Christmas tree merchants are expecting thousands of Washingtonians to turn out Saturday and Sunday to shop for evergreens for the holidays.

The average price for a cut tree is about $15 at most area stores and stands, although prices throughout the area vary widely, from less than $10 to more than $20.

That is $2 to $3 more than last year's prices, industry officials said. Unlike last year, when there was a shortage of trees, shoppers should have no trouble finding trees for this Christmas.

An estimated 400,000 cut trees have been brought into the Washington area from farms in the northeastern United States and southern Canada. Most of the trees were cut weeks ago for the long haul.

Some trees dry out prematurely due to poor handling and exposure to the weather. George Roche, a marketing specialist in the Maryland Department of Agriculture, recommends that shoppers routinely check their trees for freshness before paying for them. Roche offers these tips:

Wrap a tree needle around your finger. If the needle bends freely without breaking, the tree is fairly fresh.

Tap the tree on the ground to see how many needles shake loose. If a number of green needles fall, chances are that the tree is not fresh.

To keep the tree fresh after taking it home, cut one-half to one inch from the bottom to allow it to take water. Then place the tree in a stand that can hold several quarts of water.

Despite the special care that cut trees require, more of them will be used in homes this Christmas than artificial trees. Surveys indicate that about 42 percent of American house holds set up cut trees at Christmas, and 34 percent use artificial trees. The remaining 24 percent don't put up any kind of tree.

Because of the continued rise in prices for cut trees, many families have been attracted to the "ball and burlap" living tree that can be used indoors as a Christman tree and then planted outdoors after the holidays.

Some nurseries sell living trees for $40 or more.

But Roche, the Maryland agriculture official, warns that a living tree will die if it is planted after being kept indoors more than a few days.

"What kills it is the sudden shock of the temperature change, from warm inside to freezing outside," he said.

County offices in Maryland have a fact sheet explaining how to tend a living tree and plant it outside. The bulletin is called "Caring for Live Christmas Trees."

The offices also have a directory of Maryland's "choose and cut" Christmas tree farms, where customers can select a tree and either saw it themselves or dig it out of the ground. Many farms furnish tools, but customers should telephone ahead to be sure.

A directory of the Virginia tree farms is available at county offices in that state. The Virginia State Travel Service, 906 17 St. NW, in downtown Washington, also has the farm directory.

Most of the cut Christmas trees will be purchased from area retail stores, such as Hechinger's, which has Scotch pines and balsam firs for $11.88 at 26 locations, and from the roadside stands operated by independent tree retailers as well as a variety of civic and church groups.

In Arlington, the churchyard at St. Andrew's Episcopal Church has been converted into an open-air sales shop managed by Warren Horstman and other members of the Men's Club.

When their 400 Scotch pines have been sold at prices ranging from $8.50 to $13, the club expects to have about $1,000 to contribute to the church's general fund.

At the Chambers Flower Center Inc., 1600 Bladensburg Rd., NE, manager Sandy Morse predicts that business will be brisk this weekend that motorists will line up in the street outside his store. One reason for the crush, he said, is the modest $7.49 price tag on the trees at Chambers.

Another reason is that Chamber's parking lot has been transformed into a tree shop for 3,600 Canadian balsams.