If you're a serious berry eater, chances are you've got a preference, and that preference may be the only clue Rene and Pearl Johnson, owners of Johnson's Berry Farm, a pick-your-own place in Upper Marlboro, need to figure out just where you grew up.

Blackberry pickers are usually from the nearby country and places south, while red raspberry snobs are usually from the north, Pearl Johnson said.

And neither group wavers in its berry allegience. Pickers head straight for their favorite berry patch at the Johnson's farm, hardly casting a curious glance in the direction of the other patches separated by tiny dirt roads.

"There's a different clientele for blackberries and raspberries," Pearl Johnson said, throwing her blue-stained hands high in the air to emphasize that this is one phenomenon she cannot explain. "Seems like if you love red raspberries, you really love red raspberries." People who love the red raspberry wouldn't think of using blackberries in their favorite recipes. On the other hand, "if you love blackberries, well, that's all you want," she said.

The fact is, they can be used interchangably in any recipe you cook so long as you keep in mind that blackberries are a little more tart than red raspberries and, therefore, may require a tiny bit more sweetening in desserts. This hint makes your favorite recipes for these two berries usable now through the end of September. Blackberries are ready to pick and should be available until the end of August, Rene Johnson said. His fall crop of red raspberries will be ripening around Aug. 10 and hearty until the end of September.

The problem with these two berries, though, is that they are highly perishable, and need protection from the heat as soon as they are picked, Rene Johnson said. "They don't have a skin to protect them like apples and tomatoes." For this reason they must be picked in the morning when they are still cool from the night air and then refrigerated as soon as possible before they mold and turn to pulp.

An early morning trek to the Johnsons' immaculate farm is a lovely 30-minute drive into rural Prince Georges County. The berry patches are divided into one-acre plots that cover rolling green hills. In the spring there are strawberries to pick, and the blueberry crop has just finished a bountiful harvest. Whichever berry you decide to pick, this is a relaxing way to spend a sunny summer morning. There are picnic tables for customers who want to make a day of it -- but few do. Most people are in a hurry to get their berries home and into the refrigerator as soon as possible, Pearl Johnson said.

Thornless varieties have made blackberries easier to pick than they were in earlier years, Rene Johnson said. "In the old days you always had to fight thorns and chiggers and bugs," he said. Though people talk about the fact that blackberries attract poisonous snakes, Johnson said he has only seen two snakes in the seven years he's been growing all of his berries. He didn't wait around to get bitten on those occasions, so he couldn't tell you whether or not they were poisonous.

The canes on which the berry-laden blackberry branches grow are now supported by trellises, which saves your back from aching and knees from getting caked with dirt. Raspberries, however, still grow on low-lying bushes. Picking them does require a bit of a healthy back, and you'll want to bring gloves, since today's raspberries still have thorns.

In either case, you can pick 8 to 10 pounds of berries in about 20 minutes, Rene Johnson said. And don't worry if you get juice on your clothes; rub the stains with lemon juice and they'll come right out.

Both blackberries and raspberries hold up well throughout the season, unlike other types of berries that are at their best in the early weeks of ripening, Rene Johnson said. The perfect blackberry is as big around as your thumb, plump, glossy and black all over. Perfect raspberries should be an even dusty red or pinkish color with no white spots at all. They are about half the size of a blackberry.

In either case, be sure your berries are as ripe as you want them when picked; they do not mature well on counter tops, the Johnsons said -- they mold quickly and turn to pulp, just as they do when left in the car too long. Perfectly ripe, just-picked berries should last about one week stored uncovered in the refrigerator. Washing them speeds up their breakdown, so be sure to rinse them lightly only just before you use them, Rene Johnson advised.

Pearl Johnson does all the berry cooking at the Johnson Berry Farm. She won't tell you what her all-time favorite berry is. "During blackberry season it's blackberries, during raspberry season it's raspberries," she says with a smile. And she does keep a good supply of frozen berry pure'e of both types for a winter drink. She thins it out with water and lemon juice and sweetens it with a touch of sugar -- all to taste. On special holidays she adds some ginger ale or 7-Up to make it bubbly.

Here are some more recipes to help you start making the switch in your berry allegiance from raspberries to blackberries and back again all summer long.

