IF you want to keep your holiday plants happy, learn how to care for them before they arrive. Poinsettias take first place as a favorite plant at Christmastime. With showy blooms in red, white, and pink, poinsettias require an abundance of light. A south-facing window makes an ideal location for display. Too, they should be watered when the topsoil is barely moist. If allowed to dry out, the leaves will drop off.
The blooms are not difficult to force during the year if you follow what is called the ''short-day treatment.'' Starting 10 weeks before you want the plant to bloom, put it into a dark closet for about 13 hours each night and then expose it to as bright a light as possible during the daytime.
It's important not to expose the poinsettia to any light at all during the ''closet hours'' or buds will not develop.
Poinsettias do best when kept at a temperature above 60 degrees F. Fertilize monthly during the spring, summer, and fall months with a good houseplant mix.
A Christmas cactus makes a wonderful year-round houseplant. Although it is a member of the succulent family, it doesn't look like a standard cactus because it has no spines. Instead, rectangular ''leaves'' form the body of the plant. Red flowers will bloom at their junctures and tips during November and December.
The blossoms last about a month and may recur during the year.
A potted Christmas cactus may do well outdoors during the summer, but it must be brought indoors at the first hint of cold weather. They prefer bright sunlight and not too much water.
Kalanchoes, also called ''Tom Thumbs,'' sport clusters of red, orange, pink, or yellow blooms and round, leathery leaves with scalloped tips. While a kalanchoe will bloom year round (pinch back old growth and buds will form with the new), its blossoms are most profuse during the winter months.
Kalanchoes prefer life in a pot and will not do well if transplanted outdoors. Give them an occasional dose of good houseplant fertilizer, full sun, and light waterings for best growth.
Ornamental pepper plants are treated as annuals since they have a life span of only one year. However, during that year they make highly decorative and unusual houseplants. They are characterized by round leaves with a grouping of white blooms at the top of the plant.
As the blossoms fall, small red (nonedible) peppers develop in a cluster.
Ornamental peppers like filtered, nondirect sunlight (an east window is fine) and should be watered when the soil is slightly moist.
Cyclamens, known for their large leaves and blossoms shaped like upside-down butterflies, range in color from light to dark pink, white, and red. While a potted cyclamen lives only a few weeks, it makes a beautiful addition to decorating schemes.
To preserve a cyclamen as long as possible, give it plenty of sunlight and keep it moist. Do not overwater because cyclamens are very susceptible to root rot. It's a cool-weather plant, so keeping a cyclamen warm in the daytime and cool at night will help it to live longer.
In mild climates, you can put a cyclamen outdoors at night, but be careful you do not allow it to freeze.
Azaleas, ablaze with many delicate blossoms, range in color from pinks to reds, but are temperamental as houseplants and may not live for more than a few months. To keep an azalea healthy, give it partial sunlight and water while the soil is still moist.
The soil, which is very important to the success of an azalea, should be rich , well-drained, and acidic, high in organic matter, and low in salts. Fertilize with one of the special mixes made for azaleas.
To shape a healthy plant, pinch back the tips during a foliage period. Some types of azaleas are best transplanted outdoors in the spring. Ask your county extension office, local nurseryman, or florist.
The Department of Agriculture and county extension office have booklets and information available on request. Take advantage of what they have to offer. Consult the Yellow Pages of the telephone book under ''government, state, and county.''