THE SUMMER GARDEN is sensuous, a place where the most wicked breezes kick up. Racy red cannas call their dares to black-eyed susans and the foxglove with its speckled tongues. Herbs toast in the sun and boxwood breathes its moist, musky breath into the lush, green spaces. A garden in summer is an oasis.

"All that green stuff may stain your white trousers," says Marc Cathey, director of the National Arboretum, "but all that green stuff is cooling off the environment. The greenest time of year is summer," he says. "That's when the plants do their major air conditioning. To release water into the air they have to absorb the heat."

At the Arboretum, everything is lush and green and growing fast -- living proof that almost anything can grow in summer, given enough water. But, says Cathey, "The main thing is, it's so very quiet. You're no longer overwhelmed by 70,000 azaleas, as we have in the garden. You begin to look at the plants as sculpture. There's more time, less hurly-burly. You have a feeling at the Arboretum that it's your own personal garden. Private."

To Cathey, there is simply nothing better in summertime than to stand in the rain in a beech forest, such as the Arboretum has, and experience what he calls "the quadrophonic sound. And they will never get that on laser disc; John Cage can never achieve this. You stand in there and it's all protected and it's just wonderful. It's the ultimate design."

With 87 gardens and collections covering 444 acres, the Arboretum offers many spots for solitude. "You will all of a sudden see someone wander in the path," says Cathey, "and what you will always find is they think they are the only one there. Of course there are trysts once in a while . . ."

Romance is lustier in the sultry months. But if you can't stand the heat, getting out of the lichen is not a problem in the Arboretum: With 60,000 different kinds of plants, something is always in bloom, even in the heat of summer. Some of Cathey's favorite green scenes are Fern Valley, Asian Valley and the National Country Garden.

Fern Valley is a meander in deep shade, where the native plants go off in succession like rockets. The Valley hits you with a sudden fragrant coolness. Depending on how cool you wish to be, you dodge or stand in the path of the sprinklers. Trails wind among the silvery spleenwort and the wild sarsaparilla.

Leaves rustle when you pass by, as thrushes and chipmunks rearrange themselves. They know instinctively how far to go to discourage your pursuit: a few feet. Beside the sun-speckled walk, a patch of ivy quivers. A woodpecker beats a ratatatat above a serene pond.

Signs for "Camelias" lead to Asian Valley. Along the way, couples stop to watch geese in Green Spring Pond, and a painter works on her tan while focusing her efforts on a single magnolia branch. A car radio blares from a parking area: "Summer breeze makes me feel fine / Blowing through the jasmine in my mind . . ."

In Asian Valley, most of the rhododendrons and late azaleas have long since bloomed and abated, leaving small brown tissues in the place of flowers. But here at the end of a path sits a romantic red Japanese gazebo, so sheltered from the road you see but are unseen. Down below drops a green glen with a stream that is said to echo with laughter. It's broken and hasn't been running this year, says one of the garden maintenance men. "It's supposed to be a laughing stream, right?" asks the visitor. "Yeah," he says. "It's kind of a joke."

In the moat around the Visitors' Center, hospitable koi rise from the murk to show off their glistening yellow, gleaming black and fiery orange. Young or ancient, the fish make you feel like royalty as they cluster at your feet. Marc Cathey says there is a patriotic koi here (red, white and blue), one we should look for around the Fourth of July. Three feet long, over a hundred years old, it was given to President Reagan five years ago. "I suppose it's called 'Uncle Sam,' " says Cathey. "Unlike the Lord that names every flea, I don't know its name."

Also at the Arboretum, the National Country Garden stands as a backyard garden, growing like gangbusters. "I am gone for 10 days in July for meetings every year," says Cathey, "and it's like missing your daughter's graduation."

