Why is it warmer in the city than in the countryside? Because of all that concrete, asphalt, interlock, brick and stuff that accumulate the heat of the sun during the day and slowly release it at night. It is a “thermal mass” principle that is used in permaculture to heat homes passively (in winter) and create microclimates to lengthen the growing season or to cultivate plants that usually grow in warmer areas. But during a heat wave, this heat conservation is not necessarily desirable. So how do we reduce heat islands? How do you keep your house, your yard, your neighbourhood cool in the summer?
I speak from experience here. For the past seven years, we have been renting a house in the suburbs, where the backyard is covered with interlock and the west-facing front yard is packed with river stone, not to mention the interlocked driveway and the paved street. From noon to five on hot summer days, the rocks get extremely hot. You can’t walk on them. And they warm up the air all around. So much so that we feel like in an oven. The heat doesn’t just come from the sun, it comes from the ground, it comes from brick walls, it comes from everywhere at once. And even when the sun goes down, that heat stays until late at night.
Here are some solutions to reduce heat islands:
Reduce thermal masses
Try “unconcreting” your yard! More and more people are replacing their asphalt with green driveways. These are reinforced lawn surfaces (to prevent cars from getting stuck in the mud). In addition to absorbing less heat, these surfaces are permeable. This means that they absorb rainwater, which reduces the chances of floods and the load on city sewers. Of course, installing a green driveway can be expensive. This is something to think about when doing or redoing your landscaping.
Use light colours
It is well known that light colours reflect sun rays. Dark colours absorb them. Just try wearing a black sweater in the sun! Or sitting on a black seat: “Ouch!”
So, concrete or not, use light colours for your home and yard. Just think of the famous white villages in Andalusia in southern Spain. Even though houses are made of concrete, white really helps to keep them cool on scorching days (over 40 degrees Celsius).
Shade, shade and shade again
There are several ways to provide some shade:
Plant trees
A tree is one of the only “socially acceptable” structures in the front yard that covers enough ground to shade your whole driveway. Aside from casting shade, trees are a natural air conditioning system. They emit tiny water droplets through their leaves, which is called evapotranspiration and helps cool the air. Trees also create air circulation: the sun heats the leaves, the air in the canopy warms up and rises, attracting more air that will be cooled in the shade of the tree. That’s why it’s so much cooler when you walk under a tree in the summer!
Trees also offer a variety of benefits such as purifying the air by capturing carbon dioxide and emitting oxygen, improving water infiltration into the soil, providing habitat for local insects and animals, producing food and more. Depending on the size of tree you buy, it’s not that expensive. It can range from almost free for a seed or a cutting, to a hundred dollars for a bigger tree that will take less time to grow, but that could suffer from a bigger transplant shock.
Of course, when you plant a tree, you have to be patient. It takes years to grow enough to make a noteworthy shadow. You should also consider the following:
- Tree type: It is important to choose the type of tree to plant according to its size and properties. Deciduous trees (which lose their leaves in winter) are preferable to block the sun in the summer and let it in during winter. Coniferous trees (pine, fir, cedar, etc.) can be used to block the wind in all seasons. Fruit trees can be interesting for producing food in addition to shade, but be careful not to put an apple tree, black walnut tree or coconut tree over your driveway. It could bang up your car!
- Location: Remember that a tree can get big! Keep a good distance between trees when you plant them, taking into account their size at maturity. It will look funny at first, but you’ll thank yourself later. Also, keep trees far enough from your house to prevent branches and roots from damaging the roof and foundation. Finally, to shade your house or yard, place your trees on the sun side (which is the south in the northern hemisphere) or on the western side to protect it from the hotter late-day sun. Finally, if you have a garden, be careful not to hide all the sun from it, because most plants need a minimum hours of full sun.
Build a gazebo
A gazebo is a small hard- or soft-top shelter. It is a structure, permanent or not, which can be made of wood or metal, with or without walls and netting. In short, there is something for every taste and every budget. In general, it still costs more than a tree (a few hundred to a few thousand dollars), but you can be assemble it in a few hours or even a few days for a more elaborate structure. It’s like an outdoor living room, sheltered from the sun.
Set up a pergola with climbing vines
A pergola is a structure, usually made of wood, with horizontal beams that can be used to support vines. You can grow edible vines (nasturtium, grapes, kiwi, hops and even passion fruit) or ornamental vines (clematis, rose, hydrangea, Virginia creeper, ivy). Pergolas combine the benefits of trees (air conditioning, habitat, food, summer shade and winter sun) with those of gazebos (speed of installation, size). Of course, once the pergola is installed, it can take a few years for the vine to get established. It results in a small living room or tunnel with a green roof that let a soft sunlight seep in. Some disadvantages are that you have to pick up the leaves in the fall, and some fruits or flowers can attract wasps or other insects, which can be unpleasant when eating underneath.
Hang a shade sail
Shade cloth or shade sails are quite affordable depending on size and quality. They may be triangular or rectangular, and may let part of the sunlight and even the rain pass through if desired. They can be used above the patio, pool, garden and even the driveway. You just need to have solid anchor points to hang them.
Open an umbrella
The good old umbrella is a simple, affordable and flexible solution to relax in the shade outside. There are umbrellas that you can plant in the sand on the beach or in the middle of a table, or there are offset umbrellas with a post to the side if your table doesn’t have a hole. Of course, they do not cover a very large area and you have to remember to close them when it’s windy, but often their flexibility prevails.
What I did
Let’s go back to my rented home situation. My yard is too hot, but I don’t want to invest huge amounts of money in landscaping. What can I do?
In the front yard, I let two trees grow on the western side of the house (see my Live and Let Grow blog) to shade the driveway and cool the air before it gets to the house. I also planted and let grow ground covers to shade the rocks and prevent them from becoming sizzling stones. Cost: a few dollars for seeds. And in dry spells, the plants I let grow are often greener than the grass on the other side of the street.
In the backyard, I have made my vegetable garden in grow bags and I shelter it with a shade cloth if necessary to soften the sun rays, reduce evaporation and prevent the leaf burn. This setup reduces the surface of interlock exposed to the sun.
For the patio, we have set up an umbrella that allows us to eat outside even at noon and keeps the deck cool enough to walk on it. When we do move, we can take the garden and umbrella with us!
In short, with the addition of shade and vegetation, our yard has cooled down and will continue to do so as the trees grow. You can also help reduce heat islands. Let’s cool our planet, one tree at a time!
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