When Frederick Law Olmsted was designing the Arnold Arboretum in Boston, he drew on a sentiment he’d expressed years earlier: “We want a ground to which people may easily go after their day’s work is done, and where they may stroll for an hour, seeing, hearing, and feeling nothing of the bustle and jar of the streets, where they shall, in effect, find the city put far away from them.”

Olmsted’s sentiment, expressed in the late 1800s, rings true today. People have gained a renewed appreciation for nature as an escape from the stresses of modern life, and historic arboretums—botanic gardens focused mainly on trees, vines, and shrubs—supply a steady stream of it.

A 2022 United States Forest Service study uses hard data to confirm what Olmsted knew instinctively: Trees are good for us. Not only do green spaces improve mental health, they also clean the air, cool us off, and mitigate flooding. In the Philadelphia-based study, Forest Service scientists found that increasing the city’s tree canopy cover to 30 percent by 2025 might prevent more than 400 premature adult deaths a year.

The Arnold wasn’t Olmsted’s only arboretum design, but it’s one of his best known. Part of his Emerald Necklace group of seven connected Boston parks, the 150-year-old arboretum is mostly owned by the city but leased and operated by Harvard University. Olmsted and botanist Charles Sprague Sargent created a naturalistic landscape with winding roads and trees grouped by family and genus.

The pair used the latest thinking in plant science to make decisions, and that emphasis on innovation continues. Since 2019, the Arnold has been cultivating specially chosen plantings in its main field of solar panels, which helps power an on-site research building. The new solar meadow “combines the natural and environmental side of the arboretum together with technology,” says Arnold spokesperson Jon Hetman.

In 2021, the Arnold introduced a mobile visitor center, a portable structure that it can deploy to any of the site’s 20 different entrances. “We’re trying to think about how we can engage visitors more directly and meet neighbors where they are,” Hetman says. To further ensure a warm welcome to everyone who wants to visit the arboretum, the Arnold is redesigning and re-landscaping some of its entrances.

Like many college or university arboretums, the Arnold’s 281-acre Boston site is separate from Harvard’s Cambridge, Massachusetts, campus. Not so for Haverford College, where the school’s entire 216-acre campus attained arboretum status in the 1970s. Located in a Philadelphia suburb and founded by Quakers in 1833, Haverford boasts the oldest planned collegiate landscape in the nation.

British gardener William Carvill is credited with creating the English Pastoral–style grounds, gracing them with wide-open vistas, stately red oaks along the main roads, and evergreens on street corners. Later additions included a 13-acre pinetum (a scientific collection of conifers) assembled between 1928 and 1948, a picturesque duck pond (1931), and a student-run farm (2010) known as the “Haverfarm.”

This fall, the Haverford College Arboretum hopes to open a garden area honoring the land’s original dwellers, the Lenape people. Arboretum Director Claudia Kent says her team is working with the Lenape Nation of Pennsylvania to identify plants that the Lenape likely would have used and compose an inscription in the Lenape language. “It will be a garden of reflection,” she adds. The Lenape garden will be located along the college’s 2.2-mile nature trail, which (like the rest of the Haverford campus, as well as the Arnold Arboretum) costs nothing to visit.

Another free-admission haven for tree lovers is the U.S. National Arboretum, established in 1927 and run by the U.S. Department of Agriculture. Its location outside the central core of Washington, D.C., means it gets left off many visitors’ itineraries, but those who skip it miss an iconic sight: the National Capitol Columns, 22 freestanding Corinthian pillars that tower impassively over rolling meadows. The sandstone pillars were removed from the U.S. Capitol in 1958, when an eastern extension was added. The nonprofit Friends of the National Arboretum worked with the great British garden designer Russell Page to arrange the columns at the arboretum in the 1980s.

Other historical elements on the site include extant spring houses from the Red Oak Spring Company, a short-lived bottled water business that operated on the property in the early 1900s. The accordion-roofed 1964 Administration Building, designed by Deigert & Yerkes, pleases Midcentury Modern aficionados. But these structures are brand new compared to a small Japanese white pine tree that dates to 1625. The pine is part of the National Bonsai & Penjing Museum, started at the arboretum in 1976 with a gift of 53 bonsai from the people of Japan. “It’s one of the largest and most comprehensive museums of its kind,” says Richard Olsen, the arboretum’s director.

Scattered across the National Arboretum’s 451 acres, among other highlights such as azalea and Asian plant collections, are specimens of state trees from almost every state in the union. A mobile app contains a guide that helps visitors learn more about each one.

One of the few state trees that can’t grow in the National Arboretum’s mid-Atlantic climate is Hawaii’s kukui. But you can see kukui trees at the Harold L. Lyon Arboretum in Honolulu’s lush, rain-drenched Manoa Valley. To get there by car, visitors must drive to the northern edge of the valley, and then keep on going. “We’re at the end of the road, literally,” says Raedelle Van Fossen, the site’s education manager.

Seeing the 190-acre site is worth the effort. Used by Native Hawaiians and then by colonizers, the land had been largely deforested by the grazing of feral goats, sheep, and cattle by the time it entered the ownership of the Hawaiian Sugar Planters’ Association in 1918. The group hired plant pathologist Harold Lyon to find the best plants for reforestation efforts, which would ultimately restore the area’s watershed and aquifer.

Lyon and his successors established impressive collections of non-native tropical plants, including palms, fig trees, gingers, and “canoe plants”—those brought to the Hawaiian Islands by Polynesians 1,500 years ago. Today the University of Hawai'i–owned arboretum carefully tends these collections while also working to reintroduce plants native to Hawaii, many of which are endangered.

“We’re trying to re-establish the native canopy,” explains Līloa Dunn, grounds and collections manager at the Lyon. Over the past decade, he and his team started by planting koa trees, a larger Hawaiian species, and then moved to smaller items such as Hawaiian lobelioids, a group of flowering plants. The Lyon also houses the Hawaiian Rare Plant Program, which focuses on researching and conserving native plants in two laboratories and a greenhouse on the site.

Joy Morton, the salt magnate and founder of The Morton Arboretum in Lisle, Illinois, might have agreed with Dunn’s strategy: The Morton family’s motto was “Plant trees.” Located on ancestral lands of the Potawatomi, Ojibwe, and Odawa peoples, the site encompasses 1,700 acres, making it one of the largest arboretums in the country. Designed by landscape architect O.C. Simonds (with plant advice from the Arnold’s Charles Sprague Sargent), the Morton is celebrating its centennial this year.

As one way of marking the occasion, the site (shown above and at top) has embarked on a Centennial Tree Planting Initiative. This project, part of a longer-running push to increase the Chicago region’s tree canopy, entails planting 3,000 trees throughout the region by the spring of 2023.