Suddenly we are being subjected to real winter temperatures! Indoor gardeners may need to take some precautions to insure that plants are not adversely affected by the lower indoor temperatures we maintain to conserve energy.

Most houseplants fare well within a range of 15 degrees, from 60 at night to 75 during the day. Some measures can be taken to relieve stress if you have lowered the thermostat to 65 in the daytime and 55 at night.

Indoor temperatures may vary as much as 20 degrees in a room, from positions near a window to others near heat ducts. Space near a window may be warmer than other spots in the room on a sunny day, but cooler on sunless days.

Plants near windows will experience a lower night-time temperature than plants elsewhere in the room. In extreme cold, it is advisable to move plants away from windows or draw the curtains. You can also place cardboard or newspaper between plants and the window, especially at night.

Leaves should not be allowed to touch window panes. The entire plant may not be frozen but any leaf that touches frosty glass will be lost.

When you move plants away from windows, don't set them in front of a radiator or near space heaters.

Avoid drafts or blasts of cold air from doors frequently opened. Move plants away from entryways where there is continuous traffic.

With lower temperatures and slower winter growth, plants need less water. Reduce the frequency of watering.

Heat from incandescent bulbs can add warmth to a limited area, such as space near a window or on a plant stand. But, remember that this heat can also burn leaves. A plant placed under a table or desk lamp is close enough to the light, if your hand held just above the leaves feels the warmth of the light.

For plants that require warmth, perhaps you can contrive a protective enclosure so they will have the benefit of any heat available. Some plants in this category are gloxinias, African violets and other gesneriads, for which the temperature should never be less than 60 degrees at night.

Optimal temperature conditions for plant growth depend on the plant's place of origin. Most indoor plants come from warm climates but from elevations that are cool at night. In ordinary circumstances, when lights go out in home or office at night, a drop of 10 degrees is welcome. However, if the temperature remains below 60 for any length of time, plants will be checked in growth and may not recover. In offices that maintain temperatures of 40 to 45 degrees for long weekends, plants will suffer, especially flowering species. Tropicals such as dracaena, schefflera, philodendron and ficus will be adversely affected.

Most foliage plants are semi-dormant, not actively growing, during the short, often sunless, days of winter. Unless the low temperature is excessive and of some duration, plants will just sit there and wait for things to warm up -- as the rest of us may also be doing. Persian Violets

A good choice of houseplant that easily can be started from seed is the Persian violet. This small, compact plant bears a profusion of delicate and fragrant violet-blue flowers. However, it is not a violet. It is Exacum affine, a member of the gentian family.

Exacum flowers, about half an inch in diameter, each with a prominent yellow center of stamens, are held above the smooth green foliage on a plant about eight inches tall.

It takes four or five months for the Persian violet to come into flower, so if seeds are planted soon, indoor gardeners could have flowering plants for Mother's Day gifts or as early as Easter, which is April 15.

After danger of frost is past in the spring, plants can be set out in the garden.

Be prepared to give the young plants a warm growing place. To sow seed, fill a flower pot, any container with drainage holes, with potting soil or soiless mix. Sprinkle a layer of milled sphagnum moss on the surface. Sow seeds on top of the moss. Set the pot or pan in a larger container of warm water and leave it there until the surface moss is moist. Then remove it and drain for an hour or so.

Place the pot in a plastic bag and close it with a rubber band or tie; moisture and humidity are thus conserved inside where needed for seed germination.

Place the bagged container where there is warmth (70 degrees F.) and bright light. The seeds will germinate in about 14 days. As soon as this happens remove the plastic bag.

Seedlings need to be watered and fed to encourage sturdy growth. Use a weak solution of liquid plant food. Transplant the seedlings to individual pots when they are large enough to handle. Give them a sunny location (they should get about four hours of sunshine a day.) Feed every two weeks with a liquid houseplant fertilizer.

The Persian violet is an excellent plant for fluorescent light gardens, where 15 to 16 hours of light a day is recommended as well as feeding with every watering. Use a liquid fertilizer at one-fourth the strength recommended on the package.

Seed of Persian violet, Exacum affine, can be obtained from Park Seed Co. Inc., Greenwood, S.C. 29647.