"We gave this party to show everybody we were certified lunatics," Australia's Lady Cotton, in a diaphanous flowered garden-party dress, said yesterday. "When we started to plan the garden party, we weren't even sure there would be a spring."

"God, not the gardener, is responsible for the great display," added her husband, Ambassador Robert Cotton. "The cool weather delayed some of the azaleas, so they all bloomed at the same time."

Surely the Cottons must have sold their souls to the devil (Tasmanian, of course) to have ensured the absolutely perfect garden-party combination: a morning rain to wash the flowers' faces and make the grass green, sun in time to dry off the patios and cake the mud, a temperature warm enough to justify a show of white uniforms and decorations, and just enough breeze to ruffle the ladies' flounces but not blow away their hats.

"Ah, we at the embassy know that the ambassador has great and unexpected powers," Geoffrey Warren-Smith, the Australian first secretary for administration, said to Bruce Flatin of the State Department as they admired the red and yellow tulips around the pool.

Elizabeth McMahon, a New York interior designer down to help plan a redecoration of the house, once the home of Gen. George Patton, stood by the pool and plotted to take down the swags that obscure the ballroom's arched windows. It was a tribute to the day, that the tent by the pool, under which reposed enough oysters to stock a good-size sea, was undiscovered for some time.

Guests came in past the dining room with its table of asparagus with two sauces, cheese balls rolled in nuts, chocolate mousse in chocolate cups and tiny chicken sandwiches. Most chose to turn east to the front terrace, where blini waited to be topped with caviar, salmon and sour cream, and the crudite's were spread under a bouquet of flowers fashioned of cut vegetables.

"Heaven knows, I've been to enough garden parties in my day to know that this one is exceptional," said Caroline Simmons, social secretary to the late Janet Dulles, wife of John Foster Dulles.

Simmons was hurrying home for an expected call from her son, Huston, who is in Central America producing a television show for NBC. "When my husband the late John Farr Simmons was ambassador to El Salvador, I hated to leave so much, I cried. We loved the country," she said.

Cotton was more than willing to give his wife credit for the garden. "I only do the watering and the trimming on weekends. My wife raises seedlings."

"Three thousand summer flower seedlings," added Lady Cotton, moving on to greet the Spanish and Indian ambassadors.

After most of the 900 or so guests had arrived, the ambassador said, "There hasn't been a big garden party here for 17 years. It's about time."