Adding a final jewel to the philanthropic legacy of John and Chara Cooper Haas, their children are donating the family's Stoneleigh Estate in Villanova to the conservation group Natural Lands Trust, which will manage it as a native-plants garden, open free to the public.

The 42 acres and Tudor Revival mansion on North Spring Mill Road are to be transferred to the trust at a ceremony and luncheon Wednesday on the Great Lawn behind the house.

Emotionally, "it is a passage, for sure," said David Haas, who was 9 when his parents acquired the estate from his grandparents in 1964. "But it is inspiring to think that this place" - where he, his four siblings, and cousins played and climbed trees - "will really be open, used, and appreciated by many different people."

His father, a former chairman of the global chemical company Rohm & Haas Corp., was 92 when he died in 2011. His mother was 85 when she died the following year. They had met at a dinner party in Houston, married in 1952, and used their ensuing wealth to support many charitable causes.

The donation of Stoneleigh, which David Haas called "the final piece" in the settlement of his parents' estate, will ensure the grounds are protected green space in perpetuity, extending the couple's lifelong commitment to conservation.

"It will outlast all of us," he said, "which is part of what philanthropy does, or tries to do."

When it opens in about 18 months, the estate will be rechristened Stoneleigh: A Natural Garden.

Among its native flora are dogwoods and huge hollies and rhododendron. Some of the stately trees are thought to be at least a century old.

The landscaping blends elements of Beaux Arts formality with the naturalistic style that was brought to the grounds by the two sons of the eminent landscape architect Frederick Law Olmsted. The Olmsted-designed gardens and vistas are historically certified.

"We'll build on what's here and augment it with new materials and new science on the benefits of natives in terms of the bugs, butterflies, birds, and pollinators they attract," said Natural Lands Trust president Molly Morrison.

Stoneleigh, she said, will be a horticultural resource, a trove of ideas for what visitors can plant in their own gardens.

Founded in 1953, the Media-based trust has stewardship of 43 nature preserves across Southeastern Pennsylvania and South Jersey.

"We own many properties. They are like children, you love them all," Morrison said. "But [Stoneleigh] gives us a new opportunity to showcase the many benefits that accrue to the environment by choosing natives over nonnatives."

According to the trust's plan, new plantings will be installed, a swimming pool filled in, and the pool house converted into restrooms. The mansion's first floor will be rented out occasionally for events. The second and third floors will house the library and archives of the national Organ Historical Society, based in Richmond, Va. Pipe organs are a passion of David's younger brother, Fred.

A recently acquired three-acre parcel adjacent to Stoneleigh's northwest corner will become the entrance and parking area.

Nearby is one of Stoneleigh's most sentimental features: a family of rabbits carved into the tall stump of a tree.

A local landmark visible from the busy intersection of Spring Mill and County Line Roads, the rabbits were decked out for holidays and special occasions. The elder Haases dressed the bunnies in caps and gowns at college graduation time, festooned them with colored eggs at Easter, costumed them at Halloween.

"People really became attached to it," David Haas said.

The tradition will live on, he said.

"The whole world could change, but the bunnies are not going anywhere," he said, explaining that a variant of his surname in Dutch and German means hare.

Stoneleigh's history dates to 1877, when Edmund Smith, a Pennsylvania Railroad Co. executive, bought 65 acres in Villanova and built a home. In 1932, after the land was subdivided, Otto Haas, John's father and cofounder of Rohm & Haas, purchased a portion. So began eight decades of Haas residency.

Current tax records put the estimated market value of the property at a minimum of $9.9 million.

After Otto Haas died in 1960, John Haas became chairman of the family's charitable organization, known today as the William Penn Foundation, one of America's richest grant-making institutions.

In 2009, Rohm & Haas was sold to Dow Chemical Co. for $15.3 billion, which allowed John and Chara Haas to substantially increase their charitable activities.

Among the beneficiaries have been United Way of Southeastern Pennsylvania, the Philadelphia Orchestra, the Boys and Girls Clubs of Philadelphia, Fairmount Park, and the Opportunities Industrialization Center of America, a job-training institution in North Philadelphia founded by the Baptist preacher and civil rights leader the Rev. Leon Sullivan.

In John Haas' obituary, former Gov. Ed Rendell was quoted as saying: "He didn't care much for the trappings of wealth or success. . . . His special passion was improving human services and protecting the environment."

John and Chara Haas made their home available for nonprofit fund-raising, and each May they opened the grounds for an annual event they called the Stoneleigh Stroll-About.

David Haas said the family's vision for the estate can be found in a note his parents wrote years ago to welcome strollers:

"We hope that you enjoy your visit here, and when you depart, may a bit of the peacefulness and beauty, which is so much a part of Stoneleigh, be with you."

215-854-2541@MichaelMatza1