Slifka beach house garden in Sagaponack, N.Y., begun in 1986 by Wolfgang Oehme and James van Sweden. (Sara Cedar Miller/The Cultural Landscape Foundation)

In 1990, the innovative landscape architects Wolfgang Oehme and James van Sweden trumpeted their efforts at changing the face of the American garden by publishing a book titled “Bold Romantic Gardens.”

They showed that dynamic gardens of novel perennials and ornamental grasses sprinkled with shrubs and trees could replace static lawns and clipped hedges. They used this plant palette in all kinds of settings — small urban gardens, big civic projects, seaside landscapes and botanical gardens. Their credo seemed to be: The meek may inherit the earth, but it is the daring who should shape it.

A generation later, the D.C. firm they established lives on, but both Oehme and van Sweden have died — and so has much of their original work. Of the 21 landscape projects featured in the book, nine have been lost to time or neglect, or altered to the changing needs or tastes of subsequent owners.

Gardens are living creations, and even the best-tended of them change. It is because of this fragility that extra care must be taken to preserve the worthy ones.

Raising awareness of these forces was the major impetus behind an exhibition that closes Sunday at the National Building Museum after six months. “The New American Garden: The Landscape Architecture of Oehme, van Sweden” includes photographs of major projects, including the Federal Reserve Board garden; the Slifka beach house in Sagaponack, N.Y.; the seminal Vollmer residence in Baltimore; and the Rosenberg garden on Long Island’s Mecox Bay.

The garden at the Rosenberg residence in Water Mill, N.Y., was begun in 1981 and helped establish the firm’s reputation. (Andre Baronowski/The Cultural Landscape Foundation)

The Vollmer garden in Baltimore, begun by Oehme in the 1960s, led to his partnership with van Sweden. (Roger Foley/The Cultural Landscape Foundation)

This show is ending as another landscape design exhibition at the building museum is in full swing. “Luminous Landscapes” is a survey by photographer Alan Ward of some of our most enduring and culturally significant landscapes, including Mount Auburn Cemetery in Cambridge, Mass.; Dumbarton Oaks in Georgetown; and the Biltmore Estate in Asheville, N.C.

Through Ward’s panoramic lenses, they have an immutable quality about them. Viewing both shows in sequence offers remarkable contrasts about how spaces are given different character and how our gardening sensibilities have changed over the past century.

It is the tenuousness of Oehme and van Sweden’s creations that spurred the landscape conservationist Charles Birnbaum to stage that show in collaboration with the museum’s curators.

As founder and head of the Cultural Landscape Foundation, Birnbaum has been raising concerns for a long time about the constant loss of or threat to important designed landscapes by major landscape architects. In various institutional and civic settings, many of the gardens are aging, their founders out of the picture, and the spaces are being eyed for other uses.

If the designed landscapes “haven’t had a level of maintenance, they reach a tipping point, and when they do, there’s pressure to create something new because it’s easier to fund,” Birnbaum said.

One such work under threat is Pershing Park on Pennsylvania Avenue two blocks east of the White House, designed by the modernist M. Paul Friedberg with landscape design by Oehme and van Sweden. The central feature, a pond of aquatic plants, has been drained, the hardscape elements have decayed, and the site is earmarked for a World War I memorial.

Birnbaum also laments the loss of van Sweden’s own two personal gardens and that of his Georgetown neighbor, the late Evelyn Nef, which van Sweden designed around Nef’s mosaic by Marc Chagall. The mosaic has since been moved (and restored) and can be seen in the National Gallery of Art’s Sculpture Garden on the Mall. Oehme’s own erstwhile garden in Towson has faded away.

In addition to large images of seminal work, the exhibition includes contemporary artwork that inspired van Sweden, vivid architectural drawings and other artifacts from the firm’s practice.

Birnbaum said he hopes the show, which travels next to Pittsburgh and then to Baltimore, will do for Oehme and van Sweden what a 2013 exhibition achieved for the modernist landscape architect Dan Kiley: raise the currency, stewardship and protections for the surviving work. Oehme and van Sweden’s firm thrives and evolves, led by three principals who worked for years with the founders. But the gardens that Wolfgang Oehme and Jim van Sweden directly conceived and planted are aging, as are the residential clients who commissioned them.

Kiley’s work is among those captured by “Luminous Landscapes,” which runs until Sept. 5 and includes his seminal Miller Garden, around a house designed by Eero Saarinen in the 1950s in Columbus, Ind.

The honey locust allée at the Miller House and Garden in Columbus, Ind. From the “Luminous Landscapes" exhibition at the National Building Museum. (Alan Ward/National Building Museum)

Presented in black and white and surveying highly structured landscapes, Ward’s big images have a starkly different feel from those in the softer Oehme and van Sweden exhibition. The latter is immersive, as if you are sharing in the ebullience of the meadow. Ward’s, by contrast, is objective and cool, with the color removed “to see past what we are used to looking at in a landscape,” said the curator, Susan Piedmont-Palladino, “to probe more deeply into the structure and the light and the texture.”

Ward, who is also a landscape architect, said formal, geometric gardens in particular lend themselves to the captured image and are “canonized” by them. “In some ways, certain landscapes can become more powerful because of the way they work within the frame,” he said. We should not forget Ward’s own skill as an artist and technician working through the lens.

View from the Orangery at Dumbarton Oaks leading to garden terraces. Ward’s meticulous compositions use paths as a way of leading the eye through the image. (Alan Ward/National Building Museum)

Ward’s photographic subjects tend to be free of people, which, along with their monotones, gives them a stark and timeless quality. The pictures convey a permanence that makes the Oehme and van Sweden images fleeting. One is spiritual, the other temporal. Ward and Birnbaum caution against reading too much into this juxtaposition, noting that formal, architectural landscapes need maintenance and refreshing. The allées at the Miller Garden have been replaced several times over the decades.

The two exhibitions have different objectives but at least one linkage. “They are both highly focused on how individual people are seeing the landscape,” Piedmont-Palladino said. Their contrasts, side by side, heighten the thrill of each one. But hurry.

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