The National Country Garden offers vertical gardening for the masses, with cheerleading signs that exhort, "You can do this too." Plants spell out "USA" from their "garden" of green plastic crates with holes in them, piled into piers. Living walls, a tomato plant in every hole. "Everybody can be a gardener -- Grow it in a bag!" WASHINGTON PLANTED HERE

There's a special place in Cathey's heart for gardeners, especially for one who was father of his country on the side. At Mount Vernon, trees still stand that were planted by George Washington -- giant ash, buckeye, tulip poplar and hemlock, their bark scored with thick wrinkles. Marked by plaques and lightning rods, the trees tower over the serpentine path that borders the bowling green in front of the mansion. If you think trees can't be restored like historic houses, think again. Here is a white ash, planted in 1785, recently bricked up inside.

Here as well, the ultimate kitchen garden is packed with lavender and rosemary borders and cabbage heads in rows, allowed to be uneaten sculpture. Another garden brims with perennials and scent.

A sudden shower chases the tourists from the paths and into the mansion for a tour. After seeing the bed George slept in, the visitors watch the river and the rain from chairs lined up on the piazza, as if at a retirement home.

A few miles from the tour-bus hubbub, River Farm, also part of George Washington's former estate, now stands as the comfortable headquarters of the American Horticultural Society. Here is a host of well-cared-for roses, daisies, bleeding heart, poppies, larkspur, salvia, phlox, more than 150 cultivars of lilies, and boxwoods more than a hundred years old.

Quiet except for the whispering trees, this country estate has a wildflower meadow that rolls down to the Potomac. Swifts skim low over the daisies, searching for seeds. AVANT GARDEN

Aside from Mount Vernon, another favorite of Marc Cathey's is the park flanking the Federal Reserve Board building. A grass park, of all things. Silver grass. Big blue lily turf. A host of hosta. Japanese fountaingrass in great green tufts. While some suburbanites could mistake it for hay and sooner scythe it down than sneeze at it, think of it as sculpture. The sweet smelling grasses evoke mixtures of memories -- of walking in a field newly plowed for the circus, of turning cartwheels in the yard, of lolling in the hay inside a hot barn. Fresh summer smells.

The section known as Robert Latham Owen Park, for a senator who helped create the Federal Reserve system, sings -- the wind blowing through the grasses and the fountain sprays spilling into a moat. Life could be worse than a career of lunchtimes on these grounds, which include a tennis court. GROWING LIKE TOPSY

On the South River near Annapolis, London Town Gardens was created in the early 1970s by a set of fortuitous circumstances. In the beginning, London Town Gardens was surveyed and found to be flat. Then, when horticulturist Tony Dove was pushing away the greenbrier and honeysuckle vines, he fell to the bottom of a ravine. Instant landscaping. An Anne Arundel county horticulturist, Dove rescued most of the ferns and wildflowers here from vacant lots about to become baseball parks and recreation centers. "We hauled 30 truckloads of ferns and wildflowers from one park," says Dove. For a while the Naval Academy hustled its leaves to London Town gardens because it was closer than the dump. Middie mulch. Fertilizers aren't used in this garden, proof that the more organic matter you can throw in beforehand, the better.

On the grounds, the 1758 Publik House shows tavern life in the 18th century. It's worth a visit, but the gardens are worth several, as something is always blooming -- after spring, there are Japanese iris and daylilies in summer, camelias in fall and heaths and snowdrops in winter. There's a stream here, and a picnic area. The saltmarsh walk is temporarily closed for repair, at least till the end of the summer. But that doesn't stop the weddings here every weekend.

When Queen Elizabeth's horticulturist John Bond was touring American gardens a few years back, he gave high marks to London Town because it's not stagnant, it continues to grow (10 acres now, a new 12 being developed). And Tony Dove gives Brookside Gardens high marks for the same reason. BY THE BROOKSIDE

In the Significant Hush of the rose arbor, a young couple is having a small disagreement. "Oh, boy!" says the woman, disgustedly. "Now I see how romantic you are!" She turns and heads for the Fragrance Garden, leaving him to ponder whether a rose is a rose is a rose. He looks as if he'd rather see Brookside's terrarium of carnivorous plants right about now.

