Where appreciation for the importance of biodiversity meets the New Urbanism, you'll find the pursuit of the creation of urban wilderness.

Characterized by several traits, urban wilderness has been created by programs as varied as the New York City Parks Department's Green Streets program (which converts median strips and other microenvironments into planted areas) and small programs in such places as Davis, California and Portland, Oregon to reintroduce native species.

The nineteenth and twentieth centuries saw the creation of vast regions of concrete and asphalt, with minimal space set aside for living things beyond humans and their pets. Jacob Riis and other reformers fought for parks in urban areas, but the resulting parks, while a vast improvement, were formalized, rectilinear arrangements of artificially orderly lawns, bushes, and walkways.

While many societies had traditions of intense urban plantings, such as the famously lush rooftops of pre-conqistadore Mexico City, such traditions did not reemerge on a larger scale in the industialized world until the creation by Vaux and Olmstead and others of naturalistic urban parks. The rise of the City Beautiful Movement enhanced this trend as American, European, and other cities worked to bring natural settings back into urban areas.

In recent decades activists have sometimes seized the lead from architects, social planners, and horticulturalists as groups like squatters and Reclaim The Streets have engaged in guerrilla plantings, from work done in or on abandoned buildings to more symbolic acts like tearing holes in highway asphalt and then filling the holes with soil and flowers. These actions have been particularly effective in creating new planted zones in economically decimated areas like urban eastern Germany and Detroit, where abandoned buildings are occasionally reverting to forest. However, this trend can carry its own doom as beautified areas can work so well that they become targets for gentrification, with thirty foot (ten meter) trees grown over building foundations being torn down for yet more high density development.

Key traits of urban wilderness, the things that differentiate it from lawns and other ecologically questionable forms of plantings are:
1.) Biodiversity - a wide range of species, both of plants and animals
2.) Minimal maintenance required for viability - plants that can survive without frequent waterings, can withstand local pollution levels, and do not depend on infusions of fertilizers or other periodic soil amendments (see xeriscaping)
3.) Deep beds - deep soil allowing the creation of mature root growth, protection from drought and destructive temerature changes, and the development of a healthy colony of microorganisms, worms, and other beneficial small lifeforms
4.) Native species - considered use of local varieties rather than intrusive "emigrants"
5.) Unstructured esthetic - plants allowed to grow as they wish, where they wish, with minimal space devoted to paved walkways, trimmed grass, or other artificial environments
6.) Tolerance of groundcover and thick undergrowth - healthy ecosystems depend on "messy" microenvironments like decaying logs, thick brush, and muddy ground.