Heat Islands

Heat islands: tropical paradise or slow death for trees?

We’ve all seen it, but most of us don’t understand what it means. Naked upper branches of trees, singed leaves, neighborhood shrubs struggling to survive, our own garden and landscape plants growing less well than expected are all signs of heat island effect.

Sweet gum tree showing signs of heat and drought stress

What are heat islands?

The term ‘heat islands’ refers to land that used to be soil, bushes, and trees, and is now concrete, asphalt, and buildings. Before all that development, natural cycles and the permeability of the soil kept things cooler and healthier for plants. All the impervious, manmade materials that now cover up 3% of the earth’s landmass absorb heat, creating a zone of higher temperatures called heat islands.

Do I have heat islands?

Unless you live in a tent in the wilderness, yes, you have heat islands. Driveways, patios, extended foundations, concrete paths and walkways, roofs, sidewalks, and the street in front of your home can all create their own heat islands.

How do heat islands impact landscape plants?

Plants installed in or near heat islands must contend with soil temperatures that can be 50 to 90° hotter than the air. This means, on a scorching summer day, when the air is 95°F, your plants’ roots may be struggling to survive temperatures as high as 185°F! All this strain takes its toll. Most trees reach what is called a ’thermal death threshold’ at 115°F. Held too long at temperatures above this point and even the healthiest tree can die. Even if they survive, plants growing in heat islands have significantly shorter lifespans and are more susceptible to pests and disease. Also, things don’t cool down for these plants as the sun sets. While we may enjoy the lower temperatures of evening, concrete and asphalt hang on to all that absorbed heat, slowly releasing it into the immediate area, making evenings the hottest part of your plants’ day.

Impact of heat islands on water use

Plants growing in heat islands need as much as 50% more water than the same plants grown elsewhere. Since a mature tree can use up to 250 gallons of water a day during the peak of summer, that percentage can translate into a lot of water! Also, if you re growing edibles, such as cucumbers, peppers, lettuce, broccoli, or cauliflower, in a heat island, the stress can cause these foods to turn bitter, or bolt.

Green roof of Chicago City Hall (TonyTheTiger)

How can you help plants in heat islands?

City managers are working toward reducing the heat island effect and you can. too! Use these tips to help your plants survive the heat island effect:

1. Proper irrigation is the number one way to help these plants thrive (or at least survive). Frequent, shallow watering does more harm than good. It encourages a shallow root system that will dry out more quickly. Infrequent, deep watering will encourage a deeper root system that helps plants protect themselves.
2. Avoid feeding these vulnerable plants during the peak of summer. Adding fertilizer stimulates new growth, which uses a lot of water and puts demands on the plant’s overall system. Would you want to move everything you own into the garage on a 150°F day? Yep. You’d probably fall apart, too.
3. If you will be adding plants to a heat island, do it at a time of year when they will be able to develop an extensive root system before high summer temperatures hit. Of course, then you have the issue of potential frost. Also, be sure to select plants that can tolerate a lot of heat. This would be succulents, such as echeveria, sedums, or aeoniums.
4. Improving soil structure and condition can also help plants growing in heat islands each the water they need. Our heavy clay can turn into concrete on a hot day. Incorporating aged compost, worm castings, or aged chicken, horse, or steer manure can increase the number of macropores and micropores in the soil, giving roots a path to cooler temperatures down below.
5. Mulching heat islands with a thick layer of wood chips is a good way to reduce temperatures by shading the soil. It also slows water loss through evaporation and makes it harder for weeds to get established.

Since plants that are growing in heat islands are already under a lot of stress, be sure to monitor for pests and diseases regularly, so that you can apply treatments right away, before things get out of hand.

Reducing heat islands on your property

There are many steps you can take to reduce the number of heat islands on your property. Not only will this help your plants to be healthier, it also reduces energy bills (if you use A/C), water consumption, and emissions from energy production. Here are just a few of the ways you can reduce the number of heat islands on your property:

• Trees and plants - trees create shade that can reduce temperatures by 20 to 45°F, and all plants reduce temperatures by 2 to 9°F through evapotranspiration​
• Permeable materials - driveways, paths, and patios can now be made with pervious asphalt, concrete, or interlocking pavers that allow air and water to pass through, recharging groundwater supplies and reducing heat absorption, releasing heat back into the atmosphere more quickly
• Cool or reflective roofing materials - designed to reflect light and heat away from your home
• Green roof - rooftop gardens and green roofs use plants to create shade and lower the surrounding air temperatures; they also provide extra insulation
• Water features - ponds and fountains can offset some of the heat island effect

Traditional asphalt (left) creates runoff; porous asphalt (right) allows soil to breath and absorb rain (EPA)

Help your plants and the environment by eliminating heat islands around your home.