Stray monarch butterflies and sparrows find their way into Brookside's greenhouses, which are a popular romantic walk for the golden anniversary set. Amid fuschia festoons and more geraniums than even cliche allows, there is a fountain and a small pool with stones in the bottom. "Please do not throw stones," admonishes a sign sitting in the water. Well, actually, it had not occurred to us. But oh, this is a glass house.

These are the days when you have the Japanese teahouse to yourself, instead of sharing it with a crowd mutely staring off at the water -- days when the carp come to your Wonder Bread alone. UNDER THE OAKS

Dove recommends Dumbarton Oaks for its artful asymmetry. We recommend it for its secret places. Dumbarton Oaks does something a little differently from the other gardens. Gardening chores are done in the morning, when the grounds are closed to visitors. The gardeners have prepared the place, down to the creeping thyme nestled between slate steps, then left it for your pleasure. So easy to pretend it's yours, its weathered wooden benches so inviting.

With the spring rush over, you see more chipmunks here than people. Robins flutter away half-heartedly at the last possible moment. It is their province here, among the decorative urns and finials, the daylilies, the rose parterres, the trickly fountains, the perennial paths and the long brick walk lined with boxwood.

Dating from 1810, the Orangerie here is nothing short of elegant. And cool, with clay tile floor always wet: The gardeners have been here to water the fig that covers the walls and beams. The fig has been growing inside this room, where tender potted plants are stored in winter, since before the Civil War.

On a bench and on the pebble pond and probably in a few other undiscovered places can be found the words, "Quod Severis Metes": As you sow so shall you reap. This was the motto of Robert and Mildred Bliss, who had the foresight to hire noted landscape gardener Beatrix Farrand to oversee the effort. LADEW IS NOT ALL WET

"If you would be happy for a week, take a wife. If you would be happy for a month, kill your pig. If you would be happy all your life, plant a garden." This is the ancient Chinese proverb engraved in the stone steps leading to the Garden of Eden -- one of 15 or so small gardens at Ladew Topiary Gardens. "One of the world's best topiary gardens," as Dove calls it.

The tour guide in the former home of Harvey Smith Ladew -- horseman, rich man, gardener extraordinaire -- tells an apocryphal story about the bachelor who designed Ladew Topiary Gardens almost singlehandedly. After he admired some lilies in the garden of a friend, the friend promised to send Ladew some bulbs. But when he wrote to Ladew to say they were in the mail, the friend referred to the bulbs by the plant's nickname: "50 naked ladies arriving. Do you think you can handle them?"

From a window upstairs in a guest room, you look out on the Great Bowl -- a steamy expanse of grass that yawns around an oval pool (Ladew's former swimming pool) and fountain, bordered with sculpted hemlock and yew. Even from the house the sense of heat strikes you. "No way I'm going to the end of this garden and back," you secretly vow to yourself, and your companion does the same.

But starting with the Wild Garden, in front of the house where sometimes spiffy old Packards congregate, you move from Pink Garden to Rose Garden to White Garden to Yellow Garden, lured like a bee by color and scent. In the Topiary Sculpture Garden, the sculpted hedges are impossible, Alice-in-Wonderland chimera, and of course ever green.

There is no way around the heat. Ninety is ninety here. But you will make it to the converted stables, partly underground, with its cool brick floor. There, using the Herb Garden to best advantage, the cafe's refreshing offerings may include cold cucumber and mint soup, and salad with creamy tarragon dressing. HI LILIES

No apologies for the heat are made at Kenilworth Aquatic Gardens. The gardens are in their glory now. When other gardens are on the wane, Kenilworth waxes, its 44 ponds jammed with waterlilies and lily pads, hardy and tropical. The ones with serrated leaves are tropical, and the leaves of one South American variety may grow to be six feet across. It's said that at that point, the lily pad can hold a baby. In July, the lotus masses here. Usually a Buddha sits on that.

