Land Matters
In a preview of February's Land Matters column, Bill Thompson, FASLA, makes a case for using hearty, nonnative species in city landscapes.

Chris Starbuck, University of Missouri

This week a beautiful tree was cut down across the alley from my house in the historic core of Washington, D.C. After gracing the back patio of my neighbor's town home for many years, it was nearing the end of its life span, and drooping limbs were threatening nearby electric lines. But I'm really going to miss its spreading form, its shade, and its bright display of flowers in late summer.

The tree was an Ailanthus altissima.

Peter Del Tredici, a plant scientist at Harvard's Arnold Arboretum, broaches the idea that tough, adaptable exotics such as Ailanthus may be the most appropriate species to plant on disturbed sites in cities.

Up to now, I wouldn't have gone public with my fondness for Ailanthus. After all, this species has long headed the list of invasive "weed" trees excoriated by native-plant enthusiasts and protectors of native forests in the eastern United States. And in truth, Ailanthus is supremely opportunistic: Seedlings regularly sprout around parking lots and other leftover spaces in the city where few other plants will grow.

Now, however, Peter Del Tredici, a plant scientist at Harvard's Arnold Arboretum, has given me the courage to speak out. In "Brave New Ecology" (page 46) Del Tredici broaches the idea that tough, adaptable exotics such as Ailanthus may be the most appropriate species to plant on disturbed sites in cities.

One argument against the use of exotics is that their seedlings can invade natural areas. But remember, we're not talking about the far-out suburbs, where invasives could escape into some wildlife refuge or stream valley. We're talking about inner cities, characterized by impervious surfaces, reflected heat, air pollution, and soil compaction. In downtown D.C., for example, the chances of an exotic "weed" seed being wind-borne into some pristine forest are just about nil.

Where exotic invasions of pristine areas are likely, I agree that natives are the best choice. I disagree with native-plant enthusiasts, however, that natives are de rigueur for any and all situations. Several years ago I facilitated a design charrette for a new indoor conservatory at Longwood Gardens in Pennsylvania. One of the participants was a landscape architect well known for his designs using native plants, so perhaps I shouldn't have been shocked when his proposed plantings for the new conservatory were a limited palette of natives from a specific watershed in the South. I found myself wondering: Shouldn't a conservatory design employ a cornucopia of plants from around the world to most dramatic effect? Proposing natives as the only solution for Longwood smacked of narrow, formulaic thinking—if not outright dogmatism.

The yearning to bring back native flora to cities seems founded, in large part, on nostalgia. Isn't that an awfully weak basis for planting urban areas? Instead of asking what plants grew in our cities in the past, Del Tredici asks if it wouldn't be better—particularly in a time of looming climate change-to find out what plants will best grow there in the future.

Del Tredici thinks that the standard of deciding what to plant should be sustainability. He defines sustainable landscape plants as tolerant of conditions that prevail on a given site; requiring minimal applications of pesticides, herbicides, and fertilizers to flourish; and displaying minimal tendencies to spread aggressively into surrounding natural areas. This last criterion may rule out Ailanthus in many places—but not, I argue, in the inner neighborhoods of Washington, Philadelphia, or New York City. In those places—and yes, in my back alley—it's still the Tree of Heaven.