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By: Lindsay Hollingsworth

We’ve all seen them. The mosses creeping up foundations, the tiny leaves poking out from in between bricks, the young trees swaying merrily from their perches within gutters. And if you’re like me, you’ve probably taken secret joy in these tiny spots of green scattered throughout the city, even as those around you talk of untidiness and power washing.

While pursuing my master’s degree in Ireland, I had the opportunity to study these plants, hidden in plain sight in centuries-old stone walls. As I pursued my research, I learned that the scientific community has often overlooked wall plants as a point of potential ecological interest. Even as our understanding of cities as unique ecosystems has grown, wall plants and other urban botanicals remain a relatively underexplored topic. But there’s been just enough research done to give us tantalizing hints at a secret world of plants unfolding, like the frond of a fern, right under our noses – in the cracks and margins of our megastructures, and in the places that have become too ordinary to notice.

Wall rue (Asplenium ruta-muraria) growing on an ancient stone wall (Photo by Lindsay Hollingsworth)

Some research has suggested that, like forests, wall vegetation has successional stages. As city dirt accumulates and concrete and mortar weathered, walls are first colonized by lichens, then by mosses, and then by vascular plants. And as these tiny microhabitats form, wall plants provide shelter for insects and small animals, who in turn may carry seeds to new crannies and crevices. And in some cases, the unique environmental conditions of walls may provide a sanctuary for important local plant species, especially those that might normally make their homes in cliffs and rocky terrain.

It is unfortunate that we know so little about these plants, especially when we could learn so much from them. With so many of us living in cities, and more predicted to migrate to them in the near future, the question of how to make our cities more verdant and sustainable becomes increasingly crucial. 

And this question is perhaps especially pertinent to sprawling cities like Washington, D.C., where urban development covers so much of the surrounding landscape, and where there is an increasing push to incorporate green walls and roofs into our city infrastructure. Understanding the circumstances which allow plants to grow on walls without human aid may help us to more efficiently cultivate vegetation on our buildings. Currently, one of the biggest drawbacks to green walls is the expense and labor required to maintain them. However, better knowledge of what plants are naturally suited to wall colonization in a particular climate, and under what conditions they will do so, could help us better select plants that require little intervention to thrive. 

Green walls on an urban apartment building (WikiMedia Commons)

Moreover, an understanding of wall plants as not just a nuisance or a curiosity, but as an important part of urban ecosystems, may allow us to see and develop our green walls and roofs to support plants and animals beyond our cities. Green walls and roofs have already been deservedly celebrated for their ability to reduce air pollution, but the way in which animals seem to use naturally occurring wall plants for shelter raises some intriguing possibilities. Perhaps if we can explore the potential of green walls and roofs as refuges and habitat corridors, we could create fundamental changes to the types of animals that can use urban spaces. Perhaps instead of being obstacles for migrating songbirds and butterflies, they could be waystations. 

 The next time you see a cranesbill or hart’s tongue poking out from in between the stones of a garden wall or the bricks of the building, I encourage you to stop and take notice. Admire its fortitude, to grow in a place where so few can. Try and see what circumstances, what characteristics of this particular wall have made its small life possible. And remember that even in a city with no forests or fields, we live, always, side-by-side with nature.

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Lindsay Hollingsworth holds a master’s degree in Biodiversity and Conservation from Trinity College, Dublin, where she researched novel ecosystems, agroecology, and wall flora. She completed her undergraduate studies at St. Mary’s College of Maryland. She currently works at an environmental consulting company, and volunteers with the local Potomac Conservancy.

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