Johnson Berry Farm is located just off Route 301 in Upper Marlboro on l7000 Swanson Rd.; call (301) 627-8316. Blackberries are 80 cents per pound; red raspberries $1.50 per pound. Call your local county extension service for a listing of other PYO's in your area. BARBARA ELFMAN'S CHUNKY BERRY JAM (Makes eight to nine 1/2-pints)

Don't pick your blackberries the day after a heavy rain or the berries will be watery, Elfman warns. What makes this jam so wonderful is the fact that she leaves pieces of berry in the jam rather than processing the mixture to a fine pure'e.

4 cups crushed blackberries or raspberries

7 cups sugar

3-ounce envelope liquid pectin

Sort and wash berries, removing stems. Crush in food processor, leaving pieces of berry (for a perfectly smooth jam process to a pure'e and press through a sieve). Put berries in a large kettle and mix in sugar thoroughly. Bring to a boil over high heat, stirring constantly. When mixture is at a foaming boil, boil hard for 1 minute. Remove from heat and stir in pectin. Skim off foamy layer on the top.

Pour berry mixture into jars that have been boiled for 15 minutes, filling to within 1/4 inch from the top. Wipe rim. Seal and boil 10 minutes. Let set at least two days before using.

Note: Elfman uses canning jars with vacuum sealing lids, thereby avoiding the necessity of sealing the tops of the jars with wax. CORNISH HENS WITH BERRY SAUCE (4 servings)

10 ounces blackberries or raspberries

1 tablespoon sugar

4 whole cornish hens, split

1 stick (8 tablespoons) butter

6 tablespoons wine vinegar

1 tablespoon blackberry or raspberry jam or jelly

3 tablespoons cre me de cassis

1 1/4 cups chicken stock

Cooked hot rice for serving

Cook a heaping tablespoon of the berries in 3 tablespoons water until tender. Drain liquid into a larger pan and set aside the cooked berries for the final garnishing. Bring the berry liquor back up to 3 tablespoons with water if necessary. Put in the remaining berries and the sugar. Cook until the fruit is tender, about 2 minutes.

Begin saute'ing hens in a large pan in 4 tablespoons butter, browning evenly and cooking through, about 25 minutes. Remove the hens to a platter and keep warm.

In a small pan, reduce vinegar and jam or jelly to a caramel and deglaze with the cre me de cassis. Stir in the stock. When all is smooth, add to the pan of blackberries and simmer for 20 minutes.

Defat the cooking juices of the cornish hens and add to the berry sauce. Simmer 5 minutes. Strain the sauce into a clean pan, pushing through the juices. Be careful not to ruin the sauce by working through too much of the debris of the fruit. Reheat with the tablespoon of blackberries and whisk in the remaining butter just before serving.

Serve hens on plate with rice. Drizzle sauce over the hens and around on the base of the plate. Adapted from "Jane Grigson's Fruit Book" BERRY FLAN (6 to 8 servings)

1 1/4 cups milk

2/3 cup sugar

3 eggs

1 tablespoon vanilla

Pinch salt

1 1/4 cups sifted all-purpose flour

Butter for baking dish

3 cups stemmed and washed blackberries or raspberries

Confectioners' sugar for sprinkling on top

Place milk, 1/3 cup sugar, eggs, vanilla, salt and flour in a blender or food processor. Cover and process for 1 minute.

Pour a 1/3-inch layer of batter in a buttered 7- to 8-cup ovenproof baking dish about 1 1/2-inches deep. Set over moderate heat for 1 to 2 minutes, until a film of batter has set in the bottom of the dish. Remove from heat. Spread the berries over the batter and sprinkle on remaining 1/3 cup sugar. Pour on the rest of the batter and smooth the surface with the back of a spoon.

Place on the middle rack of a 350-degree oven and bake 1 hour. The flan is done when it has puffed and browned and a needle or knife plunged into its center comes out clean. Sprinkle with confectioners' sugar just before bringing it to the table. The flan need not be served hot, but should still be warm. It will sink slightly as it cools.

Adapted from "Mastering The Art of French Cooking," by Julia Child, Louisette Bertholle and Simone Beck