If you have kept your poinsettia from last Christmas and want it to bloom for the holidays this year, start preparing it now.

Before the first frost, bring the plant indoors to a sunny room. It needs good light during the day, preferably direct sunlight. Keep a night temperature of 60 degrees, and warmer during the day.

Beginning this week, poinsettias need complete darkness every day from about 5 p.m. to 8 a.m. Put the plant in an unused cool closet, or put a cardboard box over it, to provide this long night. Flowering will be delayed if the plant is exposed to even a few seconds of light.

During the daytime, give it sunlight. Continue this routine until the flowers begin to show color late in November.

It usually takes nine to 10 weeks to develop the red "flowers," which are really bracts, or specialized leaves.

Correct watering is important. Usually watering once a day is about right. Add enough water so that some trickles out the bottom of the pot. Then let it drain about 15 minutes, and pour off drainage water. The amount of watering will depend upon whether the plant is in a clay or a plastic pot. Clay dries out faster than plastic.

The Christmas cactus fails to bloom for many indoor gardeners. Generally this is because it does not get the long night treatment it needs to produce flower buds.

Under ordinary household conditions the plant will bloom during the Christmas season if it gets no light from sundown to sunrise, starting in October.

Place the plant in an unused cool closet or cover it with a box or black cloth so that it gets no illumination at all from about 5 p.m. to 8 a.m. for six weeks. During this time buds will form. Flowering occurs about nine weeks after the start of the treatment.

Temperatures, as well as day length, influences the blossoming of the Christmas cactus. The plant can be left outdoors as long as temperatures remain above 55 degrees. In an unlighted garden area, the nights are just long enough in the next few weeks to serve as part of the "long-night" treatment.

When you bring the plant indoors, keep it in a cool place, even in the daytime. Water when the top-soil begins to feel dry. Readers' Questions

Mrs. H. Koesick, Chevy Chase: My purple passion plant decided to give up after several years. I rooted some of it and have a small plant growing again. How do I prevent it from happening again?

A. Failure of your purple passion plant, Gynura aurantiaca, was the result of your letting it run itself out into long vining branches. Pinching back is the answer. Use your fingernails or scissors to nip out the growing points of a new branch when it has grown as far as you want it to go. Or cut off a sizable piece and root it in the soil of the same pot. Pinching back the growing tip forces branching farther back on the stem and makes the plant bushier. This plant likes a southeastern or southwestern exposure. Keep it moderately moist and reduce fertilizing in the winter.

Alice Cortese, Washington: I recently discovered small black bugs in my humidi-tray and on the bottom of pots. I threw out the gravel and used a 50 percent solution of malathion to clean the tray and bottom of pots. In a few days I noticed bugs were still on the pots. I mixed another solution of malathion and poured it on the plants. After a few minutes two of my plants wilted.

Did the plants wilt because the malathion solution was too strong? Did I do the right thing to get rid of the bugs?

A. First, and accurate identification of the insect is important in order for correct treatment to be given.

Second, washing with soapy water and a soft brush or cloth may be all that is needed to remove pests on houseplants. Use two teaspoon of a mild detergent to a gallon of water. Rinse plants with clear water.

Third, malathion is generally applied as a spray of dip rather than as a soil drench.

Pushbutton sprays for plants contain small quantities of rotenone, pyrethrins or other killing agents. They may be used to kill pests that can be hit readily with the spray. Read and follow all directions and precautions given on the label of the pesticide used. Several applications at weekly intervals may be required to control successive hatch-outs of pests. malthion sprays should be used outdoors.

You can clean your humidi-tray and pebbles with soapy water or with a diluted solution of household bleach. Rinse afterward.

The wilting of your plants was probably caused by lack of roots, the result of over-watering or standing in water at some earlier point in their culture.