We are tilling our way back to the Middle Ages. A stroll in your favorite green neighborhood will reveal that nowadays Americans grow vegetables and herbs in the front yard, and increasingly, right in the middle of flower beds--which is how it was in the Middle Ages, when people saw no great distinction between plants for beauty and plants for utility.

Talk to a back-yard putterer, and you will find a would-be farmer who admires self-sufficiency as a distant and lofty goal--as it was in the Middle Ages. We make an effort in that direction for economic or psychological reasons, or both, just as medieval people did.

Some contemporary gardeners have rediscovered the value of nasturtium leaves and marigold blossoms in salads. But few people go as far as medieval cooks who used rose petals in teas, salads and preserves; added parts of violets and calendulas to stews; and ate iris roots.

"Medieval people didn't draw sharp distinctions between plants used for ornamental, medicinal and alimentary purposes," says Elizabeth MacDougall, curator of the exhibit "Gardens of the Middle Ages" at Dumbarton Oaks in Georgetown. For instance, onions were grown for their medical and even magical properties, MacDougall says, and their flowering heads might have been looked at as having an ornamental value as well. Scarlet runner beans--a strong, prolific producer of beans--were definitely used as ornamental vines.

You may not be aware of it, but your plot is a direct descendant of one of the five types of medieval gardens:

* The kitchen garden or hortus supplying fresh vegetables and herbs for seasoning;

* The medicinal garden or herbularius--a source for a large variety of plants used as medicaments, such as rosemary, peppermint, sage, costmary and dill;

* The patrician garden--a private, exclusive space esthetic in purpose and reserved for relaxation, and filled with choice plants;

* The cloister garden--a retreat from the mundane world, a setting for the contemplative life, usually supplied with pebbled paths for walks and with a fountain of high symbolic value;

* The pleasure garden, a luxuriously appointed outdoor space, replete with benches and arbors, ornamental bushes and exotic plants, and used for outdoor gatherings and entertainment.

"Most of our Eastern Seaboard urban patterns are derived from the English medieval urban garden," says MacDougall. "The rowhouse with its garden plot goes back to Roman times."

But, MacDougall says, the English have always paid more attention to their cottage gardens in the front yard, while the American emphasis is on the back yard.

The great difference between modern American and medieval European gardens is in variety, she says. Surprisingly enough, medieval people cultivated far more varieties of plants than we do today.

MacDougall, a historian of landscape architecture, wrote her doctoral dissertation on the gardens of Rome in the 16th century. But those gardens cannot be reconstructed today, she says regretfully, because we no longer have the varieties then in use.

"Our plants have been bred extensively over the past 300 years," she says. "We have fashions in plants, as well as genetic drifts due to accidental mutations, and this leads to the eventual disappearance of recessives."

For instance, she says, there were no less than 125 different varieties of anemones listed in the botanical literature of 17th-century Italy--now we are down to three varieties.

"Varieties die out unless they aren't constantly bred and cultivated," she says. In fruits, for instance, we had roughly twenty times as many varieties in the 19th century as we have today, and the ratio is similar in vegetables.

MacDougall recently reclaimed her own Northwest Washington yard, front and back, which had been a jungle. She grows azaleas, spring-flowering bulbs, sorrel and herbs such as basil, tarragon, rosemary and sage.

Her garden does not fit any of the five categories of medieval gardens. She identifies it as a close approximation of the typical middle-class urban garden of medieval London, which, she says, is often neglected in the literature of garden history, which focuses on pure types.

If you want to see where your garden might fit into the medieval scheme of things, visit the Dumbarton Oaks exhibit, at 1703 32nd Street NW. The show runs through July 4 and is free to the public. It features 70 manuscripts, sculptures and art objects illustrating the history of medieval gardens and their relationship to science, medicine, literature and religion.