A Thai or Vietnamese dish will often have a compelling balmlike flavor you can't quite put your finger on. It's not exactly lemon, but more of a subtle lemon perfume.

The source is lemon grass, which in Indonesia is cut by young girls because of an old belief that it's most fragrant when picked by someone with a virgin's pure thoughts. Whoever is picking it in the United States, lemon grass is being harvested in California, Florida and elsewhere. Unlike many other herbs used in the Asian tropics, lemon grass will grow in moderate climates. In fact it's difficult to stop once it takes hold.

Fresh lemon grass is sold by the stalk, which is gray-green, about two feet in length and looks something like a scallion, although it is fibrous to the point of being woody. It's valued for the flavor it imparts rather than any substance it adds.

Only the bulblike, six-inch base of the stalk is used after the top is trimmed and a layer of tough outer leaves is peeled off. For soups, this is cut into coarse shreds about two inches long or sliced into little rounds. For curry-style dishes, it's pounded with other seasonings to a paste.

It's also part of a standard paste, including chili peppers and shallots, that seasons pork, seafood or chicken stuffings that are grilled or steamed in banana leaves in Southeast Asia. A wonderful Laotian dish calls for a whole crab to be steamed on a bed of freshly shredded lemon grass.

Lemon grass is also sold shredded and dried. Before it is used, it must be soaked 30 minutes in hot water. It's also sold powdered and in this form can be used like any spice. However, since tracking down dried lemon grass will take you to the same stores that carry it fresh, it makes little sense to use these substitutes.

Some sources confuse lemon grass with citronella. They're not the same although they're close relatives, and the oil of each is used commercially in food products and cosmetics. Oil of citronella is more highly valued for perfume and is highly regarded as a mosquito repellent.

Lemon grass is easy to grow if you find a stalk with vestiges of roots. Just keep it in water until the roots grow a little, then transfer it to a planter that gives it room to spread. The more closely its environment approximates its warm, moist, tropical home the better, but lemon grass will thrive anywhere there's no frost.

HOT AND SOUR SHRIMP SOUP (Dom Yam Gung) (6 to 8 servings)

This most popular of Thai soups is easy to make.

1 pound fresh medium shrimp

Few drops sesame oil

1 3/4 teaspoons salt

1 tablespoon peanut oil

6 cups chicken stock

3 stalks lemon grass, bulb part only, cut into 2-inch lengths and coarsely shredded

1 tablespoon finely shredded lime zest

3 or more small chili peppers, red or green, cut into coarse shreds with seeds

4 shallots, sliced

1 1/2 tablespoons Thai fish sauce

Juice of 2 large limes or lemons

1/3 cup combination mint and coriander leaves

Peel the shrimp, reserving the shells. Cut the shrimp in half lengthwise. Mix them with the sesame oil, 1/4 teaspoon of salt and refrigerate.

Meanwhile, heat the peanut oil and saute' the shrimp shells until they turn pink. Add the stock and bring to a boil. Skim, turn the heat to low, cover and simmer for 15 minutes. Strain the broth into a saucepan and reheat. Add the lemon grass, lime zest, chili shreds, shallots, fish sauce and 1 1/2 teaspoons salt (if unsalted chicken stock is used) and simmer for 25 minutes. Add the lime juice, cook briefly, then add the shrimp. Cook just until they separate; turn off the heat. (They'll finish cooking in the hot broth). Transfer the soup to a tureen or serving bowl, stir in the mint and fresh coriander and serve.