The Potomac Valley chapter of the Indoor Light Gardening Society of America will hold a plant show Saturday and Sunday at the National Arboretum, 24th and R streets NE, off Bladensburg Road. Show hours are 1 to 5 p.m. Saturday and 10 a.m. to 5 p.m. Sunday. It is open to the public free of charge.

The public is invited to submit entries to the show. The only requirements are that entries must have been grown under artificial light and must have been grown by the exhibitor for at least three months before the show. Entries will be accepted at the arboretum from 7 p.m. to 10 p.m. tomorrow.

In addition to the plant show, there will be educational displays, how-to information and experts who will answer questions.

For more information, call Lee Linett, 297-4526.

The Flower Show, a traditional flower and garden show, will be held tomorrow through Wednesday at the National Guard Armory in Washington. It is the first standard flower show the Washington area has had in seven years.

Visitors will be afforded an opportunity to see formal and informal gardens, exhibits of bonsai and Ikebana flower arrangements and special educational displays. Floral and landscaping films and demonstrations of gardening will be shown. Foreign governments are participating with exhibits of flowers, gardens and points of gardening interest in their countries.

The show is sponsored by the D. C. branch of the Professional Grounds Management Society. The National Capital Area Federated Garden Clubs will provide flower arranging displays and nine small gardens.

The standard flower show classes are open to all arrangers and amateur gardeners.

Admission to the show is $3.50 for adults, $2.50 for senior citizens and children under 12.

Hours are 10 a.m. to 10 p.m. tomorrow through Tuesday and 10 a.m. to 8 p.m. Wednesday.

Washington International World of Plants and Flowers will open Wednesday at Capital Centre, Landover, and continue through March 11 from noon to 10 p.m. and March 12, noon to 8 p.m.

Admission is $3.75 for adults and $1.75 for children 6 to 12.

The entire floor area will consist of walk-through gardens. In addition, there will be greenhouse and gardening demonstrations, special floral, fruit and shrubbery displays, and featured exotic plants from the U.S. Botanic Garden.

E. Pollen, Washington: After an amaryllis has bloomed, will it inhibit the future flowering success of the bulb if the leaves are cut prematurely?

I am forcing paper whites in soil and hyacinths in water. Will I have future flowering success with these bulbs?

A. Your amaryllis will not bloom again if you cut the leaves off now. Keep the leaves green and growing by feeding and watering. When warm weather comes, put the pot outdoors and continue feeding and water so that the leaves will nourish the bulb for development of flower buds within.

Some gardeners have had success with planting paper whites outdoors in a sunny, protected place, but flowering in subsequent years is not assured. You will not be able to flower them again indoors.

If you can keep the hyacinth foliage green and growing until spring, the bulb with leaves can be set in the garden to grow and bloom again. It cannot be flowered again indoors.

John Sequeira, Arlington: 1. My asparagus fern is rapidly getting brown and shedding its needles, though it grew beautifully outdoors all summer.

2. My lipstick plant, about three years old, grew well until about three weeks ago, and is now shedding leaves and drooping; the young leaves are getting brown.

3. My spider lily got yellow and brown and finally died.

4. My coleus began to shed leaves and the base of the stem became brown and spongy.

Is there a common explanation? Were they watered too little or too much?

A. The experience with your plants appears to result from too much heat and insufficient humidity. Asparagus fern reacts immediately to hot, dry indoor air by turning brown and shedding needles. Set the pot on a pebble tray or in a jardiniere in which damp pebbles can be maintained to provide humidity around the plant. Misting also helps.

Dry brown leaf margins are an indication of insufficient humidity.

Grouping your plants will enable them to benefit each other. Providing a pebble tray or container from which water can evaporate into surrounding air will increase humidity.

The coleus seems to have been overwatered. Coleus can be allowed to wilt a little, and it will recover when watered.

Watering -- too little or too much --can be the death of plants. No schedule can be established as too many factors are involved. And you cannot treat all plants alike as to frequency and amount. Watering too often may result in a soggy condition in the lower part of the pot; fine root hairs rot; the plant wilts from inability to take up available water, and it may die. When watering, add water until it comes out of the drain hole, thus assuring that the whole root ball is watered, especially important in deep pots.