Thanksgiving is now the traditional time for retailers to begin to sell Christmas trees.

The national trend is that one in three families buy a live tree. A third will use an artificial tree, and the rest will have no tree at all. If you are one of the live tree fanciers, here is how to choose and care for your large, live centerpiece.

The basic rule of thumb when purchasing a Christmas tree is to buy a fresh tree and keep it fresh. There are two simple tests for freshness. First, check the condition of the needles by placing a needle between your thumb and forefinger. Then bring your fingers together. If bent gently, the needle from a fresh tree should bend rather than break.

The second test for freshness is to lift the tree a few inches off the ground and then drop it down on the stump end. If outside green needles fall off in abundance, the tree may not be fresh. Pine trees shed their inside needles every fall and sometimes needles lodge among the branches. This is a normal process and not the sign of an old or dry tree.

Once the tree is home, care for it as carefully as you would care for cut flowers. Keep the tree outdoors, but protected from the wind and sun, or on an unheated porch or patio until you are ready to decorate. An area that provides protection from the wind will help the tree retain its moisture.

If you've purchased your tree a week or more before you intend to decorate, it's best to keep the trunk of the tree in water. Any large container will do. Simply make a fresh, straight cut across the trunk about an inch up from the original cut. This opens the tree stem so it can take up water. Then plunge the trunk end immediately into fresh water. If you allow the water level to drop below the fresh cut, a seal will form, just as it on a cut flower, and a new cut will be necessary.

Branches trimmed from the bottom of your new tree provides boughs for centerpieces and decorations, adding a delightful fragrance to your home. If possible, bring the tree into a partially heated area (basement or porch) the night before decorating. This will help it adjust gradually to the warmer temperatures in your home.

Avoid use of combustible decorations. Check all electric lights and connections. Do not use lights with worn or frayed cords and never use lighted candles. Place the tree away from the fireplace, radiators, television sets and other heat sources. These elements will prematurely dry your tree. And be sure to unplug tree lights before retiring at night and any time you leave home. Sensible precautions such as these ensure a safe and happy holiday.

A live Christmas tree, when kept watered, will be as hard to burn as a damp leaf. Be sure to keep it hydrated until the tree comes down. A tree can absorb as much as a gallon of water in the first 24 hours and one or more quarts per day thereafter. Be sure you have a stand that holds at least a gallon of water.

Follow these simple suggestions, and you can be like 35 million Americans and enjoy and several 100-year-old tradition of celebrating with a live evergreen tree.