Criteria for List

Developing the list of plants has been a painstaking process taking us well over a year to complete. The complications arose from the process of trying to develop functional definitions of the terms urban vegetation , invasive species and weed . Because the meaning of these words varies depending upon who is using it, we have had developed my own definitions that are consistent with the goals of this website. We use the term urban to refer to the core of the city, where more of the land is covered with pavement or buildings than not, and all traces of the original native vegetation have either been totally removed or drastically disturbed. This environment is characterized by high levels of disturbance associated with pedestrian and vehicular traffic, infrastructure maintenance and new construction, and the spontaneous plants that grow there are referred to as emergent urban vegetation (Zipperer et al., 1997) .

The plant list focuses on the plants that take up residence in the vacant spaces between our roads, our homes, and our businesses, that line the shores of our streams, rivers, lakes and oceans, and that take over our neglected lawns and parks. Some of these plants may be native to the region before it became a city, some may have been brought there intentionally or unintentionally by people, and some may have been dispersed there by wind, water or animals.

For the purposes of this website, the term urban does not include the more or less undisturbed natural areas found within the boundaries of the city and the species list does not include plants that are confined to such remnant habitats (Whitney, 1985). However, the list does include species that have left these natural areas and settled in the city, as well as non-native species that have established themselves inside these natural areas. And finally, the list does not include cultivated species that are characteristic of the managed habitats within the urban environment unless they have "escaped" and become "naturalized" in the surrounding unmanaged areas. See a summary of Taxonomy of Urban Habitats.

The list includes those plants that are commonly found growing spontaneously in the cities of the northeast, minimally from Boston to Philadelphia , and more broadly from Montreal to Washington , D.C. My assessment of common is based on a review of the relevant literature as well as extensive observation of the urban vegetation of the northeast. In general, the plants on the list all display an ability to colonize disturbed ground across a broad range of unmanaged, urban habitats.

In the process of developing the plant list, we have consulted innumerable books and websites covering weeds and invasive species. Most of the references on weeds focus on plants that are considered problematic in an agricultural context, where the issue of competition with economic crops is the primary concern, or in a residential context, where the term weed refers to an unsightly plant that is growing in a place where people are trying to cultivate something else. For invasive species, the defining context is usually the natural areas or minimally managed landscapes in suburban or rural areas, where the listed plants are seen as disrupting or displacing native vegetation. For spontaneous urban plants, people's complaints are usually aesthetic-they are perceived as sign of blight and neglect-or security-focused-they provide a haven for illicit human activity or habitat for vermin.

While there is considerable overlap among the plants considered weeds, invasive species and emergent urban vegetation, these categories of plants can often be distinguished from one another by the types of habitats in which they grow-the cultivated, the semi-natural, and the constructed. People are often surprised to learn that many of the plants listed as invasive species in rural areas or as weeds on fertile, agricultural soils are uncommon in the urban environment because their preferred habitats are rare or missing. Indeed, the context in which a plant is growing not only determines the label that we put on it, but also the positive or negative value that we assign to it.

Virtually all of the available literature on weeds and invasive species makes the assumption that the plants so categorized should be eradicated because of their capacity to create economic, ecological, or aesthetic damage. Inevitably, these books and websites provide detailed information on how to get rid of these "aggressive" plants through a combination of manual, mechanical, and chemical control measures. In contrast, this website refrains from making value judgments about urban plants, other than to list their potential contributions to making the city a more livable place for people, and it provides no information on how to eradicate them. This is in keeping with our two primary goals: first to make people more aware of the vegetation that is native to the urban areas in which they live; and second to counter balance the wide-spread perception that these plants serve no useful purpose or are ecologically harmful and should be eliminated from the landscape. This is not to say that we think these species should be planted in the landscape, only that we need to develop an appreciation for what they are contributing to the quality of urban life.

 

Peter Del Tredici
Lecturer in Landscape Architecture
Harvard Graduate School of Design
30 August 2005