The term vegetation is usually reserved for the natural plant communities that cover undeveloped landscapes. Agricultural lands and land enclosed within cities have generally been excluded from the term. This has led to narrow and restricted thinking about the plants of the city and has contributed to the notion that only plantings and ornamentals that require cultivation by man are nurtured in the city. The word vegetation implies a growth and maintenance unaided by man, and it can be demonstrated that the biological resources of the city can maintain themselves if the environmental circumstances are appropriate.

The concept of the vegetation of the city should not be limited to parks and playgrounds that are within the city limits, nor to the occasional open space found between city and suburb. Vegetation encompasses all of the plants of the city, including those in private yards, streetside plantings, boulevards, parks, potted trees and flowers, grass, and weeds on vacant lots. In sum, the entire vegetable component of the living environment of the city.

Plants are an important element in a city. For many cities the landscaped gardens, boulevards, and streets are their most distinguished characteristics. Although roads, bridges, buildings, and other construction make the city functional and hence workable, the plants and their arrangements, together with the arrangement of space, contribute an aesthetic value for which there is no substitute. The city is a direct effort by man to modify his environment through technology, but he has generally taken care to maintain as much of the plant life as possible, and where appropriate, to introduce new material.

In the first cities, plants were almost certainly a part of the natural environment in which the city was built. Since the maintenance of life in the city depended upon adequate supplies of food and water, no doubt considerable agricultural activity, as well as commerce, took place in the first cities. In fact, it was not long ago in the history of American cities that livestock was banned in the interest of public health, and, where the price of land does not prohibit them, one can still see kitchen gardens in cities. The well-manicured lawns and flower gardens of suburbia are the lineal descendents of pastoral antecedents.

Much of the area of the city should be capable of supporting the growth of natural elements of the city vegetation, with minimal environmental improvement. Greenbelts, parks, recreation areas, picnic grounds, city, forest, and wildlife gardens and arboretums, and most street plantings in residential areas provide suitable environmental conditions to support the growth of trees, shrubs, and turf, as well as wild and cultivated flowers. Hostile environments, if they exist in these areas, can be adjusted by design elements, such as the use of permeable paving materials and strategically located plantings. As one moves closer to its center, however, the engineering demands of the modern city diminish the prospect of plants growing as they once did. The heights of buildings, underground sewage pipes, paving, overhead utility lines, and many other structures create a hostile environment for many species.

To consider the vegetation of the city community rationally, one must begin by recognizing that whatever the origin of the plants found in the city, they are all regulated by the same life and death processes that regulate their natural community relatives. And though sometimes truncated, plants of the city form a community that, to some extent, is capable of self-regeneration, and whose vigor and vitality are dependent upon the same parameters as natural biological communities.

Among biological communities, those in equilibrium with the climate, that is to say mature biological communities, are the most desirable because they are able to maintain themselves with minimum outside maintenance, and are the most productive. It follows, therefore, that if the characteristics of the mature biological community are developed in the plants of the city, similar maintenance benefits will be enjoyed. But if we are to accomplish these goals, it is essential that we understand some of the ecological parameters of the biological community and how these apply to the plant community of the city.