Grounds supervisor Bob Dornin shows the leaves of a fern leaf beech tree at the Hill School Arboretum, one of 51 properties across the country added to the Archives of American Gardens last year. (Jim Barnes/For The Washington Post)

Thousands of trees adorn the property of The Hill School in Middleburg, providing a peaceful, natural landscape for the school campus and an outdoor learning laboratory for the students.

Some of the trees at The Hill School Arboretum look as though they could have been there for a century. But less than three decades ago, the school was surrounded by hayfields and cornfields. A gift of land and the vision of a dedicated volunteer led to the establishment of the arboretum, school officials said.

The arboretum was recently selected by the Smithsonian Institution and the Garden Club of America for inclusion in the Archives of American Gardens. The school announced in February that the arboretum was one of 51 properties across the country that were added to the archives last year.

The arboretum had its origins in the early 1990s, when Stephen C. Clark Jr. and his daughter Jane donated more than 130 acres of adjacent farmland to the private school, which educates children from prekindergarten through eighth grade.

School Headmaster Treavor Lord credited Polly Rowley, a volunteer whose children and grandchildren attended The Hill School, with providing the vision, the leadership and much of the labor behind the arboretum.

Bob Dornin, the grounds supervisor, said that Rowley designed the landscape and spent years selecting, obtaining and planting many of the trees.

“She was out here in jeans, on her hands and knees every spring for about 10 years,” Dornin said.

The arboretum has become integral to the school’s curriculum, which emphasizes “place-based education,” Lord said.

“Because we’re in the country . . . we’ve always had programs that valued getting out on the land,” he said. “Over the last decade, we have been more intentional about, ‘How can we connect kids to the land? How can we wed the arboretum and the rest of the campus to the academic program?’ ”

The place-based educational program cuts across all grades and disciplines, with each grade focusing on a different theme, Lord said.

For example, second-graders study birds, third-graders identify trees and fourth-graders learn about eliminating invasive plant species. Many of the middle school students study Latin, which is useful as they learn the scientific names of trees and other plants.

Last Tuesday, Dornin walked along the trails, pointing out the species of trees, most of which are native to the region — maples, oaks, black locusts and Virginia pines.

“Most of the native trees of the Eastern U.S. are represented here,” he said. “Unlike many arboreta, where everything is really jammed together and you’ve got everything segregated by type, this flows on the landscape a little bit more naturally.”

Five kinds of pecan trees that line the edge of the property are “more of a Southern tree,” Dornin said. Others are more exotic: Japanese maples, a paper mulberry and a Chinese oak, which Dornin said is rare in this area.

“It’s beautiful the way the trees were laid out,” he said. “It defines views of the school. It defines views of the town from the school. But as much as anything, it’s a way to define the space to create outdoor classrooms.”

One small clearing is dotted with several boulders where children could sit and watch for goldfinches and cardinals at three bird feeders. In another area, eight native trees — including black gum, sweet gum, tulip poplar and oak — were planted at each point of the compass.

Dornin gestured toward a pond that he said was “filled with frogs every spring” and also home to snakes and snapping turtles.

“We describe animals that may occupy these habitats, and what sort of adaptations they have that make it easier for them to survive in these particular kinds of habitats — woods, pond, marsh, meadow,” Dornin said. “There’s always something to learn outside.”

Lord said that the arboretum’s walking trails are open to the community and that some Middleburg residents visit every day for fresh air and exercise.

“We’re happy to do that,” Lord said. “That’s part of the spirit of both the Clarks’ gift and Polly’s vision.”