Spring has sprung, and as Arbor Day approaches, many community organizations are urging people to plant trees as a way to do something good for themselves and the planet. Unfortunately, many of the recommendations these well-intended organizations make about tree species selection and planting locations is wrong.
Yes, trees provide environmental, economic, and aesthetic benefits. But there are right ways and wrong ways to plant a tree. As the National Arbor Day Foundation says, the goal is to plant “the right tree in the right place.”
Some choices are intuitively clear: no palm trees in Colorado, no redwoods under power lines. Yet intuition has failed to take today’s building science into consideration or the implications of the expected trajectory of tomorrow’s renewable energy technology.
The world is changing, and we need to fine-tune our accepted wisdom concerning the benefits of trees. The Colorado Tree Coalition is one of the many organizations that tout the electricity-use savings obtained by shading a house in the summer.
Years ago, before the development of modern construction technique, tall trees shading your house was a good idea. Today, the numbers tell a different story.
The Coalition’s April newsletter cites a study showing that shade trees can reduce energy use for air conditioning by 214 to 642 kWh per house per year. That study was based on research from Arizona and southern California; Colorado’s cooler summers probably produce smaller savings.
In comparison, the typical rooftop solar array currently being installed in Colorado produces over ten times as much electricity as that, about 6,000 kWh per house per year! Obviously, planting a tree that will grow so tall it shades the solar panels is not an energy winner.
Of course, saving energy is not the only reason to plant a tree, but most of the other reasons for planting trees, such as sequestering carbon dioxide, reducing the heat-island effect, increasing property values, or reducing storm water run-off, can be just as easily obtained from a 30 foot tall tree (that does not shade the rooftop solar) as from a 50 foot tall tree. Shorter trees are also less likely to snap power lines in snowstorms or smash roofs in windstorms.
Just as important as choosing a tree species that does not grow too tall is the decision about where to plant it. Many sources recommend planting trees on the south and west sides of a house to cool it in the summer and using deciduous trees on the south so the sun can help warm the house in winter. Again, this advice is mostly wrong for Denver’s situation.
At this latitude, the path of the sun across Denver’s sky varies greatly with the seasons. In summer the sun is high, spending much of the day north of vertical, while the winter sun is low in the southern sky.
Building science has quantified the factors influencing the heating and cooling loads of a house, and a major factor in both summer and winter is the heat gain from sunlight entering the windows. Sunlight entering the house through the east and west windows in the summer is bad, but sunlight warming the house through the south windows in the winter is good.
Thus, trees should be used to block unwanted summer sun but allow for useful winter sun. Since the sun strikes both the east and the west sides of the house equally, both sides should be shaded in the summer. The trunk and limbs of deciduous trees can block from 20 to 60 percent of the winter sun, so no trees should be planted on the south side of the house.
While doing good by planting trees, make sure you are doing it right. We do not know what our world will be like when today’s saplings mature in 30 to 40 years. But if you plant the right trees in the right place, you will give tomorrow’s generations two gifts: a beautiful, carbon and heat controlled environment and the opportunity to heat and cool their homes, grow their own vegetables, and generate their own electricity.
Lance Wright worked with the U. S. Forest Service in Colorado, Idaho, Oregon and Alaska. He is the founder of Green Energy Man, Inc., and designs and build energy-efficient custom homes in the Denver area. EDITOR’S NOTE: This is an online-only column and has not been edited.
