James van Sweden, a landscape architect who in the 1970s successfully reinvented the look and character of the American garden, died Sept. 20 at his home in Washington. He was 78.

Mr. van Sweden died from complications from Parkinson’s disease, said Lisa Delplace, who succeeded him as chief executive of Oehme, van Sweden & Associates, the firm that Mr. van Sweden founded with designer Wolfgang Oehme in 1975.

Within a few years, the company became known internationally for its radically different approach to landscape design — replacing staid evergreen hedging, bedding annuals and groomed lawns with broad sweeps of long, flowering perennials and ornamental grasses.

The vision was a rejection of passive vegetative architecture in favor of the bold massing of grasses and perennials that placed the observer in the midst of a living tapestry. The result was a garden that actively responded to light, wind and seasonal change. “They move in the breeze and sparkle like stained glass,” he stated in “Gardening With Nature” (1997), one of five books he wrote.

The look became known as the “New American Garden” and seized the imagination of clients and the design press with its associations with the prairies of Mr. van Sweden’s native Midwest, as well as the grass gardens of progressive designers in Oehme’s German homeland.

James van Sweden, a landscape architect who in the 1970s set out successfully to reinvent the look and character of the American garden, died Sept. 20 at his home in Washington. He was 78. (Roger Foley/Courtesy of OEHME, VAN SWEDEN & Associates)

The duo’s work ranged from private homes, including Oprah Winfrey’s, to major public spaces such as the National World War II Memorial on the Mall and the Nelson A. Rockefeller Park overlooking the Hudson River in lower Manhattan. Locally, the firm did work at Reagan National Airport, the Martin Luther King Jr. National Memorial and Francis Scott Key Memorial Park in Georgetown.

In 1990, the late landscape architect John O. Simonds described their work as “unlike anything yet seen in America.” It presaged today’s emphasis in landscape architecture on naturalistic and ecologically sensitive design.

“People without knowing it were thirsty for a change,” said native plant expert and designer Darrel Morrison, explaining the immediate appeal of their work. “It was a reaction to the dullness of the designed landscape at that time.”

From the beginning, Mr. van Sweden advocated his style in even small urban spaces as a way of capturing the natural exuberance of the prairie and meadow. He also espoused the idea that gardens should be planted for year-round interest.

The design partners set out to prove their theories — and win clients — by converting the deep but narrow backyard of Mr. van Sweden’s Georgetown rowhouse into a space layered with grasses, perennials and small trees in a way that blurred boundaries and paths. The object, Mr. van Sweden wrote, was “to lead the eye deeper into a scene which is not completely revealed, even in so tiny a space.”

“I think the two of them really did help to accelerate the recognition that the landscape of the city is a planted, managed, dynamic and changing landscape,” said Gary Hilderbrand, a partner of Reed Hilderbrand landscape architects and Harvard University adjunct professor of landscape architecture.

In the early years, their work was assailed by some in a design profession who viewed plants and flowers with a degree of contempt. “People thought it was a boutique form of landscape architecture,” Delplace said. Both men later became fellows of the American Society of Landscape Architects. Oehme died in 2011.

By the time Oehme, van Sweden & Associates had made its mark, and was based out of a historic bank building on Capitol Hill, Mr. van Sweden offered his team of landscape architects some advice: Never turn down a chance to speak to a garden club or other small setting, “because you never know who’s going to be in the audience.”

Such a talk earlier had led to the firm’s first major public commission, on the grounds of the Federal Reserve headquarters in Northwest Washington.

The son of a builder, James Anthony van Sweden was born in Grand Rapids., Mich., on Feb. 5, 1935. As a youth, he was interested in gardening, art and music, creative pursuits he later credited with informing his design career.

He was a 1960 architecture graduate of the University of Michigan. The following year, he moved to the Netherlands, the land of his ancestors, to study landscape architecture and urban design at the University of Delft.

One of his early mentors was a professor in Delft who impressed on his student the social value of town planning. He also suggested that on returning to the United States, Mr. van Sweden look up a young landscape architect who was making his mark in Baltimore, an East German emigre named Wolfgang Oehme (pronounced UR-ma).

From the start, they both designed with bold gestures — for Oehme that meant large groupings of herbaceous plants, for Mr. van Sweden, who was also a plantsman, the creation of an expansive architectural framework of walls, patios and ponds.

With early success came larger commissions, which allowed Mr. van Sweden and Oehme to craft rural gardens on a big scale.

Mr. van Sweden, an art collector, said various artists informed his work: abstract expressionists taught him to mass colors, while painters such as Edward Hopper and David Hockney showed the value of creating hard edges and delineation. He also learned to play the cello and the harpsichord.

He said he returned from study trips to Japan in the 1980s realizing that he needed to devote more attention to architectural detail and to hone his sense of scale. “After seeing exquisitely proportioned Japanese pools surrounded by the simplest details of buildings, stones and plants, my approach to design would never be the same,” he wrote in his 1995 book, “Gardening With Water.”

Mr. van Sweden’s marriage to the former Linda Nordyke ended in divorce. Survivors include two sisters.

Although his work attracted wealthy clients, Mr. van Sweden collaborated with his friend Marilyn Melkonian and others on redeveloping forgotten inner-city neighborhoods, including Paradise at Parkside, a 650-unit garden-apartment community in Northeast Washington near the Anacostia River.

“He felt that gardens lifted the spirit,” said Melkonian, president of Telesis Corp., which plans and finances such projects. “He always tried to design paths in a way that people could walk side by side.”

While Oehme was introverted and taciturn, Mr. van Sweden was an ebullient promoter of their design philosophies and mixed freely in political, corporate and especially cultural circles in Washington.

In an oral history interview with the Cultural Landscape Foundation, Mr. van Sweden discussed the importance of the Federal Reserve project and the call summoning him and Oehme to a luncheon meeting. “I said, ‘Wolfgang, this is our big break. You’re going to have to wear a tie.’ ”