This article addresses and elaborates the other side of the usual argument against “invasive plants” — which, rather like “illegal aliens” — tend to move about, depending on conditions.
Did you realize that what determines the seriousness of ecosystem destruction is not the number of people living on the land but the amount of land covered over by impermeable surfaces? This and other facts startle. I guarantee that this article will teach you how to see especially degraded urban landscapes in a brand new way, and will help you learn to ask new questions, for example: if this plant is present, what does it mean about the soil? Plants are indicators, not pests.
BTW: for every “invasive” plant mentioned, I googled to discover whether it was medicinal and/or edible. And yep, every single one of the extremely hardy, adaptable, low maintenance plants (“weeds”) focused on in this article can be harvested for human use.
Celebrating the Botanical Diversity of Cities
April 17, 2014
by Peter Del Tredici
designobserver.com, via ranprieur, via Rose
“. . . ruderal or abandoned landscapes . . . consist of post-industrial or post-residential vacant land, and infrastructure edges dominated by spontaneous vegetation, either native or introduced, on relatively poor and often compacted soils. They have extremely low maintenance requirements — so low in fact that they can be considered self-sustaining.
One important research question concerning ruderal landscapes is how much land in any given city does spontaneous vegetation occupy? With the help of my Harvard Graduate School of Design students using GIS technology, we calculated that roughly 9.5 percent of the surface area of Somerville, Massachusetts, (one of the most densely populated cities in the state) is dominated by spontaneous vegetation. This is land that no one maintains, and it exceeds the land area occupied by maintained parks.
The plants that grow spontaneously in urban areas — whether native or non-native — are performing important ecological functions. Ecologists refer to these functions as environmental services and they include excess nutrient absorption in wetlands, heat reduction in paved areas, erosion control, soil and air pollution tolerance and remediation, food and habitat for wildlife, and food and medicine for people (even if we don’t use it). 
Pavement cracks are among the most distinctive niches in the urban environment. Wherever you have two types of paving material coming together, you have a seam, and the different materials expand differentially in response to summer and winter temperature to create a crack. We tend to think of pavement cracks as stressful habitats, but in fact, as the water sheets off the pavement, it flows right into the crack, making it a rich site in terms of its ability to accumulate moisture and nutrients. With oil from cars as a carbohydrate source available for decomposition by fungi and bacteria, cracks can develop significant microbial diversity.
. . . most people refer to spontaneous vegetation as “weeds” — a term with no biological meaning. “Weed” is simply a word used to describe a plant that a person does not like or does not want in the yard. It is a value judgment that reflects personal preferences. Remarkably, there seems to be no Latin word for an unwanted plant, and in many languages “bad plant” (e.g., mala hierba in Spanish) is the only available term.