As every savvy person knows there are many good reasons to never eat things that grow in the city.

We all know that: All growing things in the city are covered with an invisible but nonethless poisonous layer of pollution that cannot be washed off; if a thing is free, there must be something wrong with it, and it's the sort of thing only a Boy or Girl Scout would do. Usually, I never eat anything unless it is grown by a professional who wears coveralls.

But with mulberries, it's different.

Most people really don't know about mulberry trees. Even though there are so many of them all over Washington, it never occurs to them to eat their plump ripe fruit. Squirrels might, silkworms do, but not people.

In my back yard amongstthe garbage cans, gravel and rusty habachis grows a mulberry tree. For the past three years it has flourished, sending down its little purple berries to mess up may car, my shoes and the only pair of white slacks I have ever dared to buy.

But I have a strong affinity for the mulberry. As a boy in West Virginia, the yearly ritual of berry picking and pie baking in early June was a significant event for me.

My main tree was a huge old mulberry about, two blocks down the alley from my house. I did not need to keep a very close watch on it, however, because every year my father would announce the mulberry season for me.

He'd go out to the curb, look at the spotches of purple on his shiny car and start yelling. Then I'd know that, at last, the mulberries were ripe.

Armed with a colander I'd go out and pick enough for a pie. My mother, as her part of the ritual, would then that same day bake me a pie.

This spring the childhood memory came again. It had to - from my house to my studio there are six mulberry trees. From my job to the corner grocery story there are at least 10. Berries everywhere.I had to have a mulberry pie.

I chose the Duke Ellington Bridge mulberry tree as my source. The trunk starts way down in the park but the top - where the biggest and best berries are - bends right over the bridge railing. Also this particular tree affords a kind of natural screen that you can hide behind. That way people at the bus stop can't see and call the cops to report you for picking berries without a permit.

My picking skill was still intact. With a full bottle of berries, I went home and called my mom to find out how to make the pie. She wasn't a bit surprised.

Her instructions:

Go to the store and buy a frozen pie shell. Now my mother would never do anything as uncool as buying a frozen pie shell but she knows me fairly well and is aware of my limitations. While personally not disdainful of the grocery store pie crust, I've always preferred to con my girl friend into making one for me. It goes without saying that the ideal pie crust would be one made by one's mother but if your mother lives too far away and your girl friend is unwilling, you just have to make do.

Take the berries you've picked and put them in a bowl. (There should be enough - about 4 cups - to make a nice mound over the rim of the pie plate.) Don't worry about the little green stems still on the berries. After the pie is cooked you won't even notice them and neither will anybody else.

Wash the berries as best you can, always remembering that although you can't hope to get them really clean, they'll probably do less harm to your genes than the stuff you buy in the store that has the bug spray all over it.

In another bowl take I cup of sugar and mix in 3 tablespoons of flour. Dump this on the berries and mix it all up.

Put the result in your pie shell. At this point you can put some lemon juice on it. But my mom says that this step is optional.

Put the top pie crust on top of the pie. If you bought your pie crust at the market, you probably won't have a top for it as they seldom sell them with a top and a bottom. Don't ask my why. In this case just refer to your pie as a tart and nobody will notice that it hasn't got a top.

Carve an "A" in the top crust. That, of course, is my initial. You may prefer your own.

Put it in a 425-degree oven and bake 35 to 45 minutes.

If you follow these simple directions you can produce a real pie for not very much money that even tastes pretty good. Of course, it won't taste as good as my mother's, but them nothing ever does. CAPTION: Illustration, no caption, Billy Joe Tatum's WILD FOOD COOKBOOK, Copyright (c) 1978, Billy Joe Tatum. Workman Publishing Co., N.Y.