Jennifer Russel of Bethesda has several questions, the first about her Dracaena: My fairly large Dracaena marginata suffers from brown-edged leaves. The plant is set on a tray of pebbles and I mist it to increase humidity but the condition persists. It is growing and seems healthy. Any suggestions?

One of the most common problems with Dracaena marginata is brown tips of the leaves. Possible causes are (1) lack of humidity, (2) fluorine in the water or (3) the plant was allowed to dry out at some time. You have already taken steps to increase the humidity You cannot correct the problem of fluorine unless you save rain water for use on your plants. While the Dracaena will survive when allowed to dry out for several weeks, it really prefers to be kept constantly moist.

Russel also asks. My hanging philodendron is plagued by many brown crispy leaves which eventually drop off; new shoots which appear often become brown and crisp rather than creative new leaves. What's wrong?

It sounds as if your philodendron is located in an "air stream", probably hot dry air. Frequent misting should help. Another possibility is that the soil in the pot is too full of roots, or that the soil has become exhausted; in either case repotting is recommended.

Russel's third question is: I am planning to set up a group of plants under a grow light; would you recommend a fluorescent tube? What type of plants (medium size) flourish under a grow light environment?

For a group of plants, you will get better light distribution by setting them up under fluorescent tubes than if you use a grow light. The minimum length fluorescent tube generally recommended is the 24-inch size. Many of our most familiar houseplants flourish under fluorescent lights. Choose those with similar cultural requirements. For foliage variety, try miniature Caladiums, Maranta, Peperomias, Pileas. For flowers, Oxalis regnelli, Oxalis martiana aureareticulata. If you like begonias, small sizes are available, as is true also of geraniums. Ask at your local public library for a book on light gardening such as "The Complete Book of Houseplants Under Lights" by Charles marden Fitch (Hawthorn Books, $9.95).

James Koziol of Gaithersburg wants to know: What is the proper method of dividing my huge asparagus fern? Most of the lush green growth is occurring on the outside of the plant. Is there any way to rejuvenate the inner mass? Also I have a coffee tree that is over five feet tall. This year it is bearing coffee beans for the first time. Can I convert the beans into coffee grounds? Is it possible to propagate my coffee tree somehow?

You can divide your asparagus fern at any time. Cut off the stems at the soil line. Cut the fern into as many pieces as you want to use for new plants. On an old plant the roots are thick, fleshy and sometimes tough so that a heavy sharp knife will be needed. A packaged general purpose potting soil is suitable. Water the repotted divisions thoroughly. When the new green shoots begin to develop, you can feed the plants with a balanced houseplant fertilizer. Dividing the old plant will take care of the problem of lack of growth in the center of the old plant.

The coffee plant can be propagated by taking stem cuttings. Make the cuttings from growth in the upper third of the plant. This is also a means of pruning the plant if you think it is getting too tall. With a sharp knife make cuttings about 4 inches long from the top growth of vigorous branches. Insert each cutting in a container of moist vermiculite perlite, or a 50-50 mixture of peatmoss and sand. Put the container in a plastic bag, closed with a rubber band or wire tie, and place it in a light place such as a windowsill. New green growth at the tip of the cutting will indicate that roots have formed. Then, open the bag to let the new plant become accustomed to the air outside the bag. After about a week, the cutting may be potted in a general houseplant potting mix.

I believe coffee grinders are obtainable as household appliances.

Rita Roe and Mrs. G. W. Arthur of Arlington are owners of Ficus benjamina trees which are being rapidly denuded of leaves. Both are using moisturemeters to determine need for watering, and ask what else can be done.

The tree is probably trying to adjust to a new location, compensating for reduced light and humidity by dropping leaves. Give the plant bright indirect light, or morning sun. Allow it to become moderately dry between thorough waterings. There should be no drafts either from heat or air conditioning vents, or from the opening and closing of doors. Day temperatures of 75 to 85 degrees are ideal, and 10 to 15 degrees lower at night. The 'Exotica' form of this Ficus, especially, tends to react to cold and to temperature changes by dropping its leaves.

Mary Kay Streck of Fairfax and Julia Ann Breidenstein of Burke ask if there is anything that can be done to get rid of small gray gnats and the little worm-like creatures in the soil.

Two of the most common insects associated with houseplants are fungus gnts and springtails.

Fungus gnats are small black or brown flies, sometimes mistaken for fruit flies. They fly around potted plants and near windows when disturbed. The files do not feed. The larvae (small white worms) of these files feed on organic matter in the potting soil; they rarely attack healthy stems or roots. Both the larvae and adult gnats are attracted to moist soil. Allowing the soil to dry out for several days or a week is probably the best control measure.

Springtails are very small, usually white insects that crawl and hop over the surface of potting soil. They often come to the survace when plants are watered. They live in moist soil nearly everywhere in the world and frequently infest potted plants if soil from the yard or garden is used. They do not harm healthy plant roots. The adults and young feed on organic matter in the soil. Allowing the soil to dry out will help to control them.

For both of these pests, a 50% Malathion houses-plant spray insecticide may be used; spray the soil surface, saucer and pot. This type of spray may also be directed at the adult fungus gnats swarming in windows near the plants.

Miss Catherine Johnson of Washington writes: I have a problem with mites. They look like little balls of cotton. I have tried washing them with plain water and tobacco water; they just keep coming back. What do you recommend?

"Little balls of cotton" insects are mealy bugs. Use a cotton dab on the end of a toothpick, dipped in rubbing alcohol, to touch each little critter. Try to keep the alcohol off the foliage and stems of the plant. The mealy bugs will die and fall off the plant. Then wash the plant with warm soapy water and rinse. Do not set a wet plant in bright sunlight. A severely infested plant can be sprayed (outdoors if possible) with malathion houseplant spray.After the spray dries, rinse the plant with clear water.