by David Hopman, ASLA, PLA
All cities in the United States have undesigned areas that develop what is called “spontaneous urban vegetation”—plants that establish themselves without human intervention or planning. These areas can be large, such as abandoned or vacant building lots, former farms and ranches, and river corridors. They can also be small opportunities for plants and plant communities in sidewalk cracks, between paving and buildings, or anywhere enough soil has accumulated to allow the sprouting of seeds, as was the case on New York’s High Line elevated railroad before it was so famously developed into the urban amenity it is today.
In well-developed cities, 5-10% of the total vegetation or more can be spontaneous. In Detroit, the amount of area abandoned to this undesigned vegetation is about 40% as the city has depopulated and thousands of homes have been removed.
Spontaneous urban vegetation has been widely touted by scientists and landscape architects for its environmental benefits that include but are not limited to:
- excess nutrient absorption in wetlands,
- heat reduction in paved areas,
- erosion control,
- soil and air pollution tolerance and remediation, and
- food and medicine for people.
However, there has been very little discussion, or appreciation, of the role that this vast amount of urban vegetation can have on native pollinators.
We know from the research of Annie White and other pollinator scientists that many pollinators are exquisitely sensitive to plant morphology (size, shape, and structure), and the color and the timing of blooms, even with variants of native plants, or “nativars.” It is curious that vast communities of exotic plants in undesigned urban areas are still celebrated as “novel ecosystems.” An analogy would be celebrating global warming and the climate crisis as a “novel weather system” or celebrating water and air pollution as “novel environmental chemistry.” Fortunately, there is a path forward that all landscape architects can use to improve urban ecology for our native pollinators.
The early successional plants that find their way into spontaneous urban plant communities are often referred to as “ruderal” species—ecologies that spontaneously inhabit disturbed environments. For a thorough discussion of ruderal species in various types of disturbance conditions, see Plant Strategies, Vegetation Processes, and Ecosystem Properties, by J. Phillip Grime. For our purposes, we will focus on all species that readily self-seed. Currently, most of these plants are exotic species that have escaped from farms, ranches, and from ornamental horticulture through many years of planting invasive exotic plants. At the same time, most of the plants that are currently available in the horticulture industry are both exotic and sterile. This leaves the field wide open for the exotic invasive ruderals.
We can begin to change the composition of spontaneous urban vegetation by including readily self-seeding native plants in our urban plant palettes. The seeds from these plants will propagate through the air, in water, and on animals to inhabit the niches formerly occupied only by exotic invasive species without the biological connections with the local pollinators that have evolved over long periods of time. I refer to this as “guerilla gardening” and have seen its effects in areas where I have implemented polycultures of plants that include ruderal species (see photo below).
Let’s commit this Pollinator Week to adding self-seeding native ruderal species to our planting designs—species that can be a critical asset for the survival of local pollinators and local native ecosystems in spontaneous urban communities. Then we can truly appreciate spontaneous vegetation for its benefits to BOTH humans and to our fellow travelers on planet earth!
A longer post on spontaneous urban vegetation by David Hopman will appear on The Field soon as a future Planting Design PPN post, so stay tuned for more!
See The Dirt for more on Park(ing) Day—the theme is pollinators for this September—and ASLA social media for more Pollinator Week coverage.
David Hopman, ASLA, PLA, is an Associate Professor of Landscape Architecture at The University of Texas at Arlington and an Officer and Past Chair of ASLA’s Planting Design Professional Practice Network (PPN).