Energy-Wise Landscape Design – “evapo-transpiration”? Tree Sweat?

suereed | August 21, 2011 | 0 Comments

Here in the last precious month of summer, there’s still a lot we can do in our gardens and grounds to help our houses stay cool… and save energy!

A shade tree plus lush foliage below equals maximum cooling.

We all understand the energy-saving value of a good shade tree, right? (for a reminder, see my post “Gotta Love a Shade Tree). But beyond keeping the sun from heating up our houses, a large tree can also directly cool the air itself. How? Well, you probably know that perspiration is our body’s way of cooling us off. But do you know how it works?

Yeah, neither do I, not in any real scientific way, but basically what happens is that the evaporation of moisture off our skin involves breaking the chemical bonds of that moisture, and this is a process that’s powered by energy. Some of that energy comes from the heat of our skin itself, with the result that heat gets removed from our body.

Same with trees, and all vegetation for that matter. You probably remember the term “evapo-transpiration” from a biology course somewhere along the way. That’s the plant world’s equivalent of our sweating. And it works pretty much the same way.

Even without a big tree, a lot of smaller plants will still help cool the air.

Plants suck water up from the soil or absorb a little bit through each leaf surface, and then when temperatures are hot the plant breathes – transpires – some of that moisture back out to the air. Some of the energy needed to power this process comes from the plant itself, but a lot of it comes from… guess where…. yep, from the surrounding hot air. And the result is that the air itself cools down. Okay, enough science.

All plants transpire. So here’s today’s big tip: the more vegetation you have around your house, the cooler the air will be outside the house, and this will make the inside cooler too. Which translates into less running time for the air conditioner. And voila: you save money!

Now are you asking: if all plants transpire, can’t I just have lawn all around the house? Well, yes, you can. But here are a couple of things to consider. First, lawn doesn’t transpire as much as plants with bigger leaves, so you’ll get more benefit from larger, lusher plants with lots more foliage. Second, you can have some lawn but you can have even more plants in the layers above it – perennials, shrubs and trees – and some of these may create enough shade that a different ground layer would be better than lawn, like maybe moss or ferns or a shady meadow. And all of these alternatives are energy-wise choices for a bunch of other reasons.

Any house will be cooled by the transpiration of abundant plants growing nearby.

So remember: more vegetation equals cooler air… cooler air equals a cooler house… and a cooler house is more comfortable for us inside, so we need to use less electricity to cool ourselves off!

To find out more about why and how this works, check out my book, Energy-Wise Landscape Design. You can find the book everywhere: in libraries (but you’ll probably want to buy it); on Amazon and all the usual online booksellers; at bookstores big and small; as an e-book in any platform of your choice; or you can buy a signed copy at www.energywiselandscape.com.

Energy-Wise Landscape Design