Olmsted recalled that the Birkenhead site had not been in much better condition before that park’s creation—“a flat, sterile, clay farm.” In Central Park, Olmsted applied the lessons he had learned there, re-creating the winding paths, the variety of shrubs and flowers, the vast open meadows, the irregular clustering of trees. He developed a series of rules that he would follow in his subsequent projects, which included not only dozens of municipal parks but college campuses (Stanford, UC Berkeley, Gallaudet, Trinity College); private estates (George Vanderbilt’s Biltmore and John D. Rockefeller’s Kykuit); national sites (the grounds surrounding the U.S. Capitol, and Niagara Reservation, the country’s oldest state park); and Riverside, Illinois, one of the nation’s first planned suburbs. Olmsted’s success helped create not only a profession, but an aesthetic.

His first principle was that a park should complement the city to which it belongs. If a city is cramped, crowded, and rectilinear, its park should be composed of sinuous thoroughfares and a variable topography that includes large open spaces. The “comparative largeness” of Central Park was essential, since a park should “be a ground which invites, encourages & facilitates movement.” The giddy impulse you feel, upon arriving at the Great Lawn or Sheep Meadow, to burst into a full-out sprint—that is by design.

A park should also be faithful to the character of its natural terrain. It was in “bad taste,” for instance, to grow lawns in the arid western United States or palm trees in New England. Beauty was to be found not in decorative plants, as one might expect from a florist’s display window, but in general effects. Trees should be grouped in such a way that “their individual qualities would gradually merge harmoniously.” In one of Olmsted’s earliest memories, he planted a seed from a honey locust tree and, returning to the site a year later, discovered a sprig of leaves. By the time he was 12, it had grown into a sapling. Decades later, he found that his honey locust tree had been chopped down. After a momentary sentimental twinge, Olmsted concluded that he was glad the tree was gone, “for its individual beauty was out of key with the surrounding[s].”

Man-made structures were also out of key. When bridges or buildings were absolutely necessary, they should be built from local stone, heavily camouflaged with shrubbery and vines. One of his most remarkable technical achievements in Central Park was to make its four major crosstown thoroughfares disappear: He sunk them into the ground and hid them with foliage. Much of the park’s charm derives from the alternation of rolling expanses and hidden passages, such as those that thread through the Ramble, which create the illusion of privacy and mystery.

An unmistakable irony creeps vinelike through Olmsted’s landscape theory: It takes a lot of artifice to create convincing “natural” scenery. Everything in Central Park is man-made; the same is true of most of Olmsted’s designs. They are not imitations of nature so much as idealizations, like the landscape paintings of the Hudson River School. Each Olmsted creation was the product of painstaking sleight of hand, requiring enormous amounts of labor and expense. In his notes on Central Park, Olmsted called for thinning forests, creating artificially winding and uneven paths, and clearing away “indifferent plants,” ugly rocks, and inconvenient hillocks and depressions—all in order to “induce the formation … of natural landscape scenery.” He complained to his superintendents when his parks appeared “too gardenlike” and constantly demanded that they “be made more natural.”