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 semi-dormant state from about November through February. During this period 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 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 the 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. Is that possible?

Everybody always wanted 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, but during the darkest days they will not be very attractive plants.

I bought a beautiful philodenron and in only a few weeks it began to wilt. No matter how much I watered it, it never perked up and it finally died. Why?

This is a typical example of watering a plant to death, and the problem is reported in frequent questions to this column.

Over-heating is the most frequent failing of indoor gardeners. Plants can wither and die because too much water has been given. True, a continuous stream of water must flow through the plant from root to leaves and thence to the air. Roots take in plant nutrients dissolved in water. There must be roots. But air spaces in the soil are essential to good root development. If the air spaces are clogged with water, root development is inhibited and the delicate roots rot. The plant begins to wilt. More water only aggravates the situation, and soon no roots, no plants. Finis to the philodendron.

There is no time schedule you can follow for watering your plants. Just keep in mind some basic points.

Always use tepid water, not cold.Cold water is a shock to many species.

When watering, apply enough water so that it comes out the drainage hold of the pot. The water should drain away from the soil surface in a couple of minutes, indicating good drainage. Then the soil should be allowed to get rather dry at the surface before you water again.

The soil will dry out more rapidly in a clay pot than in plastic or ceramic.

Always discard the drainage water from saucers within 20 to 30 minutes to prevent soil at the bottom of the pot from becoming water-logged.

A plant in active growth, showing sprouting buds and young leaves, will need more water than a plant that is resting. In bright light, in warm rooms or under supplementary lighting, more water will be required.

Remember that the health of plants depends not only on the water in the soil but on the moisture in the air - the humidity. When you use pebble trays to increase humidity around plants, be sure that pots sit on pebbles, not in the water. Greenhouse Seminar

The public is invited to an informal seminar presented by the Hobby Greenhouse Association of Northern Virginia on Saturday Sept. 9 from 9 a.m. to 5 p.m. at Fairfax High School, 3500 Old Lee Highway, Fairfax.

The seminar is designed for all plant lovers, whether they garden in a greenhouse, a cold frame or on a windowsill. Greenhouse supplies, plant propagation methods, plant pests and disease and plants, of course, are a few of the horticultural topics to be discussed by speakers from Virginia Polytechnic Institute and State University, the U.S. Botanic Gardens, George Mason University and industry.A question and answer period, displays and exhibits and drawing for door prizes of plants and supplies are part of this day-long event.

The seminar is sponsored by the Cooperative Extension Service, VPI&SU and the Department of Extension and Continuing Education in Fairfax County in cooperation with the Hobby Greenhouse Association. For further details, call Sharon L. Nolan, extension agent, 691-3456. 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), Pittosporum tobira.

The U.S. National Aboretum, 24thand 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 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. 'm just writing because I think people should be warned that birds can d