Recently listed in Time Out as the world’s most beautiful botanic display, Longwood Gardens, in Pennsylvania’s Brandywine Valley, might be thought of as a thousand-acre gallery of gardens. An international roster of designers has shaped its evolution, and next Friday brings the unveiling of another transformation: Longwood Reimagined, a magnificent, 17-acre addition to its collection of gardens and conservatories. The cost: $250 million.

For well over 200 years, these gently rolling hills and meadows have continuously been home to horticultural display. What began here as an 18th-century hobbyist’s collection of native and exotic trees grew to be one of the continent’s finest arboretums and a pioneer public garden by 1850.

Longwood’s modern history begins a half-century later, when the industrialist Pierre S. du Pont purchased the arboretum to save it from the logger’s ax. Du Pont took up residence and across the property created a potpourri of gardens and playful fountains. In 1921, the year he opened the grounds to the general public, he completed a grand, ridgetop garden sheltered by a monumental, neoclassical conservatory. Today, Longwood is known for its efforts in research and conservation as well as its beauty.

Views of the West Conservatory, home to nearly an acre of Mediterranean gardens curated by the landscape-architecture practice Reed Hilderbrand.

The centerpiece of Longwood Reimagined is the new West Conservatory, which holds nearly an acre of Mediterranean gardens designed by the architects Weiss/Manfredi and the landscape architects Reed Hilderbrand. It joins a range of existing conservatories that overlook the spectacular Main Fountain Garden and its six acres of choreographed, dancing fountains.

The new glasshouse floats ethereally above a quiet reflecting pool. The parallel glass gables of its roof rise and fall like an abstract hillscape. Says Marion Weiss, a principal at Weiss/Manfredi, this new structure completes the string of conservatories across the site’s Crystalline Ridge. The evocation of landscape is no accident. It is part of a deliberate design strategy to weave together new and old in a hundred different ways.

Pierre S. du Pont, the founder of Longwood Gardens, standing in front of the property’s neoclassical conservatory, circa 1920s–30s.

According to Kristin Frederickson, a landscape architect and principal at Reed Hilderbrand, the design couldn’t begin until they understood “the palimpsest of the full history of the place—what is the essential DNA and how to bring that forward.”

Inspiration drawn from throughout Longwood’s lengthy history was pulled into the project: new ridgetop allées echo the tree-lined promenades of the original arboretum; water meanders through new gardens and bubbles up in new fountains, an homage to du Pont’s aquatic delights; and the West Conservatory, with its swooping, pleated roof, is a free-spirited fraternal twin of the staid East Conservatory, at the opposite end of the ridge. Even geology is brought into the mix. The ridge itself is a timeline across the landscape, linking today’s elaborate gardens to the east with sunset views over the primeval topography of conserved lands to the west.

Longwood, then and now.

When I asked Frederickson what she learned from her work at Longwood Gardens, she replied, “A sense that gardens are joyful places, that they are places of transformation and revelation. I think that is part of Longwood’s DNA.”

Longwood Reimagined, in Kennett Square, Pennsylvania, opens November 22. Timed tickets are required, with hours varying by date

Lewis Jacobsen keeps an eye on art and design from his aerie in upper Manhattan while overseeing his New York architectural practice

Photos: Albert Vecerka/Esto/Courtesy of Weiss/Manfredi (West Conservatory with walkway, Main Fountain Garden); courtesy of Hagley Museum and Library (du Pont); courtesy of Longwood Gardens (construction); Ngoc Minh Ngo/Courtesy of Reed Hilderbrand (all others)