In waterlilies, there are daybloomers and there are nightbloomers, and that's why the park rangers here recommend the 9 a.m. tour. "You get to see the nights just before they close and the days just as they are opening," says ranger Carol Borneman. "You get to see the wildlife too."

Muskrats, turtles, frogs, snakes. Northern watersnakes sun themselves along the banks of the ponds. Says Borneman, "Lots of people see them and yell 'Cottonmouths!' But we don't have any poisonous snakes around here. They have a head somewhat triangular, and whenever you see a triangular head you know it's poisonous. But a lot of snakes will puff up their head just to scare people."

W.B. Shaw bought the slice of land along the Anacostia River in 1880. Starting with a few species of water lily imported from his home state of Maine, Shaw turned his hobby into a family business. His daughter Helen Fowler succeeded him. But in the 1930s, the Army Corps of Engineers considered the ponds a haven for mosquitoes and malaria and moved to condemn them. That was when the Interior Department bought them.

"We don't have a problem with mosquitoes at all," says Borneman. "We put fish to work. We give them room and board and they eat the mosquito larvae."

During the summer, green herons stand on top of the pads fishing for frogs and fish. There are spotted salamanders, huge snapping turtles and burping bullfrogs. Sensing perhaps that the fauna here is more interesting to most than the flora, the rangers offer talks on such homey subjects as "good snakes," "eating weeds," "a rotten place to live" (dead trees) and "filling the bill."

Every so often, as you walk among the ponds at the Aquatic Gardens, the grass mysteriously flutters underfoot.

"Oh, look at them all!" says Borneman.

"Baby toads! We've probably stepped on a hundred of them!" WITH FAIREST FLOWERS WHILE SUMMER LASTS BROOKSIDE GARDENS -- 1500 Glenallan Avenue, Wheaton, Maryland. Open daily 9 to 5. Free. 949-8230.

DUMBARTON OAKS -- R and 31st Street NW. Open daily 2 to 6. Admission $2; children and senior citizens $1. 338-8278. GRASS GARDENS -- Between Virginia Avenue and C Street NW at 20th and 21st streets NW. More specifically, the Robert Latham Owen Park and the Edward J. Kelly Park, in front of the William McChesney Martin Jr. Federal Reserve Board building. Free.

KENILWORTH AQUATIC GARDENS -- 1900 Anacostia Drive SE. Open daily 7 to 3:30. Hour-long guided tours at 9, 11 and 1, through Labor Day. Free. 426-6905.

LADEW TOPIARY GARDENS -- 3535 Jarrettsville Pike, Monkton, Maryland. Tuesday-Friday, 10 to 4; Saturday-Sunday noon-5. House is open for tours during garden hours on Wednesday, Saturday and Sunday. Gardens only: adults, $2.50; students and senior citizens, $2; children under 12, 50 cents. House and gardens: adults, $4; students and senior citizens, $3; children under 12, $1. Sunday evening concerts, 6 p.m., call for information and prices. 301/557-9466.

LONDON TOWN PUBLIK HOUSE & GARDENS -- 839 Londontown Road, Edgewater, Maryland. Tuesday-Saturday 10 to 4; Sunday noon to 4. Admission: adults $2; students 6 to 18, $1; children under 6 free. 301/956-4900.

MOUNT VERNON -- At the southern end of George Washington Memorial Parkway, Virginia. Open 9 to 5. Admission: $4; senior citizens, $3.50; children 6-11, $2. 780-2000.

NATIONAL ARBORETUM -- 24th and R streets NE. Open weekdays 8 to 5; weekends 10 to 5. Information center, weekdays 8 to 4:30, weekends for special events only. Free. 472-9100.

RIVER FARM -- American Horticultural Society, 7931 East Boulevard Drive (off the George Washington Memorial Parkway), Alexandria. Check in with the receptionist on your arrival for a map of the gardens. Picnic tables. Free. Open 8:30 to 5 weekdays only, except for special events such as Dahlia Day, Sept. 6, when $2 admission is charged. 768-5700.