Enthusiasm for herbs is growing, but few can top Frank Copp of Falls Church, who bought his first herbs 10 years ago. He's now having the swimming pool removed and replacing it with a kitchen-herb garden.

Copp was among the herb lovers at a workshop given the other day by Holly Shimizu, curator of the National Herb Garden at the Arboretum.

"The way to make associations with these plants is to taste them," said Shimizu, as she passed around cuttings of rosemary, mints and lemon balm. "Once you know them that way, you never forget them."

Most of the students were well beyond growing a row of parsley. Although this was Copp's first class, one woman was taking it for the third time. Whenever Shimizu mentioned an herb, however exotic, someone would call out his or her use for it: "I stuffed a chicken with it!" "I put it in Bloody Marys." "We use the stem for straws."

"There are so many ways to use them, but it's a lot harder to grow them," said Shimizu.

Still, if you want to try it and you've got an unoccupied patch of ground outside the kitchen door, or just a windowbox, this is the time of year to start.

And the place to start is the soil. In the best of all possible gardens, you'd have the National Herb Garden's divine dirt: perfectly pH-balanced at 6.5, half of it good loamy topsoil and the rest mostly organic matter -- well-rotted manure.

"You would use horse or cow manure," said Shimizu. "We get ours from the zoo, so it's a little bit of everything. Everyone here calls it 'zoo doo.' It's a wonderful combination. Who knows what's in there." The manure is sterilized -- don't want any strange seeds sprouting at the National Arboretum.

Shimizu advises against peatmoss: "It's a sponge in wet weather and a wick in dry weather." And perlite rises to the top and floats off. The herb gardener should use small granite chips or sand.

To improve drainage -- herbs hate having wet feet -- raise the bed, which happens naturally if you prepare the soil right.

A proper preparation is to double dig -- shovel to a two- spade depth. This aerates the soil. Then, if you mix in some topsoil, sterilized manure and compost, you'll have more dirt than you started with, so the herb bed will be higher than the yard around it.

Location, location, location is essential: Herbs need at least six hours' sunlight daily. A few, like mint, will grow in shade, but this reduces the flavor when herbs are harvested. Chemical fertilizers often have the same effect.

Here's a checklist of culinary herbs that Shimizu wouldn't want to live without. ANNUALS -- This foursome is started in place from seed: DILL, CORIANDER, FENNEL, SUMMER SAVORY. For BASIL (which rhymes with dazzle), seedlings aren't planted in the garden plot until May 15. But you can start seeds outside now. They won't germinate for a little while; then the danger of cold is gone. When plants are grown for their leaves -- such as summer savory and basil -- you prolong the life and peak of flavor if you don't let them flower. And then there are PARSLEY (really a biennial with a poor showing in the second year) and MARJORAM (a perennial grown as an annual here). BIENNIALS -- If started now from seeds, CARAWAY will give you only leaves this year, but it will put forth flowers and seeds next year. Start ANGELICA now as a plant. (Don't wait; it resents hot weather.) Next year it will provide seeds. HARDY PERENNIALS -- True FRENCH TARRAGON isn't grown from seeds. Beware the Russian tarragon, which is, easily: It's a weed. CHIVES, SAGE, LEMON BALM, LOVAGE, MINT. Confine the mint's sociable roots, or plant it off by itself. Of course, THYME is of the essence, as is OREGANO. "The genus thymus is a mess, which is also true of oregano," said Shimizu. The plants can hybridize themselves beyond recognition, and even when they are named, growers often confuse them. So buy them from someone who specializes in herbs. TENDER PERENNIALS -- ROSEMARY, LEMON VERBENA, BAY (a nice house plant) and ROSE GERANIUM. These are bought as plants and come in from the cold in winter. Here are a few local sources for herb plants. BITTERSWEET HILL NURSERIES -- 1274 Governors Bridge Road, Davidsonville, Maryland. 301/798-0231. EARTHWORKS HERB GARDEN NURSERY CO. -- 923 North Ivy Street, Arlington. 243-2498. NATIONAL CATHEDRAL GARDEN SHOP -- Wisconsin and Massachusetts avenues NW. 537-6200, ext. 258. STILLRIDGE HERB FARM -- 10370 Route 99, Woodstock, Maryland. 301/465-8348. AN HERBAL POTPOURRI The annual Herb Fair at the National Arboretum, 3501 New York Avenue NE, is happening Thursday, May 19, from 10 to 3. Sponsored by the Potomac Valley Chapter of The Herb Society of America and the Friends of the Arboretum, the fair will be selling live plants, herb breads and books. There'll be wreath-making demonstrations and lectures on herbs in food and potpourris. Admission is $2. Holly Shimizu will be holding workshops on harvesting herbs later in the season. For information, call 475-4857.