At this time of year you need to be especially careful about feeding houseplants. Don't succumb to the temptation to feed your plants just because they don't seem to be growing or you think they look as if they need food.

Winter is a time of rest for many plants. Lights is life to plants, and in fall and winter, when there is less sunshine than in summer, most plants grow less vigirously.

In their semi-dormant state, plants need less fertilizer. Most foliage plants can go four months (November to March) without feeding. Flowering plants may need feeding every two weeks when in bud and bloom, but even these require smaller amounts between November and February.

If plants is overfed, nutrients in the fertilizer are not used Instead, the chemicals accumulate in the soil, burn the roots and cause damage to the whole plant, sometimes killing it. Misuse of fertilizer causes more problems than using none at all.

Three elements essential to good plant growth are nitrogen, phosphorus and potassium. They are the main ingredients in fertilizers you use for houseplants. Plants require a diet of at least sixteen chemical elements, or nutrients. Most houseplant fertilizers contain small amounts of some of the other nutrients, somethimes referred to as trace elements.

The main nutrients in the fertilizers are listed on the package by the initials N (nitrogen), P (phosphorus). A formula on the package indicates the proportion of each in that order, N-P-K. A balanced fertilizer has a formula such as 20-20-20, which means 20 per cent by weight of each of the three basic elements; the remaining percentage is inert matter. Plant food with a formula 10-20-10 contains, by weight, 10 per cent nitrogen soluble potash. Examples of some other formulas: 23-19-7, (a preponderance of nitrogen) and 12-30-14 (a preponderance of phosphorus).

Nitrogen promotes green growth. A lack of it generally results in yellowing foliage. An excess may cause rampant leaf production at the expense of flowers. Nitogen is recommended for foliage plants more than for flowering ones. Fish emulsion is a fertilizer rich in nitrogen; the formula is 5-1-1, with trace elements also present.

Phosphorus is important for flowering, fruits production and seed set. For flowering plants, a fertilizer with a higher P number is used, for instance, 5-10-5, or 12-30-14. Potassium promotes strong structural growth, good roots and hastens maturity. Lack of potassium may cause yellowing or dropping of leaves.

Trace elements are required to be listed on the fertilizer label; some commercial brands contain trace elements, some do not. To be sure your plants receive all the nutrients needed, it is a good idea to alternate different brands.

Any powdered, liquid or tablet fertilizer for houseplants can be used. Fish emulsion and seaweed fertilizers are considered "organic" in the gardening sense, that is, derived from once-living organisms.

The following are guidelines for using houseplant fertilizers:

Read the label on the package so you know what you are giong to use, and why and how.

Liquid fertilizers are easiest to use. Some brands are concentrated liquids, for example Schultz and Hyponex for African Violets, and call for a few drops added to a certain amount of water. Some are water-soluble powders, such as Peters and Miracle-Glo. When preparing, use tepid water, and be sure the powder is completely dissolved.

Frequently of application depends on the season and stage of growth of the plant. Many of the manufacturers' instructions are based on plants growing under optimal conditions and active growth.

Indoor gardeners must consider their own situations and make appropriate allowances in estimating frequency and strenght of application.

Use the product at half the strength recommended on the label. In any case, several light feedings are preferable to one heavy does. From March to October, feeding every four to six weeks is a general practice. Newly purchased plants will not need fertilizer for about three months; the grower has incorporated in the planting mix sufficient food for that period.

Keep a record or schedule of feeding dates.

Timed-release fertilizers now are generally available for houseplants. Keeping a record is essential if you are using this new granule-type fertilizer, tiny capsules of plant food which dissolve gradually in the soil; a single application nourished aplant food which dissolve gradually in the soil; a single application nourishes a plant for an extended period, usually three or four months. Timed-release fertilizer simplifies the feeding schedule if you are growing plants, such as African violets, which require constant food supplies. But it can complicate your life if you are growing a number of plants, including some that require a rest period. It means that you must keep an accurate record for each plant on which timed-release fertilizer is applied. These special fertilizers were developed for nursery use and are indeed convenient for commercial growers of large numbers of plants.

Occasionally, perhaps one in three months, omit fertilizer and apply fresh tepid water liberally to flush out unused fertilizer.

Finally, the basic rule for feeding is - when in doubt don't.