America's vast territory and changeable natural landforms make its landscape design almost naturally pursue naturalism. Olmsted and his followers advocated naturalism in the planning and design of a series of urban park systems, and opposed classicism which pursued solemn and clear structure. The beautiful natural landscape of the park is in sharp contrast with the harsh environment of the big cities at that time, which meets the social needs of returning to nature. However, American landscape architecture has experienced the flashy post-Victorian eclectic landscape, urban beautification movement and neoclassicism revived by classicism, and has not found a way to adapt itself to modern society in the decades of style swing.

2 Modernism

Before and after World War II, under the influence of modern art and modern architectural theories and works, American modernist landscape architecture gradually took shape after the so-called "Harvard Revolution". Modernism's most positive contribution to landscape architecture is not the use of new materials, but the concept of taking function as the starting point of design. Modern landscape architecture has thus got rid of some beautiful patterns or landscape transcendentalism, adapted to the reality of the site and the times, and endowed landscape architecture with rationality and greater creative freedom. As james ross, the third person of Harvard Revolution, said, "We can't live in paintings, but the landscape designed as a group of paintings deprives us of the use of living areas." He is most concerned about the use of space, not the planning mode or the so-called landscape order. What really inspires people in ThomasChurch's works of California School is not the order of composition, but the free design language and the delicate balance between design itself, site and employer's requirements. GarrettEckbo, another master of modern landscape design, emphasized the social scale in landscape design and the role of landscape architecture in public life. In his view, "if the design only considers beauty, it is a luxury that lacks internal social rationality." Dankley, whose works have the most sense of geometric order, also thinks that design is the mapping of life itself, and the pursuit of function will produce real art (Figure 3). The landscape architect who can best symbolize the landscape design concept and environmental concern in this period is Chaplin. His works reflect all aspects of the progress of modernist landscape architecture, including the social function of design, the emphasis on adapting to the natural system, and the importance of function and process to form. His series of urban public landscape designs take nature as the source of inspiration for drama landscape planning, which is not only a beautiful urban landscape, but also a place for people to relax, thus becoming a humanized open space in the city (Figure 4). Since the 1960s, the system of public participation in decision-making brought by social democracy has promoted changes in all aspects of American society, and so has landscape architecture design. Chaplin is the direct advocate and advocate of this change, and it is Chaplin who adapts his company's design program to the new social reality. Public participation in design through seminars and information feedback makes social will be reflected in landscape design. Modernist landscape design has stepped onto the road of synchronization with social reality by further emphasizing social factors and functions.

3 ecological ethics

Since 1970s, people have paid more and more attention to the ecological environment. LanMcHarg, a professor of landscape architecture at the University of Pennsylvania, holds that landscape should be regarded as an interconnected whole, including decisive factors such as geology, topography, hydrology, land use, plants, wildlife and climate. It emphasizes that landscape planning should follow the intrinsic value and natural process of nature, and perfects the ecological planning method with factor hierarchical analysis and map overlay technology as the core, which Macharg calls "thousand-layer cake model". Its theory and method endow landscape architecture with a certain degree of science, and landscape architecture planning has also become a clearly defined subject that can withstand various objective analysis and induction. Machar's research focuses on large-scale landscape and environmental planning, but for landscape architecture practice of any scale, it means an important message, that is, landscape is not only an aesthetic system, but also an ecosystem. Compared with those artistic design methods of arranging plants and terrain, a more comprehensive design idea is environmental ethics. Although the concept of natural determinism is only a hypothesis in the diversified practice of landscape design, when the environment is in a fragile critical state, the most important significance of the emergence of Machar and Payne School is to promote the rebirth of professional work as the ideological basis of landscape design, and their vast information has laid an indelible mark on the potential structure of landscape designers' thinking. For modernist landscape architects, ecological ethics tells them that all people have a natural connection with the earth's ecosystem except the social connection between people.

4 Postmodernism and Landscape Art Exploration

While large-scale landscape planning has turned to rational ecological methods, small-scale landscape architectural design has been influenced by environmental art and inspired by postmodernism since the 1960s, and many new explorations have been made on the relationship between art and landscape. PeterWalker, a new generation of landscape architect, combines minimalism, classicism and modernism to create a unique minimalist landscape. In his mysterious landscape design works, Walker uses simple physical, repetitive and geometric structures to bring natural materials together in a way that is divorced from their original natural structures, bringing a new visual comprehensive experience in the new structure. Artistic scenes with mysterious atmosphere metaphorically express the mysterious features of nature and the relationship between man and nature, and garden architecture is endowed with meaningful artistic temperament on the basis of function and beauty. However, MarthaSchwartz's landscape design works deny the authenticity of its materials, replacing seriousness with banter and simplicity with complexity, and the rigidity and rationality in modernist landscape are abandoned by designers. Designers express their new understanding of landscape with artistic conception and form: landscape is a collection of artificial or artificially decorated spaces, the foundation and background of public life, and a work of art related to life. Postmodernists reflect the complex and contradictory social reality of American postmodern society with almost grotesque novel materials and staggered composition system, embody the multiple sources of social values with diverse images, and express the postmodern social ideal of giving the disadvantaged groups the right to speak in this complex society. In terms of performance style, these positive experiments are similar to the neoclassical landscape architecture in the19th century, and are also inspired by visual art, and they also emphasize the use of geometric circles instead of the so-called naturalistic style. But here, the individual's imagination is integrated with the perfect functional concern of modernism, and artistic thinking regards the social elements in modern landscape as opportunities for creation rather than constraints. The role of art in creating a unique landscape environment has been re-established and deepened, but at this time, art is an incentive to design, not a priori formal domination.

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After a century's extension and development in art and science, American modern landscape architecture has drawn a watershed with its own social and ecological scale, and has become a multi-dimensional and multi-value practical specialty with continuous expansion and change. However, as the medium of human perception of nature, the three potential concerns of landscape architecture-aesthetics, environment and society-have not faded with the passage of time in the development of modern American landscape architecture. On the contrary, emphasizing the harmony between man and nature, the embodiment of social justice and the appeal of human spiritual pleasure in the landscape has become the cornerstone of its value system more and more clearly. From small-scale courtyards and block gardens to large-scale national parks, the values they care about may be different in practice, but every excellent design is the balance and synthesis of the above three values, not just the simple satisfaction of patterned forms or functions. Today, with the economic development of China, our urban construction and landscape design and practice have entered a period of rapid development. However, the neglect of comprehensive landscape architecture education by the competent authorities and the messy and fragmented landscape architecture practice make us unable to determine who we are or how to call ourselves, landscape architect or environmental designer in the face of kaleidoscope reality. The emerging superficial and formal random landscape design lacks real care for people and the environment, which profoundly reflects the poverty and paleness of the value care of China landscape architecture. When our landscape architecture is still full of confusion and confusion, olmsted, one of the founders of modern American landscape architecture, and many pioneers of modern American landscape architecture insist on creating a sustainable, life-saving and just landscape theme, and endow this clear idea with imagination, which may give us a lot of inspiration and enlightenment. This theme is the pursuit of American modern landscape architecture in a century, and it may also accompany us into the modern landscape architecture in the next century.