The Permacultivator - Journal of Cool Climate Permaculture
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Plants and Comfort

Some insights into using plants in Permaculture design by Rod McMullan

Plants have an amazing way of keeping us humans comfortable, and perform it very simply.

Have you ever been standing in an open field in the late afternoon, and felt a coolness in the air? You then walked over to a large group of trees and slowly noticed the air becoming warmer. In under the canopy, the air is warmer and remains fairly constant, only becoming cooler when the outside temperature is hot. This is natures reverse cycle air conditioner, without the running costs. Nature has a plan to keep everything in balance. If you observe and listen to her, you will benefit.

Keeping Cool

How do trees and all plants produce this effect? "Transpiration and the Convective Transfer of Heat", is the answer. To put it simply, plants draw up water from the earth with their roots and transport it to the leaves where they need it. The water is turned into vapour because of the heat, (approx. 10 degrees hotter than the air), in the leaves. About 80% of the radiant energy of sunlight that falls upon the leaves may be absorbed by them, and it is this energy that is changed to heat which raises the temperature of the water in the leaf and turns to vapour. This vapour is under pressure and is released by the plant to the atmosphere. If the temperature is hot and dry, then more water vapour will be lost through the leaves. This effect produces a cooling of the air around the leaf and the whole tree. This air flows downwards and under the canopy, where combined with the shade from the tree, produces a cooler environment.

Keeping Warm

When night falls, and the air temperature drops the plants shut down thus preventing theescape of water vapour. The canopy of the tree acts as an umbrella over the ground keeping the air warm. The mulch on the surface insulates the earth’s thermal mass, producing an average ground temperature of approx 10 - 15 degrees higher than above the canopy.

Human Comfort

How does this relate to our living? A canopy of leaves can change the temperature of the ground by either cooling or warming it, depending if its night or daytime and in extremes of climate. The snow will melt around the trunk of a tree in cold climates and keep us cool in hot climates. The shade cast by a tree can cool the ground by at least 20% and trees with dense foliage can filter 80% of the sun’s radiation. Trees can vary in the percentage of shade cast, either depending on the amount of leaves a tree grows or the size of them. Deciduous trees can cast shade and vary from one tree to the next. For example an Alnus casts 88% shade, Gleditsia 40%, Morus (Mulberry) 45% and Ginko 30% Remember this for the north side of your house.

Design Strategies

Growing vines on a pergola can be a highly cost effective way of cooling your house and estimates show a 1/2 to 2/3rd reduction in costs. As with the forest the vines cool the area beneath by as much as 10% to 20% than the air above. Vines can also be used on trellises to produce the same effect. They can also be grown on the walls of your house, keeping you cool and prolonging the age of your bricks etc.

Vines and shrubs grown close to your building can also keep the heat in the walls from escaping by means of an insulating pocket of air.

By shaping the area around the house with earth mounds and wind- breaks, you can stop unwanted dry air from reaching your cool areas and direct afternoon breezes where needed. If hot air is a problem in the short term then using water features such as ponds or misting can cool the air before it reaches you.

So you can see that by observing nature and working with her you can have an attractive, cost effective lifestyle. Plants will comfort you, if you comfort them!!!