Which life form on Dumbarton Oaks' 16 terraced acres is toughest to maintain in August?

"Grass," says Donald E. Smith, superintendent of gardens, who's kept the Georgetown retreat growing for some 30 years. "Any gardener who says otherwise, I want to know his secret. See the foot tracks? There's so much traffic. The White House has the same problem."

In fact, the wide sweeps of green look fresh rather than trampled. It's the few visitors who are wilted, sticking to the shade of the American hornbeams on the Ellipse and dallying beside the mosaic Pebble Garden.

"This is not really a summer garden," Smith warns. But it beats summer sidewalks. For a dollar, it offers enough cooling- off spots to temper a tour.

Dumbarton Oaks (its fifth name) was built in 1801, purchased by Mr. and Mrs. Robert Woods Bliss in 1920, landscaped in 1922 by Beatrix Farrand, and is now owned by Harvard. Clearly, it's at the elegant end of the local garden spectrum, probably our best example of an 18th-century European back yard. Elaborate stone clusters of magnolias and oak leaves with acorns mimic the surrounding nature. Delicate ribbons of stone and brickwork lace the paths; the formality lessens as one moves farther from the house.

In the front yard, a Japanese maple with a spread of more than 70 feet towers over R Street. It's the largest Smith has seen since he started gardening 50 years ago in Bar Harbor. Beside it a katsura lunges toward the house, stretching horizontally across the lawn. It drops its leaves early, like a gingko, seemingly at once. In the Orangerie next to the house, built in 1810, creeping fig planted when Lincoln was president circles the walls.

Just beyond the formal Zodiac Garden in the back, Smith shakes his head in amazement over the oldest and biggest (though not the tallest) tree on the property: a black oak, well over 300 years' worth. "It's been dying for 30 years," he says. Eventually, it will have to come down by hand, a piece at a time, as there's no way to move a crane into the enclosure. The oak shadows the Green Garden, designed to show off the colors in Victorian ladies' dresses at tea time.

By the off-limits swimming pool, there's a Portuguese tile-and-granite mosaic on the bathhouse wall. The scene, by Allyn Cox (who executed many of the Capitol's murals), depicts Diana and her maids, after she's turned a Peeping Tom into a stag.

Each section is discrete: the Lovers' Lane pool and amphitheater are out of view of the rose garden, which holds 37 varieties plus the ashes of Mr. and Mrs. Bliss encased in a wall. The Box Walk and the Ellipse, the Urn Terrace and Crabapple Hill are also distinct settings, tended by the fulltime staff of 13.

"We're lucky to have what we have in the summertime," says Smith. The 3,000 chrysanthemums for this fall will be planted next month. The 1,000 tulips for spring follow ("We mix our colors"), along with a number of perennials. But for now, the marigolds, impatiens, deep-blue ageratum, red veronica, salvia, daisies, gawky cleome ("nothing but a weed, but that son of a gun will take the heat") and tall hollyhocks are holding out against the season. "If you believe everything you read, you'd never put these impatiens out in the hot sun," Smith said of a potted array of pink. "But this one took right off."

This is the second summer for a double row of pear trees overlooking the cut-flower garden with an expansive view of Rock Creek Park. "Last year we had to pick the fruit off, they were leaning over so much," Smith said. Eventually they will form a thick hedge like the double row of hornbeams in the Ellipse.

Will we be around to see it?

"Well, that's the fun of it," Smith said, hedging. "A garden is different from an art gallery. You don't just put something on a pedestal. It's changing every minute." DUMBARTON OAKS -- At 31st and R Streets NW. Open 2-5 daily; admission $1 April through October, free November through March. Call 338-8278 for a recording of what's blooming and where to park. Admission to the Byzantine and Pre-Columbian art collections in the museum is free..