C.R. Brown, Washington: I live in an air-conditioned apartment with good sunlight. My problem is that my schefflera drops leaves each time it is watered - once a month. It gets good indirect sunlight. What is wrong?

A. A sudden rise or fall in temperature will cause leaves to turn yellow and drop. Move your plant away from air conditioner drafts.

Over-watering is a frequent problem with schefflera. Allow your plant to become moderately dry between thorough waterings. If you can manage it, it should be a good idea to remove the plant from the pot and examine the roots. If the roots are healthy, prune them to keep them in balance with top growth and repot with fresh soil; be sure to provide for good drainage.

No time schedule can be set for watering (for example, once a month).

E. M. Tallard, McLean: The potting earth in many of my plants is bursting with toadstools. I can see the toadstools forming on all levels of the soil in clear plastic pots. I have repotted some plants after thoroughly washing off the roots. I am unsure if larger plants will survive this treatment. What could I do with the larger plants? How can I avoid this infestation in future?

A. Fermate is an excellent general fungicide. Apply it to the soil according to directions on the container.

In the future, use sterilized potting soil. Readymade, sterile potting mixes are available in convenient packages at garden shops and variety stores.

Since your plants have been in the same potting for two years, all undoubtedly need reporting. If the large plants are of such great size that you cannot manage them, at least dig out the upper two or three inches of soil and replace it with fresh sterilized potting soil.

If you are using garden soil as part of your potting mix, sterilize it before combining it with other ingredients (peatmoss, vermiculite, perlite or sand). To sterilize, spread a layer of moist soil in a shallow pan and bake it in a 180-degree oven for an hour. Allow it to cool before mixing it with other ingredients.

Garden soil alone is rarely suitable as a growing medium for plants grown in pots.

Lorraine S. Sulmer, Columbia, and others who send in samples of plants for identification:

A. The plant sample you sent attached to a postcard by transparent tape is insufficient for identification.

For identification of plants, do not send pieces of plants (or flowers) through the mail in plastic wrap, wax paper of plastic bags. The samples deteriorate to uncrecognizable slime.

To send plant specimens satisfactorily through the mail, it is necessary to prepare a pressed, dried piece that is representative of the whole plant. The piece should be mounted with tape on paper or cardboard. Give as much information as you can about size and age of plant, description (or sample) of flowers (if any) and time of bloom. Don't forget to add your name and address.

Portions of a plant can be pressed and dried between layers of blotters, newspapers or in pages of a telephone directory.

In the Washington area such specimens can be sent for identification to the Taxonomist, U.S. National Aroboretum, 24th and R Streets NE., Washington 20002.

A small plant could be readily transported to a nursery in the area (there are several in the Columbia area) or to a plant store (for instance on the Mall at Columbia) for identification. Miscellany

Some people mix coffee grounds on the top soil of houseplants because they think it looks pretty and gives a rich brown color. But nutrient analysis of coffee indicates that it adds little of food value to the potting soil. Moreover, coffee grounds are strongly acid and could have an adverse effect on any plant's growth. The grounds have more practical use for acid-loving shrubs outdoors or as an addition to the compost pile.

Poinsettia - Discontinue pinching back your poinsettia after mid-August. Keep it in bright light, and water and feed regularly until time to take it indoors for special handling to bring it into bloom for Christmas. Horticulture Courses

The U.S. National Arboretum has announced its fall 1978 horticulture series. Courses are conducted at the Arboretum in cooperation with the U.S. Department of Agriculture Graduate School.

Of interest to indoor gardeners are the following:

Introduction to Bonsai: Held at National Arboretum greenhouse, Saturdays, Sept. 30; Oct. 7, 14, 21, 28. Time: 10 a.m.-12:30 p.m. Fee: $19 plus $20 for materials. The Japanese art of creating miniature trees and landscapes in shallow containers.

The Home Greenhouse: Held at the Arboretum activity center, Mondays, Oct. 16, 23, 30; Nov. 6. Time: 6:30 p.m.-8:30 p.m. Fee: $19 plus $20 for materials. Designed to provide the fundamentals of building and caring for the home greenhouse and plants. Telephone 447-6337.