The weekend of Sept. 29 to Oct. 1 is the latest safe date for leaving house-plants outdoors. Most species grown as houseplants cannot survive even a few degrees of frost. They should be inspected for pests, 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 amarylis 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 folage 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 dward types of impatiens, 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 to 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 houseplants 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.