Looking for a Christmas tree? Try 'bumping' it a few times

President Reagan won't bother to bump his Christmas tree up and down when it is delivered to the Blue Room of the White House Dec. 9. For one thing, a 20 -foot-tall Fraser fir isn't all that easy to pick up; for another, the grower has guaranteed it will be fresh.

However, you might consider doing a little tree bumping yourself if you're among the 30 million US families that will decorate a real Christmas tree this year.

Some 400 years ago German families first began trimming evergreen trees as a way of celebrating the Christmas season. Thanks to the British choice of Hessian troops during the Revolutionary War, the custom came to this country.

A German immigrant, Charles Minnegrode, introduced the idea to Williamsburg, Va., in 1842 with a tree ''splendidly decorated'' with strings of popcorn, gilded nuts, and lighted candles. It was an eye-catching sight that others were soon trying to duplicate. So quickly did the custom catch on that by 1851 a Pennsylvanian by the name of Mark Carr saw enough profit in Christmas trees to haul appropriate specimens all the way from his Catskill land to the sidewalks of New York City.

When Carr first realized that he could bring a new form of Christmas cheer to New York City residents and run a profit at the same time, he went out and, in all probability, cut Balsam firs, which remained the most popular tree for the better part of a century. In the early 1950s, however, Christmas tree farms in the Northwest turned to Douglas fir trees, while their counterparts in the East turned to Scotch pines.

These trees grow rapidly into the appropriate shape (helped on by pruning) and, when cut, they ship well. But inevitably some dry out more than they should. And that's where the bumping comes in when you go out to make your selection.

When you've found the tree that looks the right size and shape for your living room, lift it off the ground a few inches and let it drop. Keep your eyes on the outside green needles. If they fall off freely it is a sign that your tree is not as fresh as you would want.

Remember, however, that it is the outside leaves you are interested in. Evergreens naturally drop their inside leaves (nearer the trunk) every fall, and some of them may let go immediately when you try this trick. For the same reason , you might see some dead leaves lodged in the bows inside the tree. This is quite natural, and is no indication that the tree is old.

Another option is to place a needle between the thumb and forefinger and then bend the two ends slowly together. If the leaf is fresh, it will bend rather than snap in two.

Even without roots, Christmas trees drink a lot of water, just as cut flowers do. They will take up between a quart and a gallon of water a day, depending on their size and how warm your home is. The chances are, however, that when you get your tree the cut stem will have dried over, closing off the capillary tubes , which effectively stops the tree drinking.

If you've ever known the frustration of a clogged drinking straw at the soda fountain, you will understand. The solution is to make a fresh cut by sawing an inch off the bottom of the tree.

If you have to store your tree for a week or so before trimming it, leave it with its stem in water, where it is protected from drying winds and hot sun. If possible, bring the tree into a partly heated room (basement or porch) the night before decorating. This allows the tree to adjust somewhat before being brought into the much warmer environment of the living room.

Check the water level in the container every day. If the bottom dries out, you will have to take another inch off, which isn't all that easy once the tree is trimmed. Moreover, you won't want the brightly packaged presents to be covered with needles on Christmas morning.

After the festive season is over, take your tree outside and anchor it in some way so that it won't blow around. Trim it again with orange slices, bread, and suet, and the birds will enjoy a festive season of their own.

Meanwhile, Hal Johnson of Lansing, N.C., has chosen the ''absolutely perfect'' tree for shipment to the White House. As the National Christmas Tree Association's ''grand champion grower'' for 1982, that's his privilege.