Landscape designer Sally Boasberg, photographed in 2004 in her wooded garden, died March 29 at 74. (Marie Poirier Marzi/For The Washington Post)

Sally Boasberg, a landscape designer and dogged advocate for the District’s green spaces, died March 29 at her home in Washington. She was 74.

The cause was ovarian cancer, said her husband, Emanuel “Tersh” Boasberg III.

Mrs. Boasberg took up the plight of the city’s tree canopy in the late 1990s, penning a critical report revealing that as many as 5,000 trees were being lost annually to neglect and incompetence by the D.C. government. She later formed her own nonprofit group, Green Spaces for DC, that helped organize and fund beautification projects.

Her efforts helped secure passage of tougher laws to protect old trees on private property and encouraged the city to increase significantly the funding and staffing for the planting and maintenance of public trees.

Another of her major causes was the welfare of the U.S. National Arboretum, the federal botanical park in Northeast Washington. As a board member of the Friends of the National Arboretum (FONA) since 1996, Mrs. Boasberg organized the annual spring plant sale, which draws thousands of visitors in search of rare plants. The event is a major fundraiser for the institution.

In order to find worthy plants for the sale, Mrs. Boasberg developed a large network of specialty growers, said Kathy Horan, the executive director of FONA. She pressed suppliers to donate trees, shrubs and perennials for the sale, “which made it much more profitable for us,” Horan said.

In spite of her illness, Mrs. Boasberg spent the last months of her life urging arboretum administrators to complete the flowering tree walk, a tree-lined promenade encircling the heart of the arboretum that had been stalled for years because of funding and administrative problems. In 2008, Mrs. Boasberg and others successfully killed a proposed major budget cut at the arboretum.

She was also founding co-chairman of the Cultural Landscape Foundation, a national nonprofit organization based in Washington that was established to raise awareness about the value and fragility of historic landscapes. She also served as board chairman of the American Horticultural Society from 1993 to 1996.

“When you look at all the different types of organizations she was involved in, they were all about creating spaces that were well designed, well maintained, joyful, and brought communities together,” said Charles Birnbaum, the president of the Cultural Landscape Foundation.

Sarah Margaret Szold was born Aug. 1, 1937, in Brooklyn. She spent part of her childhood in Washington, where her father, Harold J. Szold, worked for the U.S. Munitions Assignment Board. She spent weekends at the family’s farm in Washingtonville, N.Y., where her parents kept a dairy herd and cultivated a large vegetable garden that she credited with inspiring in her a lifelong love of horticulture.

She received a bachelor’s degree in philosophy from Smith College in Northampton, Mass., in 1958 and married Tersh Boasberg two years later.

The couple moved to Washington in 1964 when her husband, a Harvard-trained lawyer, was recruited by Sargent Shriver for the Office of Economic Opportunity.

In addition to her husband, of Washington, survivors include four children, James E. Boasberg, a U.S. District Court judge in Washington, Thomas A. Boasberg, the Denver public schools superintendent, Margaret E. Boasberg of Newton, Mass., and Melissa A. Boasberg of Arlington County; and 12 grandchildren.

Mrs. Boasberg also served on the governing board of National Cathedral School and the board of trustees of the Washington Opera, and she was a past chairman of the Washington Opera Guild.

She loved to travel and visit gardens in Europe and Asia, said her husband. She studied landscape design at George Washington University and with the noted English designer John Brookes, and designed residential gardens for clients in Washington.

In June, on her last overseas trip, she visited 15 gardens in England and France, including the preserved landscapes of two of her design heroes, Edwin ­Lutyens and Gertrude Jekyll.

Over 40 years, she transformed the wooded and steep terrain of her own garden in Cleveland Park into a sylvan delight full of shade-loving perennials, bulbs, shrubs and trees. In December, she was hospitalized and told she wouldn’t see the new year.

But she returned home to her living room, with its view of the garden, and enjoyed an unusually early and prolonged spring. She made sure the paths were freshly mulched for her visitors.

One of her close friends, Sherry Houghton, said she reconnoitered the garden before entering the house so that she could tell Mrs. Boasberg what was sprouting and growing. Friends cut daffodils, quince and other early blossoms, and surrounded her bed with the flowers she had carefully chosen and grown.

“She was determined that spring would be as spring should be,” Houghton said.