Monday, February 28, 2011

Wild Urban Plants

Ever since hearing Peter DelTredici speak about Wild Urban Plants and his ideas on urban ecology last November at the 13th Annual Trees in the Urban Landscape Symposium at Tower Hill Botanic Garden, I have been wrestling with how that fits in with my approach to the use of native plants in urban and suburban environments. This talk explored the plants that are flourishing in the wild, unmanaged spaces in our cities, where they came from and why they are there. While 25-35% of these species are native to North America, many more are introduced species from other parts of the world. The difficulty for me was the apparent acceptance of many invasive and potentially invasive species as the new order of things. To help sort things out and not jump to conclusions, I read over a copy of his book, Wild Urban Plants of the Northeast.

I’m of two minds about this book. On one hand it is an excellent field guide for plants growing without cultivation in an urban setting. It is organized in the same way as Uva, Neal and DiTomaso’s Weeds of the Northeast, but with additional historical and cultural notes on how and why the plant has gotten itself established in the urban environment. These notes were very interesting to me in gaining a better understanding of why many of these introduced plants are so common. DelTredici includes over 200 species that are found growing in cities in the Northeast, from Detroit to Boston and Montreal to Washington, D.C. In the introductory sections this book points out the difficulty of doing strict botanical restoration on an area where the original conditions no longer exist and restoration of those conditions is unlikely, if not impossible. The plants described in this book are examples of ones that have proven to be adaptable to the built environment that is the modern city.

Where I began to have real differences with this book was its description of the ‘Brave New Ecology.’ Here it states that the cosmopolitan collection of plants (natives and introduced species) is the default vegetation for cities in the Northeast. While most would agree that having some vegetation is better than nothing (with some exceptions), I believe that we could be operating at a higher level of ecological function if there were more native species filling the urban niches, than introduced species. The major ecological function that natives support better than most introduced species is supplying nutrition to insect larvae, one of the most basic levels on the food web. I would refer you to Douglas Tallamy’s Bringing Nature Home for a fuller discussion of the role of native plants in the food web.

Also in this book DelTredici tends to treat the urban ecosystem as isolated from more rural, less disturbed ecosystems. If it were the case that the cosmopolitan plants were ill-adapted to life outside the city and did not spread beyond highly developed areas, I would have fewer issues with these conclusions. But the seed from many of the introduced species do not respect any borders, being spread by wind (black swallowwort) or birds (porcelainberry, buckthorns & barberries) into the wider environment.

Many of the introduced species in the ‘cosmopolitan collection’ got their foothold 200-300 years ago as new settlements and, later, cities were being built. These plants were imported along with the new settlers and were preselected for these new urban habitats being built. This made me think, what would happen in an urban site without the addition of exotic plants? If instead of the European mixed seed banks that were brought into the developing urban areas of the North American continent, there were only indigenous plants present to fill the urban niches. We may then see a greater array of regionally native plants in the urban environment. One reason this has not happened is that the native seed banks were not as large or as close to the urban environment as those of the cosmopolitan plants.

For nearly any habitat on earth there seems to be a plant or two that are able to grow there, as this book documents. In the case of the urban environment the ‘problem’ is that many introduced species were given first shot at these newly created lands, while otherwise appropriate native species were nowhere nearby. It would be interesting to see how some natives fared in the city when better matched to the actual growing conditions. Perhaps some more native species that grow on calcareous outcrops or cliffs would be found growing in sidewalk cracks. Looking at the plant community for calcareous outcrops in Massachusetts (found in west and central Massachusetts) there are a number of desirable plants, including red columbine, purple clematis and downey arrowwood.

A roadside planting of Seaside Goldenrod
and Appalachian Blazing Star
with some indigenous Milkweed
What I would like to see is a resource that looks for similarities between habitats in and around urban areas (the built environment) and natural areas that have similar chemical and physical attributes. In addition, there are many plant species that require disturbance to survive, in fact, many of these are on the threatened and endangered lists dues to a decline in ‘natural’ disturbance like fire and agriculture in the Northeast. Some of these may be candidates for native urban plants.

One example of a native plant that is expanding its range, included in his presentation but not cited in the book, is Seaside Goldenrod, Solidago sempervirens. This seaside species has been spreading inland along highways that have similar cultural (growing) conditions as in its native habitat, that is, sandy, salty soils with lots of wind and other disturbance.

Looking around the foundation and at cracks in the pavement around my house I have several native species flourishing. These include the understory tree, Wafer Ash (Ptelea trifoliata) and herbaceous species, like pink tickseed (Coreopsis rosea), swamp verbena (Verbena hastata), a variety of asters and the ever-spreading bearded-beggarticks (Bidens aristosa).

These have been speading in cracks in my driveway
for over 5 years.

Introduced plants are not evil, they are just growing and reproducing as they were made to do. But, as the dominant sentient species, we have choices of what to use in our landscapes. Selecting plants that appropriate to the location and support the local ecology is more responsible than just picking plants for use as ornamentation. With a little work, and trial-and-error, we can find more native species to enrich our urban and suburban environments.

If you would like to hear more about native plants and urban ecology check you the Ecological Landscaping Association conference this week (March 3rd) in Springfield, MA. There will be a panel discussion titled "Native, Introduced, Invasive and Endangered Plants in the Landscape:Untangling the Roots of the Problem", the panel features three experts including Peter Del Tredici, Bill Cullina, a native plant expert and author and Jono Neiger a leading figure in permaculture.


Curbstone Valley Farm said...

Excellent post Curtis, and you raise some very important points. In my opinion it's critical even for urban gardeners to exercise some degree of gardening responsibility when selecting plants. It's easy to fall into the trap of planting something just because it's pretty. On the west coast, that's how we've ended up with invasive broom species inundating our woodlands, Vinca major displacing native woodland species, Cotoneasters running amok, and the like. All of these species were commonly planted in urban gardens, and it's alarming to see where they've ended up. Our property is only two miles outside of the city limit, and there's now a continuous band of French broom between the urban and rural landscapes here. I wholeheartedly agree it's a dangerous presumption to think that urban gardens are isolated from their rural surroundings.

Laurrie said...

Very interesting reaction and commentary on this book and the issue of what to accept as the "new order of things" with introduced plants. It's complicated, and this was a thought provoking post.