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It's Too Hot!!!

How hot do common landscape surfaces get in summer?

By Todd Layt

Over the last couple of years on 40 degree days we took the temperatures of many landscape surfaces. The results may surprise you. There was a great deal of variation between surfaces, from grass to gravel the temperature varied by 32 degrees. This important research could help contractors and designers create cooler landscapes. Some hard surfaces were much cooler than others. Choosing the right product can greatly reduce the surrounding temperature of the beloved backyard entertaining area. With fear spreading about the possibility of rising temperatures, is it time to consider using cooler landscape alternatives?

Not surprisingly the coolest temperature taken was the surface water of a large pond, which was exactly 40 degrees, the same as the air temperature. The next coolest surface in full sun was that of green grass on a sports field, which measured 45.65°. Dry grass on the same field was 51.5°. Green grass growing near gravel was 52.15°, whilst dry grass next to gravel was surprisingly 62.2°. Dry grass further away from the gravel was cooler. Shaded turf near concrete was 47.2° whilst concrete in shade was 49.8°. Gravel in full sun was 77.78° and grey concrete on a tennis court reached an amazing 78.27°, with concrete surfaces around a house in full sun 74.75°. Fake grass did not fair much better reaching 66.95°.

So the first trend that can be seen is that grass in full sun is much cooler than concrete and gravel, but so is shaded concrete. The shade of a tree reduced the temperature of the concrete by 25°. Lomandra Tanika growing in gravel that was 77.78 degrees just a few metres away cooled the gravel that was right next to it down to 53.9 degrees Celsius. It’s amazing the plants can even survive in that heat, but I guess next to the plant is cooler so that’s why they live. Even wood mulch was much hotter without green life. Fine grade wood mulch was 64.15° in the full sun. Bare soil was 68.05°. An important lesson learnt from this research is that green life significantly cools down a landscape. Turf, strappy leaf plants, shrubs and trees are truly wonderful things. The more green the cooler the landscape.

Many timber surfaces were tested. I was quite surprised by the results, as I thought timber would be cooler than it was. There is no relaxing on a painted wooden park bench on a 40° day as the recorded temperature was 67.9°. Treated pine was 62.35°, and timber decking recorded 66.9°. Hardwood seemed to be even hotter at a surprising 72.6°. But even for timber, life seems to cool it down. The temperature of live tree trunks exposed to full sun was 53.25°. Life has moisture and circulation, even if it is slow like a tree. A perfect example of this is my head compared to my research partner’s head. I thought my head of hair would have made my head’s surface temperature cooler, but a head of hair recorded 58° whilst a bald head was only 47.8 degrees in full sun. Blood flow near the surface and sweat obviously had a cooling effect, where hair does not have circulation, just like dead wood, or concrete has no moisture and circulation. Green life has an amazing cooling effect on this earth as it stands today. Plants help control the amount of carbon dioxide, a greenhouse gas, in the atmosphere. They protect the soil from wind and from water runoff, helping to control erosion. In addition, they release water into the air during photosynthesis. This water, along with the rest of the water on the planet, takes part in a huge cycle that the sun controls. Without plants we would have no oxygen to breath. The only reason we have an atmosphere with significant amounts of oxygen here now, is because of photosynthetic life constantly replenishing the supply. If all photosynthesis were to suddenly stop, the oxygen levels would drop pretty rapidly and CO2 levels would climb even faster. Eventually we would get back to an atmosphere that has much higher levels of CO2. So bring on the plants.

Most hard surfaces in this research were very hot; however a few saved the day for hard landscapes. Although scientists believe hard surfaces and the heat they generate is not contributing to alleged global warming, it most certainly contributes to the heat island effect that rings cities. Cities are hotter than rural areas, due largely to the abundance of hard surfaces and lack of plants.  So on the 40 degree day in full sun the hottest hard surfaces included concrete, with a  dark concrete driveway 74.75°, an even darker concrete at 78.27° , a light concrete paver 67°, and a dark concrete paver at 70°. So myth confirmed, darker surfaces are hotter than lighter coloured ones. A lighter coloured clay paver was 67°, a mid coloured one was 69.1° whilst a dark clay brick paver was 73.65°. A brick wall had reached 67.2°. I decided to measure a piece of flat steel plate, thinking it would be the hottest, but it was cooler than some surfaces at 70.6°. Bitumen on a road was 69.55°, whilst gravel was even hotter at 77.78°. People are always saying natural rock is cooler, but we found that a piece of sawn sandstone was only a little cooler at 66°.

We tried putting our bare hands on the different surfaces above, but on all but the Sandstone and the lighter coloured pavers we could only keep our hand there for 4 seconds, whilst the former allowed our hands to linger for 7 seconds. Imagine how hot these surfaces must feel to the tender bare feet of a small child.  Natural limestone had a temperature of 64 degrees, and did not burn our hands as quickly, but was hotter than I thought Limestone would be. Dry compressed reconstituted limestone was 61.4°, and wet cast limestone was 61.9° whilst wet cast Quartz was quite cool in comparison at 59.1°. We could keep our hands on the wet cast Quartz paver as long as we wanted. Wet cast Quartz is a less dense crushed and blended volcanic rock manufactured using a wet cast process. The myth that limestone is a much cooler surface is partly true if compared to Concrete, but compared to the 2 coolest hard surfaces you would say it is moderately cooler. Being a natural product I am sure the temperature of limestone could vary depending on where it is quarried.

The coolest surface was Timercrete. The darker Timbercrete pavers were 56.75°, whilst the lighter pavers were 55.8.  We could keep our hand on these without discomfort for as long as we wanted. Timbercrete pavers also have some other environmental benefits, as 114 square metres of pavers lock up 1 tonne of carbon. This is because well over half the paver is made up of wood waste, which would otherwise have its carbon released back into the atmosphere. As there is still a good amount of cemetuous product in the paver, they are still very strong and tough, termite proof and also very light to lift. We also measured the temperature of Timbercrete blocks used in a retaining wall, and in the full sun they only reached 49.6°, whist a clay brick retaining wall had a temperature of 67.2°. Based on this research a cooler hard surface and a cooler backyard is possible if you use either Timbercrete or reconstituted wet cast Quartz. The other piece of good news is that Timbercrete is no more expensive, and is in fact better value than concrete pavers. A cooler landscape will not cost you anymore.

While we were at the park and sports field we thought we would test playground equipment. The plastic playground equipment was 70.25°, and surprisingly the metal play ground equipment was 62.35°. Either way a little too hot for kids to play on when it is a 40° day. Our last measurement was just for fun, a vinyl seat on our outdoor vehicle was a staggering 88.4°. Talk about a hot seat.

In testing these temperatures we took between 10 and 20 measurements using a fully calibrated temperature gun. We waited till 2 in the afternoon, and on that day it did get hot early in the day, so all the surfaces were treated equally. We have since tested the same surfaces again, and noticed that if the day takes longer to get hot the temperatures are not as high. So the temperature depends not just on the raw temperature, but also the duration of the hot temperature. Cloudy cover or a cool breeze also quickly dropped the surface temperatures. But even on slightly cooler days the Grass, wet cast Quartz and Timbercrete were still significantly cooler than the concrete.

The next time you landscape in this hot and rugged country, think of the temperature differences of materials. Consider larger lawn area, more gardens, more shade trees, and where hard surfaces are used consider Timbercrete or reconstituted wet cast Limestone.