by Maureen Jett
Heat Island Effect is a man-made phenomenon in which air temperatures and surface temperatures are higher in urban areas than in the surrounding rural settings.
A team from the Master Gardener's Climate Change and Soils committee recently recorded temperatures of different surfaces in one neighborhood in Napa (see Surface Temperature Chart). Readings were taken on two different days, March 4th and June 21st. On June 21st, two recordings were done, one at 10:30am and the other at 3:30 pm. The temperature reached 104° that day.
Soil organic matter is what keeps soil alive and healthy, in other words, the living world below. A teaspoon of healthy soil contains millions of living organisms. This graphic from the USDA displays the threshold temperatures that expose your soil to potential “death”, in other words turning your soil into plain dirt. Read the chart from the bottom up to see what happens to Soil Organic Matter (SOM) at different temperatures.
140° F — Soil bacteria die. (We're talking about good bacteria).
130° F — 100% moisture is lost through evaporation or transpiration.
113° F — Some bacteria species start dying.
85% — moisture lost through evaporation.
90° F — Plant growth slows.
70° F — 100% moisture is used for plant growth. The ideal range plant growth and planting is (65 to 86 F).
Plant more trees that will protect against high winds, erosion and flooding, as well as keeping carbon in the ground and keeping groundwater clean. The big plus is the shade provided for our homes thereby reducing energy consumption.
Cover the ground with living plants to absorb CO2 rather than reflect the sunlight. Given, turf (grass) is green and it is cool, and in many circumstances needed for pets and children, but an expanse of green lawn requires heavy watering for maintenance and it and contributes little to soil health because the roots are shallow. On the other hand, plants and shrubs provide abundant roots, creating healthy soil and sequestering carbon in the earth.
Where plants are not growing, try to cover the ground with organic mulch. In the temperature readings, notice that the soil under the mulch is significantly cooler than the mulch itself. But be aware that all mulch is not created equal. Some dyed mulches may leach toxins into the soil and dark/black mulch heats up and can be detrimental to tender plants. Rock may seem like a good choice and in some cases may be appropriate, perhaps around cactus, but large areas covered with rocks/stones in the sun will absorb and retain heat, later releasing it back into the atmosphere. Keep rocks/stones in shady areas under plants, trees or next to the house.
Use permeable hardscapes (including permeable pavers) instead of concrete and other solid surfaces. Rain is absorbed into the soil rather than running off into the gutters and streets and will not retain the heat like non-porous and darker surfaces.
Resist the urge to put down artificial turf. Not only is it hot to the touch, but it kills anything living in the soil underneath of it. There is truly nothing beneficial about it. Notice it is the hottest reading on the Surface Temperature chart.
If you want to learn more about Heat Islands, what can be done, and what's been implemented or being proposed in some cities to mitigate heat island effects, check out these US government's websites:
Heat.gov (National Integrated Heat Health Information System/ NIHHIS) https://www.heat.gov/pages/urban-heat-islands
And for children: NASA has a website, Climate Kids, chockfull of great information and age-related activities: https://climatekids.nasa.gov/heat-islands/
Napa Master Gardeners are available to answer garden questions by email: email@example.com. or phone at 707-253-4143. Volunteers will get back to you after they research answers to your questions.
Visit our website: napamg.ucanr.edu to find answers to all of your horticultural questions.