At this time of year some outdoor gardeners become indoor gardeners, eager to get an early start by producing seedlings to transplant into the garden as soon as the soil and air are warm enough. At this time, too, the indoor gardener may want to start plants for use on balconies or terraces or in window boxes.

To sow too early can be a mistake more serious than to sow too late. Seed can be planted indoors about six weeks before it will be sage to put plants outdoors. In these area, April 1 is soon enough to plant indoors.

No expensive equipment is needed, but there are certain requirements - soil, warmth, moisture, light or absence of light - to assure getting the seedlings off to a good start.

Some household items, such as aluminium loaf pans, cottage cheese cartons, plastic refrigerator dishes, all with proper drainage holes punched in the bottom, are simple and adequate containers. Jiffy-7s and Fertil-cubes, sold by seedsmen and garden centers, are pressed peat pellets into which one to three seeds can be sown. Handle these according to directions on the packages.

It is important that the seed sowing mix be sterile so the seed can sprout and grow without the danger of disease. Ready-made sterile sowing mix, such as Jiffy-Mix, Pro-Mix or Redi-earth, is sold at garden centers and by mail order seedsmen.

When you have assembled your containers, sowing mix, labels, and water, you are ready to plant. Fill the container with the mix to within one-half inch of the top and set it in a tub or sink containing 2 inches of warm water.

Let it soak until the top is wet, then set it out to drain.

Seed should be sown thickly. Most packets contain more seed than is needed for one planting. Save the excess in case some dire happening brings about the loss of the first planting.

Seed may be sown in rows or all over the surface of the mix. When sowing very fine seed, such as petunias, begonias, streptocarpus or browillia, it is a good idea to mix a tiny amount of seed with some milled sphagnum in a cup and spread this mixture over the seed bed or thinly in rows.

Depth of planting depends on the size of the seed. Very fine seed should never be covered. Gently press it into the surface with a tongue depresser or the flat side of a pot label. Other seed should be barely covered, and the largest need to be covered just so that they cannot be seen. Milled sphagnum can be sifted through a kitchen sieve to cover the seed; press it down gently to be sure it is moistened by contact with the mix.

As you plant, label the container or each row with name and date. You may think you'll remember when and what you planted, but chances are you won't.

After sowing the seed, water by placing the container in the tub or sink again until you are sure the surface is moist. Drain and cover with a sheet of plastic or glass, making sure that the cover does not touch the soil surface.

Warmth hastens germination. You can place the container in a sunny windowsill. Other warm spots are the top of the refrigerator or the top of a fluorescent light fixture.

Many seeds catalogs give information about how many days are required for germination and whether light or darkness is needed. Check your seed catalog or seed packets for details. Lettuce, Petunias, begonias and impatiens germinate best in light. Most vegetables require light to germinate. Some seeds which require darkness are protulaca, phlox and calendula. Darkness can be provided by placing folded newspaper or lightweight cardboard over the plastic or glass cover.

Check the container frequently to be sure the seeding mix is moist. If needed, water from the bottom as before. Condensation on the cover can be drained off by slightly tilting the container so the water drops run off inside and into the mix.

As soon as germination occurs and little green sprouts start to grow, they will need as much light as you can possibly provide - daylight, sunlight or electric light for 12 to 14 hours a day. A lack of sufficient light, together with high temperatures, will cause the seedlings to be weak and spindlu. Crowding, because the seed is sown too thickly, also weakens the seedlings.

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