Some questions seem to trouble indoor gardeners more than others. Here are some of the most frequently asked questions, and the answers.

How often should I water and feed cacti and succulents in the colder months when they are dormant?

Proper watering of cacti and succulents depends on the type of plant, type of container, size of the plant and time of year. Most species of cacti go into a semi-dormant state from about November through February. During this period they should be watered only enough to keep the plants from shriveling. Under usual home conditions this would mean about every two or three weeks. In plastic or glazed containers, less frequent watering would be needed. Especially with cacti, avoid the temptation to water them as much as you would your other houseplants. Give them practically no feeding during this period.

My beautiful, huge fern got through the summer on our covered patio with lots of misting. Now it's dying, with leaves falling all over. What do I do?

My three Boston ferns are slowly turning brown near the soil and on many of their tips. They are not in direct sunlight and get daily misting. What can I do to save them?

Boston ferns observe a brief resting period each year. The first sign of resting is yellowing of the older leaves and the dropping of leaflets. Clip off old leaves, omit feeding and lessen watering at such a time, but never allow the roots to become dry. Maintain high humidity by misting or by providing a pebble tray under the pot.

I bought sterilized potting soil for my plants, mixed a little vermiculite with it, but they all seem to be getting mold on top of the soil. What am I doing wrong?

Fermate is an excellent general fungicide which should take care of the mold. Apply it in solution to soil according to directions on the container. Physan, one teaspoon per gallon of water, can be sprayed over seedlings or over pot tops to prevent growth of algae and some forms of fungus.

Could you tell me how to "hold over" geraniums? I have heard of a method whereby the plants are unpotted and hung up to dry.

Everybody always wants to know if they can hang up their old geraniums by their toes in the basement and plant them out the following spring.

It is possible if you have a dry, very cool basement, but in our modern houses, as contrasted with grandmother's dirt-floor cellar, it is more likely to be wasted effort.

The best way to preserve a special geranium is to take cuttings in later summer, and grow the small young plants as houseplants through the winter.

Or, if you have sufficient indoor winter garden space, you can salvage this summer's plants by cutting them back to about 3 inches when you bring them indoors; reduce watering and omit feeding. As days lengthen after the New Year, signs of growth may appear. This is a sign to start feeding and increasing the amount of water. Toward the end of February or early in March, in a sunny growing area, they should flower again.

and R. Streets NE, Washington, will offer a free bonsai class that is designed to introduce students to the basic techniques of developing and growing bonsai. The classes will include films, lectures and workshops. The course will consist of four, 2-hour sessions on Tuesday and Thursday mornings from 10 a.m. to 12 noon, Sept. 12, 14, 19 and 21. It will be taught by Robert Drechsler, curator of the National Bonsai Collection. There is no charge for the class, but a $15 materials fee will be charged to cover cost of text, plants and containers.

For reservations, call the education office at the Arboretum, 399-5400 ext. 71. Advance registration is necessary. Bonsai Class for Beginners

Many indoor plants are excellent subjects for bonsai. Examples are: Citrus species such as calamondin orange, mistletoe fig (Ficus diversifolia), dwarf pomegranate (Punica granatum nana), Pottosporum tobira.

The U.S. National Arboretum, 24th and R Streets NE, Washington, D.C., offer a free bonsai class that is assigned to introduce students to the basic techniques of developing and growing bonsai. The classes will include films, lectures and workshops. The course will consist of four, 2-hour session on Tuesday and Thursday mornings from 10 a.m. to 12 on, Sept. 12, 14, 19, and 21. It will be taught by Robert Drechsler, curator of the National Bonsai Collection. There is no charge for the class, but a $15 materials fee will be charged to cover cost fo text, plants and containers.

For reservations, call the education office at the Arboretum, 399-5400 ext.71. Advance registration is necessary.