Urban landscapes are an intrinsic part of human and planetary health and well-being. The ability plants have to capture and convert light into energy is fundamental to the food and fuel on which the earth depends. Additionally, plants within our landscapes have the potential to improve air quality, reduce urban heat islands, purify wastewater, limit runoff, and reduce stress. As communities and individuals, incorporating green infrastructure into our urban landscapes will make our cities more livable, resilient, and healthful.
Clean Air and Cooling Summer Heat
As our cities grow, plants offer a natural solution for mitigating air pollution and cooling cities. Trees and other greenery may be eight times more effective than previously thought in their ability to reduce air pollutants, respirable particulate matter 10 micrometers or less in diameter (PM10), and other toxins known to impact human health. One study demonstrated that a street row of birch trees removed greater than 50 percent of PM10 in adjacent indoor environments.1 Another study concluded that judicious placement of grass, climbing ivy, and other plants in urban canyons can reduce the concentration at street level of NO2 by as much as 40 percent and PM by 60 percent.2 So how do we bring this to our home landscape?
Hackberries, lindens, elms, oaks, birches, and maples are some of the best trees to remove air pollutants. When planted on the south or west side of buildings, their shade helps to mitigate summer heat and lower cooling costs. Well placed parks and trees have been shown to have a cooling effect that reaches well beyond their boundaries. On the other hand, trees may trap air pollutants under their canopy where air flow is limited by glass and concrete canyons. Here, green roofs and walls may help cool cities and improve air quality. Ivy and Virginia creeper have been covering walls for centuries. Their leaves are excellent at removing PM10 and shade buildings to reduce heat absorption. Where vines attaching directly to walls are unadvisable, hops, sweet autumn clematis, and other twining vines can clad buildings with simple green support systems. In addition to mitigating air pollution and cooling ambient air temperatures, trees, green roofs, and gardens take up rainwater to protect lakes and waterways from excessive runoff.
As cities and populations expand, water quality and water scarcity become more problematic. To address these challenges, buildings and landscapes are being designed to conserve clean water, recycle grey water, treat wastewater, and reduce runoff. In the Port of Portland building, a Living Machine installation utilizes striking inside plantings to provide advanced wastewater treatment and reduce water use by 75 percent. While exciting new technology using plants and solar energy to turn human feces into potable water may soon be available, time-tested methods of harvesting and storing are being revisited as a source for potable water.
For centuries, homes were built with cisterns to collect rainwater. In Madison, many were functional into the 1960s. Now, in areas where groundwater pollution and unsafe wells are a problem, large cement cisterns are being built to provide potable water. Plastic tanks and expandable water bladders are also being integrated into decks and beneath patios to collect rainwater for irrigation and grey water. Similarly, catch basins are being designed under patios and driveways to reduce runoff and recharge groundwater. Rain gardens, trees, green roofs, and water harvesting structures all contribute to protecting our fresh water.
While large-scale hydroponic vertical growing systems and rooftop farms are just beginning to catch on in the United States, they hold the promise of a solution to the problem of a growing population and the consequent need to increase food production. Moving food production closer to city centers makes healthier foods more available and reduces transportation costs. Community and educational gardens are sprouting up all over and inspiring children and adults to grow, prepare, and eat fresh vegetables. Changing brownfields into community gardens is providing healthy food for low-income families and transforming neighborhoods.
The possibility that urban greenery and green infrastructure have positive effects (direct and indirect) on human health has been confirmed by more than three decades of psychology and ecology research exploring the relationship between nature contact and human well-being.3 To keep our urban cities healthy we must design livable landscapes that meet human and planetary needs for a healthy environment. In addition to providing access and space for outdoor activity, its imperative that green infrastructure be integrated into urban landscapes to ensure clean water, fresh air, and food security for all.
1 Maher, B., Ahmed, I., Davison, B., Karloukovski, V., & Clark, R. (2013). Impact of roadside tree lines on indoor concentrations of traffic-derived particulate matter. Environmental Science & Technology, 47(23), pp. 137373-13744.
2 Pugh, T., Mackenzie, R., Whyatt, J., & Hewitt, N. (2012). Effectiveness of green infrastructure for improvement of air quality in urban street canyons. Environmental Science & Technology, 46(14), pp. 7692-7699.
3 Santamouris, M., Ban-Weiss, G., Osmond, P., Paolini, R., Synnefa, A., Cartalis, C., Muscio, A., Zinzi, M., Morakinyo, T., Ng, E., Tan, Z., Takebayashi, H., Sailor, D., Crank, P., Taha, H., Pisello, A., Rossi, F., Zhang, J., & Kolokotsa, D. (2018). Progress in urban greenery mitigation science assessment methodologies advanced technologies and impact on cities. Journal of Civil Engineering and Management, 24(8), pp. 638-671.
Joan W. Ziegler is a horticulturist and garden designer and winner of the 2015 Perennial Plant Association Merit Award for Residential Landscape at ZDA, Inc. Landscape Architecture, 4797 Capitol View Road, Middleton. Call (608) 831-5098 or visit zdainc.com .