DON'T SQUISH the mulberries when you walk the streets of Washington these days. It is hard not to, there are mulberry trees and bushes everywhere. During May, June and July if you look up you will see the gleaming black and purple berries in the trees: Look down and the sidewalk will be littered with them. If you aren't looking either up or down you will feel them squishing under foot, making a purple stain.

Mulberry trees were so profuse in the Capital Hill area in the early 1900s that a group of enterprising citizens decided to go into the silk making business. They imported silk worms from Japan, but the worms did not do well in the Washington climate and died off. The silk making business likewise died off, but the trees continued to flourish and spread throughout the city, year after year shedding their berries for pedestrians to either squish or gather.

If you are under a healthy mulberry tree it takes only about 10 minutes to gather enough berries for a 9-inch pie, or a batch of jam.And a few quarts will do for a starter on mulberry wine.

When passersby see me gathering the berries they assume I'm going to make wine. What few of them realize is that mulberries have a much more practical use. They make delicious berry pies, tarts, cobblers, muffins and jam. They add an interesting note to fresh or stewed fruit compote. Combined with sliced peaches they make a delicious topping for ice cream and and alone or in combination with other fresh fruit are excellent with yogurt and with breakfast cereal.

To eat fresh, mulberries are improved if sweetened to taste and allowed to stand for at least 10 minutes before using. The fresh mulberry, like the fresh apricot, is much more bland than the cooked fruit.

When you are in a berry-gathering mood go out with a big bag or pan and gather enough to store in your freezer for use next winter when fresh berries are unavailable. They freeze as easily and well as blueberries.

Wash thoroughly to remove dirt. Drain. Then fill plastic bags or containers and freeze. Sugar is not needed for freezing. Add the sugar as you use them since different uses call for different amounts of sweetening. MULBERRY PIE (6 servings) Pie crust for 9-inch pie plate 4 cups mulberries 3/4 tablespoon flour 2/3 cup sugar 1-1/2 tablespoons lemon juice

Line pie plate with pastry, prick all over. Put in a 400-degree over for 5 to 8 minutes or until slightly glazed. Remove from oven, fill the shell with berries.Combine flour and sugar and distribute over the berries and then sprinkle on lemon juice. Cover with pastry, pricking to make small holes for steam to escape. Return to oven, lower temperature to 375 degrees and bake for 35 to 40 minutes. Crust must be golden and the juices slightly syrupy. Serve at room temperature. Excellent as is, it is even better a la mode. MULBERRY COBBLER (6 servings) 5 cups mulberries 1 1/2 cups sugar 1 tablespoon flour 1 1/2 tablespoons lemon juice 3 tablespoons butter or margarine 2 eggs 1 cup sifted flour 1/8 teaspoon salt 1 tablespoon baking powder 1/2 teaspoon vanilla

Mix mulberries with 1/2 cup sugar, 1 tablespoon flour and lemon juice and pour into a 9-inch baking dish. Set aside while mixing the batter.

Cream together the butter and sugar. Beat the eggs and add to mixture. Sift together 1 cup pre-sifted flour, salt and baking powder. Then stir into the butter mixture. Add vanilla and stir until just blended. Bake at 375 degrees for 1 hour. Serve hot, warm or cold with whipped cream. MULBERRY JAM

If you are an advocate of commercial pectins, by all means use them; just follow the directions for blueberry or blackberry jam.

If, however, you prefer the old-fashioned boiled-down method, pick over berries, combine with 2/3 as much sugar by measure and 2 tablespoons lemon juice for every 4 cups berries. Let stand overnight or at least 5 hours, stirring from time to time. Then stir the mixture over the heat until it comes to a boil. Boil rapidly until it has a thick consistency. When the jelly stage is reached, after about 20 minutes, remove from heat and allow to cool and thicken before bottling or putting into cartons for the freezer. The jelly stage is when the juices drop from the spoon with two drops clinging together. Note that the boiled-down jam takes much less sugar. MULBERRY TOPPING FOR CHEESECAKE OR CREAM-FILLED TARTS 3 cups fresh or frozen berries 3 tablespoons water 1 tablespoon lemon juice 1/2 cup sugar 2 teaspoons cornstarch dissolved in cold water

Put the berries, water, lemon juice and sugar into a saucepan and place over moderate heat. Bring to a boil and then turn down heat and simmer until berries are soft and have given off enough juice to cover them. Drop by drop stir in the dissolved cornstarch, stopping when you have a consistency just spreadable. Remember that it will thicken several degrees as it cools. When cold spread on top of the cheese cake or cream-filled tart.