• Brenna Pavan

Beating the Heat: How Plants Help Us Keep Our Cool

In recent years summers have been getting warmer, and though we had a wet, cool spring this year on Vancouver Island, the summer months have brought us many weeks of weather reports containing heat warnings. Last year especially, with the heat waves we experienced, many of us realized that the conditions where we live are starting to shift. With hotter temperatures becoming the norm it is important to recognize how our own lives may need to shift in response. That may seem daunting, but there are many ways to battle the heat that provide large-scale, long-term solutions, and these solutions often involve plants and how we utilize our urban green spaces.

In dense urban areas pavement, buildings, and other man-made surfaces absorb and retain heat. These areas then trap heat much more effectively than either a natural ecosystem or rural area can. “Urban heat islands”, as these are called, lead to hotter average daytime temperatures and reduced cooling at night. In turn, this hotter average temperature in the city leads to more use of air conditioning in homes, or sprinklers in lawns to beat the heat. However, there are better ways to cool our homes and our cities that allow us to save money on hydro and water bills. The trick is that we want as many plants as possible covering our urban spaces.

Natural ecosystems are often significantly cooler than urban or even agricultural areas, and the greater the biodiversity, the greater the cooling effect. Larger trees are especially beneficial in urban areas as they provide shade. In cities, trees can help drop the outside temperatures around them by up to 10 degrees. If you have ever walked through an exposed, sunny area in the summer and then walked into the forest, you will be familiar with just how drastic the temperature change between those two areas can be. Plants offer more than just shade however, and while trees are a great start, the more biodiversity we can introduce to a space, the better the cooling effect can be. Plants cool the area around them through transpiration, a process where water is released through the above-ground parts of the plants. This released water then evaporates and causes the air immediately around the plant to cool. Transpiration, combined with the shade provided by larger plants, can aid people in creating cooler temperatures in our urban spaces. Even simply having a tree that shades your home can dramatically impact the temperature of your home. That being the case, imagine what a whole native ecosystem working together to cool the surrounding area could do.

While all plants transpire, and those that are tall enough can provide shade, native plants in particular are the best choice when we are deciding what species to utilize for cooling. They are better adapted to the local conditions, even when those local conditions include more heat waves than usual, and require far fewer inputs of fertilizers and particularly water in the summer months. Utilizing native species more in our gardens and other green spaces, especially a diverse mix of native species including trees, shrubs, and smaller plants, helps to simulate those cooler conditions we often experience when walking through natural areas on a hot day. Trees often provide the initial cooling effect through shading and the remainder of the species can transpire and further cool the area. Even areas filled with native grasses or meadow species with minimal shade cover, offer more cooling effects than large areas of pavement. Including healthy native plants as much as we can in our urban spaces is a key solution for cooling our urban areas.

For the best results planning ahead when planting is required. Plants should ideally be put in the ground in the fall, to give them time to adjust and establish in their new area. If fall planting cannot be done, early spring can also give plants enough time to adjust before the tougher summer conditions. The summer months often are tougher on plants as it is so hot and dry, so allowing them time in the wetter months to establish and grow gives them the best chance for survival. Planting in summer can be done, however a new transplant in the summer is less likely to survive and will require more attention and inputs than a more established fall transplant would.

When thinking of the best way to help cool your space, remember to include not just trees but other native plants like shrubs, bunch grasses, and of course flowers. Creating diversity and structure in our green spaces will create both aesthetically pleasing and functional areas in our cities. If you are interested in how we can create better green spaces geared towards cooling, please contact us with any thoughts or questions! Together we can work with nature to utilize our urban green spaces.

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