If you live in town, you probably notice that it seems a lot hotter in the center of the city than it does in more rural areas. You may have even noticed the differences in your own garden. Temperatures in cities tend to be higher due to something called the urban heat island effect. Besides making life uncomfortable for humans and pets, it also can affect your plants.
What is the urban heat island?
The urban heat island effect is due to the sheer amount of paved surfaces present in our cities. The paved areas, consisting of buildings, roadways, sidewalks and other similar surfaces tend to absorb and reflect larger amounts of heat energy than natural landscapes (even in the desert). According to the EPA, this effect results in average daytime temperatures 1-7 degrees F higher, and nighttime temperatures 2-5 degrees F higher in urban versus natural areas.
Vegetation and bodies of water tend to cool the air above them, so the less of those there are in a city, the hotter the air above it will be. This is due chiefly to the fact that vegetation provides shade, and also transpires water from its leaves, cooling the air around it. Water bodies evaporate water, which also cools the air. Concrete and pavement simply absorb and reflect the heat coming in from the sun. In addition, waste heat from cars, air conditioners, and other machinery heats up the air more. Large structures (such as tall buildings) can also prevent air circulation and wind, which results in even more heating.
Heating is particularly extreme over large artificial surfaces such as roads and roofs. According to the EPA, a conventional roof can be up to 60 degrees F hotter than the surrounding air on a 90 F day. That means your roof material can reach 150 F. As you can imagine, those temperatures put a lot of stress on your air conditioning systems and your house. They also affect the plants surrounding your house.
How does the urban heat island affect your garden?
Having more heat in the garden means your plants will be more stressed. They will use more water and potentially grow less vigorously due to the higher temperatures. This is partly due to the effects of high temperatures on photosynthesis. Above around 104 F, the enzymes that make photosynthesis possible lose their shape, which means they stop working.
Your plants aren’t the only ones to suffer. If your yard is a hot, paved frying pan, you will not want to use it. Humans are particularly sensitive to the effects of heat, with unpleasant symptoms starting with just a few minutes’ exposure. For more vulnerable groups, such as older people, those with chronic illness, and children, it takes very little heat exposure to cause significant health effects. In addition, higher temperatures result in higher ozone levels in the lower atmosphere. While ozone is important in the high atmosphere, its presence at ground level causes lung and throat irritation and can worsen asthma and other respiratory diseases, and cause lung scarring with repeated exposure.
One potential plus is that you may be able to grow some tropical plants that technically shouldn’t be able to survive here. Frost sensitive citrus varieties, jacaranda trees, and other similar plants may do well in a warm location in your yard, thanks to the few degrees of extra heat provided by the urban heat island. You can measure the maximum and minimum temperatures in your yard with a simple, inexpensive digital thermometer to see what temperature ranges different areas of your yard experience.
Reducing the impacts of the urban heat island in your garden
The best thing to do to help mitigate the effects of the urban heat island is to plant trees. Several trees near each other will have a cumulative effect, so the more the better. Maybe you can encourage your neighbors to plant a couple? Or perhaps you can get some trees in green strips or roundabouts in your neighborhood’s paved areas? The trees’ canopy will cool things off by providing shade, and, as already mentioned, the transpiration of water out of the leaves will help cool the air above the trees as well. Studies by the Department of Energy show that temperatures under trees are as much as 25 F cooler, while the surrounding air around the tree is 6 F cooler on average. Trees in your yard will also create cool microclimates for your other plants, while also protecting them from frost and wind.
Other plants will also help cool your yard. Anywhere you can have plants instead of hardscaping will help cool things off. Green walls, vines and narrow plants can help shade vertical structures. Keep in mind that plants in these areas will need extra water to survive the higher temperatures and sunlight, and choose your plants carefully. Whatever plants you choose for your yard, make sure they’re heat adapted, drought-adapted, and that you have regular, efficient watering (such as a drip irrigation system) installed.
Shading with structures will also help. For example, you may have areas of the yard where a tree or other plants aren’t a possibility. Consider shade structures like arbors, trellises, and (for larger spaces) pergolas to increase the amount of shade in your yard. Shaded surfaces are between 20 and 45 F cooler than unshaded ones.
Water features also help cool off the surrounding air, but in our climate there is an obvious tradeoff between that and water use. Having small ponds with vegetation that covers the surface will help reduce evaporation while still providing a cooling effect and aesthetic appeal. Refilling these features with rainwater when possible will help conserve water; rainwater is also best for plants, animals and ponds due to its lack of harmful salts.
You will likely need to have some hardscaping in your yard, but you can reduce its heating effect by using permeable hardscaping like gravel or brick and pavers that are set in sand. With impermeable surfaces, water runs off quickly and the pavement soon returns to its previous temperatures, which can reach 120 to 150 F. Permeable hardscaping allows water to soak in, which wets the hardscape area and the soil below it and cools it off via slower evaporation.
Because a large portion of the built area in your yard is your house, the type of roof you have can have a tremendous effect on the surrounding temperature (and your energy bills). In Tucson, green roofs with planted vegetation are not very practical, due in part to the extremely high temperatures mentioned above as well as native plants’ need for relatively thick soil layers. However, having bright, reflective roof surfaces will help reduce the heat absorbed by your roof and house, and can lower your energy bills by as much as 70%. Most flat roofs in Tucson already have white reflective coatings, but you can make this work with sloped roofs as well. For example, if you have a shingle roof, it might be worth converting to light-colored asphalt shingles with reflective granules. For tiled roofs, you can explore glazed tiles with more reflective surfaces and specially engineered pigments.
Enacting some of these suggestions will not only make your yard more usable and comfortable, but it will make for happier plants and lower energy bills. Your yard will also provide a cool oasis for urban wildlife, an important service as we lose habitat for various small animals that form the crucial backbone of our ecosystems.
For more details on how to mitigate the effects of the urban heat island, take a look at the EPA’s extensive handbook, the Heat Island Compendium.
