The weekend of Sept. 29 to Oct. 1 is the latest safe date for leaving houseplants outdoors. Most species grown as houseplants cannot survive even a few degrees of frost. They should be inspected for pest, cleaned up and moved indoors before it is necessary to close the windows and turn on the heat.

Don't forget to take in your amaryllis before frost. When amaryllis foliage has yellowed, cut it off and stop watering; lay the pot on its side in a cool place (40 to 50 degrees). When you want it to start growing again, probably just before or after Christmas, resume watering and set it in a sunny spot.

Reduce watering of caladiums that you expect to hold over for next summer. Bring them indoors. When foliage dries, store them either in their pots or in paper or net bags in a cool place. Don't forget labels if you want to keep the colors separate. Repot in the spring.

Plants such as geraniums, lantana or fuchsia that have grown in the garden, not in pots, are difficult to lift and move indoors. It is best to raise new plants from cuttings made in the summer.

Think twice about growing plants indoors that attract white flies; lantana and fuchsia are examples. White flies are almost impossible to get rid of.

Some annuals can be brought indoors after putting them through the cleaning and adjustment recommended for houseplants that have been outdoors. Trim flower and foliage severely when transplanting annuals to pots. For most of these to have flowers, a sunny south window is essential. Try this with dwarf types of impatients, french marigolds, wax begonias and compact petunias. In good light you can extend the flowering period for several weeks after frost has blackened outdoor blossoms.

With good light, pieces of coleus, wax begonia and impatiens can be grown in water all winter - a convenient way to preserve a special one selected for its color. Add some charcoal chips ot the water to keep it sweet. Keep the water level up in the container, and occasionally when fertilizing other plants add a few drops of the solution to the water. If water tends to become discolored, change it monthly.

There is still time to take cuttings of some of the window box and garden plants, such as begonias and coleus. Rampant growing houseplants, such as wandering jew, velvet plant, swedish ivy and pink polka dot, are a source of trimmings from which additional plants can be started now.

Plant supplies are much more plentiful now than they will be during the winter. Decide what soil mixes, pots, fertilizers and labels you will need for houseplant care during the winter and purchase them now.

Ease up on watering and fertilizing houseplants as the days become shorter and cooler. You will undoubtedly notice that the plants require less frequent watering.

Here is a question frequently asked by indoor gardeners:

I bought a beautiful philodeudron 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-watering is the most frequent failing of indoor gardeners. Plants can wither and die because too much water has been given. True, a continous 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 are clogged with water, root development is inhibited and the delicate roots rot. The plant begin 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 hole 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 dry at the surface before you water again.

The soil will dry out more rapidly in a clay pot than in a 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 pebbles trays to increase humidity around plants, be sure that pots sit on pebbles, not in the water.

Indoor gardening questions may be addressed to Jane Steffey at The Weekly, The Washington Post, 1150 15th St. NW, Washington D.C. 20071. Please include your address and telephone number.