Philip Johnson's influence on architecture during the 20th century is impossible to overstate. He brought Ludwig Mies van der Rohe into the American consciousness, he became one of the most famous practitioners of the International Style (then angered and confused critics with an abrupt mid-career shift into Post-Modernism), and he created one of the best-known private homes in the last 100 years. His brand of glass-and-steel architecture—and his bespectacled appearance—are familiar to anyone with even a passing interest in buildings and the people who design them.
Still, even given his great fame and exhaustively documented life, it might be surprising to learn that the designer of the Glass House and Manhattan's AT&T Building considered himself an architect of the landscape as well as one of glass cathedrals and columned performance spaces, once stating that for him, architecture and landscape architecture were "one art."
For an architect like Frank Lloyd Wright, whose philosophy of organic architecture led him to build some of his most famous designs literally into the landscape, that kind of statement would make sense as a logical extension of his famous quote on the relationship between a house and its land: "No house should ever be on a hill or on anything. It should be of the hill. Belonging to it. Hill and house should live together each the happier for the other."
For someone like Johnson, though, with his Modernist background, iconoclastic bent, and love of glass-and-steel construction, the sentiment either reflects an unexpected facet of the architect's sensibility, a deliberate provocation, or both.
Johnson surely was somewhat influenced by the rural Ohio landscapes of his youth, but beyond that there's little to suggest a deep-seated love of nature seeking expression in landscape architecture. The fact that he did not start his architectural career until well into his 30s, combined with his wide-ranging lifelong interests in philosophy, music, and the visual arts, seem the hallmarks of a young man who struggled to find an appropriate outlet for his restless, energetic intellect (a search that perhaps led to—but does not excuse—his troubling involvement with Nazism), and who gratefully threw himself into his avocation as soon as he stumbled upon it. It would seem that he'd have little energy left to devote to the landscape as a design form.
But if the man himself believed he was a landscape architect, then maybe it's best to take him at his word. After all, under the broad Wikipedia definition of the discipline, which states that "landscape architecture is the design of outdoor public areas, landmarks, and structures to achieve environmental, social-behavioral, or aesthetic outcomes," he does squeak in. So perhaps the question that should be asked is not "Was he a landscape architect" but rather "Did he have any aptitude at all for it?"
To answer that question, it's probably best to start with a look at what is inarguably his most successful created landscape, the Glass House in New Canaan, Conn.
Constructed in 1949, the heavily Mies-influenced Glass House was designed to serve as Johnson's personal residence and, just as importantly, as a way of trolling the traditionalists like Wright, who, upon entering the house for the first time, sniped, "Here I am, Philip, am I indoors or am I out? Do I take my hat off or keep it on?" Sited on what was then a lot of five acres, amid the ruins of old farmhouses, the home and its glass walls would come to preside over the many changes Johnson swept over the landscape as the estate expanded to 47 acres and 14 buildings over the years.
The most dramatic of those changes was the creation of what Johnson called "events on the landscape": the buildings and pavilions he built around the estate grounds as he hacked down trees and went about opening sightlines and creating vistas. While the quality of the various buildings—a group that includes an unusual underground art gallery and a lake pavilion—vary, the site as a whole is generally considered a success, and represents a competent job of landscape design by Johnson.
Whether he ever successfully designed another landscape project is a question that can be partially answered by looking at a public space by Johnson located in Texas, the 1974 Forth Worth Water Gardens. Meant as a cooling oasis in the heat sink of the city center, the Water Gardens consist of three pools of water, an aerating pool with fountains, a meditation pool surrounded by a stand of shade trees, and a waterfall pool with cascades that tumble down steep concrete steps into a central pond, all of which are surrounded by landscaped terraces and planters.
As a design, it's very much of its time, its concrete forms and rigid channeling of water a reflection of the paradoxically anti-urban neo-Brutalism of urban development in the 1970s, as inorganic and reflective of anxiety about urban life as any of John Portman's hermetically sealed corporate monoliths. The fact that it was prominently featured as a location in the dystopian Sci-Fi film Logan's Run puts it in the same unhappy company as other contemporaneous Modernist hellscapes like Century City (which itself played a starring role in another bleak Sci-Fi feature of the 1970s, the apocalyptic Conquest of the Planet of the Apes).
It's not entirely fair, of course, to form overarching judgments about an architect's oeuvre from only two of the numerous designs produced over the course of a prolific career that spanned over 60 years. It is fair, however, to say that if Philip Johnson is also to be considered as a landscape architect, his record is decidedly mixed.