Christmas planning begins long before Thanksgiving, and after Thanksgiving Day the indoor gardener can begin thinking of ways to include houseplants in holiday decorations.

Lucky is the indoor gardener who owns a Norfolk Island Pine. This handsome houseplant adapts very well as a Christmas tree and continues to be an attractive indoor plant after the holidays.

If you don't already have a Norfolk Island Pine, this holiday season may be the time to buy one. Choose a tree that has a straight trunk with a good balance of horizontal branches. The size will depend upon your pocketbook. A small tree should come with a stake in the pot to assure that the tree will grow erect.

Since the branches of a Norfolk Island Pine are soft and flexible, you should use lightweight ornaments. The smallest light bulbs on a strand of fine wire can be carefully draped on the branches; heat from the miniature bulbs will not damage the plant for the brief holiday use. Other decorations can be left on the tree for a longer time.

Red velvet or other bows tied on the trunk between branches produce a stunning effect. A string of popcorn and cranberries, or a handmade paper chain, would be light and lively.

This amount of trimming may be sufficient for the size of your tree and the effect you want. Do not overload the tree or you may burden it and mask its natural symmetry.

If your taste runs to other decorations, a variety of ornaments is available in local shops. Some of the following types and combinations which I have seen may appeal to you.

Clear, blown-glass ornaments are relatively expensive, but with a string of cleat miniature lights the total effect is an appropriate crystalline inciness.

Mexican or Scandinavian straw figures or cornhusk dolls, combined with calico bows reflect regional Christmas traditions.

Japanese shell ornaments - red, green or white stars, bells or angels - are lightweight, unusual and not gaudy.

I especially like the colorful and lightweight 3-inch glass birds and peacocks (made in Czechoslovakia) reminiscent of my childhood Christmases. All have clamps and are easily attached to branches.

At first sight. I thought the peacocks were turtle doves - they are not, and they certainly are not partridges - so one peacock on a tree is probably enough.

Other combinations can be made - perhaps of filmy angels and the smallest of the usual baubles.

If your tree is small, put it on a pedestal, buffet or end table to emphasize its importance as part of the festive atmosphere.

After Christmas, place your Norfolk Island Pine where it will have good light, such as an unobstructed north window or a south window during winter. Turn it regularly so it does not become unbalanced from growing toward the light.

Norfolk Island Pine prefers a cool environment. The temperature should not be higher than 70 degrees. The needles will turn brown in hot dry air. Keep the soil evenly moist.

In summer, set the tree outside in light shade or on the patio or porch protected from sun and wind. Indoors, move it away from a sunny window to a cool, bright location.Summer is the growing season, so feed the tree with an all-purpose, water-soluble houseplant fertilizer every two or three weeks.

Norfolk Island Pine is a slow grower and probably will not require repotting for two or three years. Thus, you have acquired a perennial Christman tree which can become part of your own Christmas traditions and still be a satisfying houseplant the rest of the year.