors, take up and pot them now so that they will be settled in before it is time to take them indoors.

With this late summer gardening you can enhance your own collection, and also can be prepared with gift plants as occasions arise from now until Christmas.

All the plants should be ready to move indoors two weeks before the heat is turned on. Gradually getting them back indoors will be discussed in a later column. Readers' Questions

Frances Plunket, Washington: My new apartment will have a lovely set of south windows in the bedroom. Since I will not be heating it much. I will need plants that will be happy in a relatively cold room. Could you make some suggestions?

A. Most of the plants we enjoy growing indoors can get along very well with temperatures lower than are comfortable for us. A drop of 10 degrees of temperature at night is desirable, but a nighttime temperature of 55 degrees should be the minimum.

You are fortunate in having south windows which increases the opportunity to have flowering plants. The following will thrive in full sun:

Citrus lemon and orange trees, Hoya, Lantana, Bellflower (Campanula), geranium - both flowering and scented, purple velvet plant, shrimp plant, hibiscus, coleus, cactus.

While the quality and quantity of light the plant receives is vital, humidity is probably the most variable condition affecting indoor plants. Moisture is rapidly removed from indoor air by our central heating. Few plants do well when relative humidity is less than 20 percent: higher (50 percent or more) is essential to many exotic species. A humidifier or vaporizer is one solution. Alternatively, waterproof trays containing gravel to a depth of three-quarters of an inch and one-half inch of water will release local moisture around plants. Grouping plants together is also beneficial as it helps to concentrate moisture in the vicinity of all the plants.

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