Photo credit: Ted McGrath
Everyone knows it’s cooler in the shade and warmer in the sun’s rays. Even animals instinctively know this; you have seen cows gather under trees in the fields when the sun is high in the sky. We can also take advantage of this natural shelter for our homes, farms, and businesses. Let’s review the basics of how trees can conserve energy.
If you live in a warmer climate, such as the southern United States, shading your home from the hot afternoon sun may be one of the best energy-saving choices you can make. Research conducted at Auburn University has shown that well-placed shade trees can save up to 20 percent annually in air conditioning costs. Trees cool structures in several different ways (How do trees cool the air?). Shade is the most direct way, and some types of trees are better than others when looking for shade.
Oak trees are often a great choice for long-lived shade.
Photo Credit: Raina Sheridan
Ideal shade trees have the following characteristics:
Tree height is dependent primarily on species but also on the quality of the site (soil type & moisture availability). Some species, such as flowering dogwood or crape myrtle, are characterized as small trees, meaning no matter how big they are when planted or how old they get, they will rarely exceed 20-30 feet in height. Other than for playhouses and dog kennels, these small-stature trees will not cast meaningful shade to cool buildings.
Not all deciduous species are created equal, though. Some trees that generally fall into the deciduous category are not “fully deciduous.” For example, some oak trees, in some circumstances, tend to hold on to their dead leaves for months after leaf fall until snow, ice, wind, or something else knocks them loose. White oak (Quercus alba) is probably the most common offender. It is difficult to know if a given white oak will be a “leaf holder” until it has been growing in the landscape for several years. Thus, for good summer shade and minimal winter shade, white oak and other trees that retain their leaves through part or all of the winter are not recommended.
For summer shade, choose tree species that can live a long time. Resist the temptation to select trees that grow fast at the expense of longevity, which is a downside of many fast-growing cultivars and species. Tree shade is a long-term investment. As a rule of thumb, the faster growing the tree, the shorter lived it is. The time invested to get a tree of sufficient height for good shade is best rewarded with the hope of summer shade for decades to come. If speed is a must, fast -growing trees can be planted at the same time as the slower-growing trees, with the plan that these more durable trees will begin providing long-term shade when the faster-growing trees are starting to decline.
If you live in a cooler climate, energy-saving trees can offer their ability to block, slow, and/or divert wind to save heating costs to your home, farm, or business. The best types of tree for blocking cold winter winds are evergreen, either needle (for example, spruce, pine, or fir) or broad-leaf (for example, magnolias or hollies) varieties. Cold winds will sap heat from buildings by physically entering the structure through small cracks or actual openings, such as your homes' dryer vent.
Ideal wind-break trees have the following characteristics:
Tree height is important and related to the height of the structure you wish to shelter from cold winds. Maximum protection occurs at a distance from the tree of 5-7 times its height, so the taller the trees, the greater the zone of protection will be (Texas AgriLife Extension Service).
For the same reasons as noted for “cool trees,” the value of long-lived trees holds true but is less critical. The best windbreaks are groupings of several rows of staggered trees, which can be managed as a small forest with regular replanting as another row or interplanting to continuum of ages to ensure continual function. A good strategy for planting windbreaks is to plant a row or two of fast-growing conifers to establish the benefits as soon as possible. If room permits, plant two or more rows of staggered evergreens of longer-lived, mixed species to function as a more permanent energy savings planting.
For maximum energy savings, plant both deciduous trees for summer shade and evergreen trees to block cold winter winds. Visit the National Arbor Day Foundation's Energy Saving Trees website for tools to help you save money with trees around your home: http://energysavingtrees.arborday.org/ For specific species recommendations for your location, contact a local consulting arborist, your local Extension agent or State Forestry Agency forester.
Pandit, R. and D.N. Laband. 2010. Energy savings from tree shade. Ecological Economics, 69: 1324-1329.
Landscaping for Energy Conservation, Part II. http://aggie-horticulture.tamu.edu/earthkind/files/2010/10/wind.pdf. (accessed September 14, 2012).
By: Adam Downing, Forestry & Natural Resources Extension Agent, Virginia Tech University, and Jay Banks, Four Oaks Consulting, Winchester, Va.