This is not the largest bequest ever to a public garden in the Philadelphia area, but it's Scott's biggest since 1929, when the arboretum was founded by the family of Arthur Hoyt Scott, class of 1895.
"It feels wonderful, like it completes the circle of my life," says Frederick, whose political science and botany studies at Swarthmore - interrupted by his Navy enlistment in 1944 - put him in contact with John Wister, the garden's founding director; Gertrude Smith, the assistant Wister would later marry; and British-trained Harry Wood, buildings and grounds superintendent.
Those relationships would change Frederick's life, eventually persuading him to abandon his family's dictum that he practice law - "they were shoving me in that direction" - and follow his heart. The Dickinson School of Law grad, keen on gardening since age 8, went off to Cornell University with his bride, Nancy Greenewalt, to study landscape architecture.
"I thought I was in heaven," Frederick recalls during a lengthy interview at the beautiful home the couple designed and built in 1965. It's in the familiar countryside between Centreville and Hockessin, where the families of both Bill and Nancy go back more than three generations.
Claire E. Sawyers, Scott director since 1990, calls Frederick "a passionate plantsman" whose gift represents "a very powerful expression of the pivotal relationships and pivotal moments" he experienced so long ago at Swarthmore.
The $800,000 is "a fabulous endorsement of the arboretum's mission, which is to display plants suited for home gardens in this area and to educate the public."
"Bill was clear that he wanted to revisit those early mission statements," Sawyer says.
Frederick also wanted, as he says, to "provide something substantial to help stabilize the organization in the future."
Paul W. Meyer, executive director of Morris Arboretum in Chestnut Hill, has known Frederick since he was a Longwood Gardens graduate student at the University of Delaware 40 years ago. (Besides a long association with Scott, Frederick was a Longwood board member for 36 years.)
"Bill has always cared about the education of young horticulturists and landscape architecture students," Meyer says. "We were meager students and he was chairman of the board, but he would reach out to us, invite us to his garden, show us around, take all of us to the Flower Show and dinner afterward, and introduce us to everyone."
Meyer suggests that Ashland Hollow, Frederick's garden, accurately reflects its designer - the fine design, the eclectic plant list, the blending of naturalistic and formal styles, serious elements and humor.
Meyer also cites Frederick's influential 1975 book, One Hundred Great Garden Plants. "He had paperbark maple in there. Forty years ago, you'd see that in botanic gardens but not much beyond," Meyer says. "Now you see it in relatively ordinary places as a street tree."
When contacted for this story, Rick Darke, former Longwood curator whose latest book, with Douglas W. Tallamy, is The Living Landscape: Designing for Beauty and Biodiversity in the Home Garden, was in York, Pa., touring a private garden that "probably includes the most beautifully executed examples of Bill's designs."
Darke credits Frederick with bringing innovative designers to Longwood, hosting salonlike gatherings of horticulture stars and students alike, and "being a catalyst for a tremendous amount of good work."
"If we're lucky, we have great mentors in our life and Bill is certainly one of mine," says Darke, who calls Ashland Hollow "pretty magical."
Touted by the Washington Post as "one of the most admired private gardens on the East Coast," it's designed as a stroll garden, a horticultural experience that reveals itself slowly, from far to near, and an art form that occupies "a middle ground between nature and culture."
It's a naturally hilly site, which both Bill and Nancy, 85, still navigate with gusto, guiding visitors from stream valley garden to game lawn and orchard; swimming pool and vegetable garden; plant nursery, meadow and quince hillside; green, gray and white shrub path; wisteria walk; frog steps; and persimmon grove.
There is so much to see and admire - earthen island in a pond, towering Japanese umbrella pines, gazebo on stilts - to say nothing of its wonderland value for the Fredericks' four children, six grandchildren, and one great-grandchild.
Besides the 1975 book, Frederick has written The Exuberant Garden and the Controlling Hand (1992) and a new self-published opus due out in January, called Wrestling With Angels and Singing With Dragons: The History of the Making of a Garden Across 45 Years.
It's a history that might never have happened, but for a young man's serendipitous arrival on the Swarthmore campus six decades ago.
"I didn't even know there was an arboretum there," Frederick says.