The last reasonable date for a spring freeze is now past for the Washington metropolitan area--except for Beltsville, College Park, Frederick, Glenn Dale and Germantown where a danger exists as late as April 27. The great outdoors is now safe for seedlings of all herbs as well as carrots, celery, chard and any member of the onion family. It is a good idea to sow another row of lettuce and radishes, and to start something as exotic as radicchio and arugula (also known as roquettes).

But it is still too early to set out such hot-weather favorites as beans, corn, eggplant, peppers and tomatoes. Their recommended planting date is between May 5 and May 15. Exposing them to the rigors of outdoors earlier is likely to set them back, instead of giving them a jump on summer.

The smartest thing to do in these weeks before the heat is to get your herb garden going.

"For herb gardeners, the latter part of April is a time of digging, dividing, transplanting and sowing," says Holly Shimizu, curator of the the National Herb Garden of the U.S. National Arboretum, at 3501 New York Ave. NE. "Some people wait with sowing till the first days of May to be completely sure, but I like to get an early start. I take the risk. I am an impatient gardener."

Shimizu says that in the four years of tending the National Herb Garden, she has not had reason to regret her impatience. Last week she sowed her caraway, salad burnet (a delicious, cucumber-flavored herb), parsley, chives, dill, fennel, sage, sweet cicely (also known as sweet chervil, and useful in sweet dishes), coriander, thyme, tarragon and angelica (a showy herb that grows up to seven feet, likes cool weather, does not mind a bit of shade and its leaves, stalks and roots are used as flavoring in liqueurs and brewed as teas).

The latter part of April is "a very important time for the herb gardener for another reason," Shimizu says, because it is the right time for cutting back perennial herbs, such as thyme (she recommends heavy pruning, down to half its growth), sage, lavender and santolina.

Shimizu says that visitors expecting to see "just a beautiful garden" should wait until May 19--official Herb Day at the National Arboretum--when the herb collection will be blooming, "showing lots of color." But gardeners interested in growing herbs will benefit from a visit now. "They can see what we are doing," Shimizu says. "They can see what's poking up, and they can see what we prune and how we prune. For instance we just pruned our old roses that produce rose hips for jams, jellies and teas."

"All your herbs can be set out in the garden by April 20," says James Horne, a retired Air Force colonel who is in charge of the Washington Cathedral's crowded, mist-laden, multiscented greenhouses just off Wisconsin Avenue NW. "But make sure that before you set out your herbs, you keep them for two days in a room without any heat or some other cool spot. This gives them a chance to acclimatize."

This year the cathedral's greenhouse shop offers more than 90 kinds of herbs for sale, most of them grown either in the two greenhouses on the cathedral grounds or on Horne's Annandale farm. The profits go to the cathedral.

Horne says the herbs most popular in the Washington area are basil, parsley, chives, oregano and marjoram, in that order. Over the past few years, he has noted "a great upsurge of interest" in growing culinary herbs. Close to half of the cathedral greenhouse sales are now herbs, he says.

He is especially proud of being able to offer four kinds of parsley (French, Italian, curly and coriander), five types of thyme (English, French, golden, silver, woolly) and 20 varieties of scented geraniums.

Horne has been gardening for 42 years. What he enjoys most is the midwifery of plant propagation: He breaks off cuttings using both hands by balancing each thumb against the index and middle fingers and snapping the stalk. He never uses scissors or a knife. He then judiciously sinks the cutting in a growing medium, with a lot of delicate tapping and tamping.

"The hand has to know," he says, "and the heart has to feel."

But, he says, he does not believe in something as mystical as a green thumb--which is what people think he has. He also doesn't believe in talking to plants. "But I can't help talking to them," he says. "I treat them as if I did talk to them."

"Herb cuttings are real easy to root," he says, and they are good to train with. Evergreens are tough and some of them are impossible to root. Or nearly impossible. "I keep trying," he says. He shows trays full of evergreens he has raised from cuttings, including such hard-to-root kinds as blue spruce, Japanese cypress and boxwood.

"You should have faith that they will grow," Horne says. "Then you have to make sure that they have the right environment--good soil and plenty of moisture. You have to watch them every day. And you have to like to grow them."