"It is sort of like a scavenger hunt of the best of English gardens," says Callaway.

One night, around a fire at Fielding's Dorset home, she posed a tough question: "After all this, what's your favorite garden?" Callaway balked. There were more than 50 on his list, private and public spaces that they'd traipsed through together from early morning until sunset through the years. But at last, he shocked his friend with his final answer: Hidcote Manor Garden.

Fielding had to agree it's a lovely place. But she, who reveres British female garden designers such as Gertrude Jekyll, Penelope Hobhouse, and Rosemary Verey, was surprised, outraged even, because the quintessential English garden of Hidcote was designed in the early 20th century by, of all things, an American. True, Major Lawrence Johnston was born in France, was educated at Cambridge, became a British subject, and fought in the British Army, but he was American nonetheless, part of a wealthy Baltimore family.

"Jackie said, 'Not only was Hidcote designed by a man, it was designed by an American man!' She thought my choice was the height of cheap," Callaway remembers, chuckling.

But together the pair had walked through Hidcote, marveling at Johnston's ingenuity and how his ideas have been maintained through the years. Located near Chipping Campden in Gloucestershire and purchased by Johnston's mother in 1907, Hidcote just celebrated a century since Johnston transformed 10 acres of estate grounds into a masterpiece of garden design. "He was among the first to create gardens as a series of 'rooms,' each dramatically different from one another," Callaway explains. "Johnston was a genius at providing strong architectural elements, such as boxwood and yew topiary, to give the gardens great structural interest even in winter, let alone during the milder months."