Practical Permaculture – Albedo and Absorption of Solar Energy in the Garden

First of all let me say that Spring is almost here, but it is still very cold outside… I am running out of photos to use and I’m sorry,  but for now these will have to do… I promise I’ll be taking many new photos as soon as I can… But I am starting to run out of steam as far as Winter is concerned… I really need Spring to get here…

Albedo, or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it. Being a dimensionless fraction, it may also be expressed as a percentage, and is measured on a scale from zero for no reflecting power of a perfectly black surface, to 1 for perfect reflection of a white surface.

The opposite of albedo is absorption, which is defined in this case as the interception of radiant energy. Either one of these principles can benefit the garden when working alone, but when you team the two principles up, you can bend the rules… If not break them…

Essentially, it all boils down to bright colors reflect light, and dark colors absorb them… Simple enough… Now the question becomes, how do we adapt these effects into the permaculture based garden? Since forests are generally attributed a low albedo, (as the majority of the ultraviolet and visible spectrum is absorbed through photosynthesis), it has been wrongly assumed that removing forests would lead to cooling on the grounds of increased albedo. Through the evapotranspiration of water, trees discharge excess heat from the forest canopy. This water vapour rises resulting in cloud cover which also has a high albedo, thereby further increasing the net global cooling effect attributable to forests.

In seasonally snow-covered zones, winter albedos of treeless areas are 10% to 50% higher than nearby forested areas because snow does not cover the trees as readily. Deciduous trees have an albedo value of about 0.15 to 0.18 while coniferous trees have a value of about 0.09 to 0.15.

In winter landscapes, conifers can also be heat magnets, they provide a relatively flat surface that is often the only dark color in a snow-covered area. Conifers can absorb enough heat to melt the snow around them, this is often evident in the amount of bird nests you will find in a conifer in the winter. Properly selected and planted conifer trees can help you cheat zones, this will allow you to experiment with plants that may not be suited to your area…

A variety of factors influence the ability of plants to reflect sunlight. At the most simplistic level, dark coloration provides the greatest absorption and hence the lowest albedo. However, leaf shape is quite important, with leaf shapes that are planar providing a higher reflectivity. Furthermore, leaf aspect is also contributory, with leaves that have surfaces parallel to the ground surface having the highest albedo.

A common way of hardening off seedlings and extending the growing season is the use of cold frames, in the Northern Hemisphere these structures should always face south to take advantage of the low winter sun. A few properly placed concrete pavers, when situated on the ground to the south of your cold frame, will reflect energy onto the cold frame that would have otherwise been lost…

Furthermore, a semi-circle of conifer trees, with the opening facing south, creates what is known as a heat dam. The conifer trees not only reflect the energy towards the center of the circle, they also absorb some energy to be released over night and serve as a wind break from the cold winds that commonly blow from the north. A cold frame or garden can be placed inside of the heat dam, that is just one example of how a thrifty permaculturist can get away without having to purchase row covers or other expensive season extending products.

I have always dreamed of having a south-facing stone face, the possibilities would be immense. I am not going to have my own cliff any time soon, but my house does have a south-facing wall… And I’d be willing to bet yours does too… The reflected energy from the sun can and will melt snow in the middle of winter, I have a patch of canna Lilies in my front yard that keeps surviving through the winter… The roots have become so massive that I wish it would die just so I could remove the damn thing…

Some objects have variable effects dependant on season. Ponds for instance, have a reflective quality that can focus the low winter suns energy in a specific location. This same pond, in the Summer, absorb the sun’s energy creating a cooling effect to the area directly around it. Think of it like this… If you take a rock, and throw it at an angle to the water… It will skip… But that same rock… When dropped in the water… Falls right through… Sunlight basically does the same thing…

Several years ago I had a permaculture epiphany while sitting on a hillside… What I found was that the hill I was sitting on had slopes that equally faced both East and West, the side of the slope that faced East was bone dry… But the side that faced West was wet and swampy… Both of these slopes received equal hours of daylight, but one side was considerably drier. What I realized was that the side that received the morning sun would absorb that sun for the first part of the day, then have the afternoon heat to help retain that warmth for the rest of the day. The side that did not have the morning sun only received direct heat in the afternoon, the night air makes quick work of eliminating the heat absorbed in the afternoon.

So you may be sitting there scratching your head wondering what in the hell does this have to do with my garden?.. When looking for a property for your farm or homestead in the northern hemisphere, never buy on a slope that faces north or west… The morning sun is the key element when designing your garden…

Many of the older houses in my neighborhood have the majority of the windows facing south and east, this is not by accident, it is by design… West and north facing windows are very inefficient, losing more heat than they absorb… Avoid them in your designs when possible…

Furthermore, many of the local houses have aluminum awnings overtop of the windows, believe it or not, these often tacky additions serve an ingenious purpose… In the winter when the sun is low on the horizon the awnings let the sun in the window… But in the summer, when the sun is high, it shades the window and reflects much of the heat away…

A proper understanding of albedo and absorption can be powerful tools in any gardeners repertoire. It will allow you to grow what I like to call “WTF” plants, turning the heads of your garden visitors every time… This is the type of information that will set you apart from the rest, and that is what I hope to build on from now on… The stuff that will set you apart from the rest…

This graphic is just a simple example of using the reflected heat from the pond, as well as from the conifers behind. The conifers also block the wind when it is blowing from the north, typically cold wind blows from the north and warm winds blow from the south.

Not bad for a dude who barely graduated high school… I hope you find this information helpful in your garden this year…

peace – chriscondello

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

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