Evergreen Windbreak
Creating a windbreak or windscreen is not as simple as just planting a row of evergreen trees
If all you want to do is create a privacy screen a row, or staggered rows, of evergreens, trees or shrubs will work just fine. But if you are trying to block or diffuse wind from buffeting your home or reduce wind that batters plants or a sitting area, you need to do some planning. A windbreak can help to reduce heating costs in winter, protect plants and property from wind damage, prevent snow from drifting and eliminate wind from recreation or personal use areas.
Evergreens planted in a row do not simply block wind, they re-direct it. Wind will hit the “wall” of evergreens and flow up. Once the air reaches the top it flows not just horizontally but back down. And if you only plant a few evergreens the wind will diffuse through and redirect around the trees too. Just in front of a dense line of young evergreens is a nice quiet zone, virtually free from wind. But a short distance further and the wind has begun to flow back down to the ground. The wind speed is reduced from when it hit the line of evergreens but certainly there will be wind and turbulence where it has flowed back down to ground level. So, it is very important that you plant accordingly and that you understand what to expect. I am going to outline a few pointers about creating a windbreak, but you may want to consider hiring someone who thoroughly understand all the range and angle formulas involved if this is an important or large scale project. (Note that planning a wind and snow break in the plains or rural areas can get substantially more complicated.)
Benefit will be minimal while the trees grow
First you must accept that a few brand new small evergreens will not do much to block wind. It takes time for them to grow tall, to protect a larger area, and to fill in the width of the expanse you have planted. Your patience and need will help you to decide if you are willing to invest in larger balled and burlapped trees or just wait for potted trees to grow. In either case it will take several years to fully accomplish your goal, likely as much as twenty years. The growth rate of the trees you select will determine just how many years. Keep in mind that typically a fast growing tree is not as strong as the slow growers and may be susceptible to wind damage.
Even before your evergreens have grown tall and filled in between the plantings, you will begin to benefit with reduced winds. The wind will be redirected and somewhat filtered by even small trees.
Large evergreen trees are ideal for protecting from strong winter winds. A deciduous tree will have no leaves (and has no foliage to the ground level) when you really need to stop cold winter winds from hitting your home. If your objective is only to protect a patio area from wind in the summer, a combination of deciduous trees and shrubs will not only work fine, but there is greater opportunity to include a variety of textures, colors and blooms to enhance your landscape.
How tall should the evergreens be
Evaluate the space you have for planting, the space you want to protect from wind, and the distance between the proposed planting site and the protection site. That distance will tell you how tall you will need your evergreens to be. The calmest zone occurs in an distance of 2 to 5 or more times the height of your tree. So a 10 foot tall evergreen tree will create a calm zone in the first 20 to 50 feet. Turbulence will gradually increase after that point but diminished wind speed will be enjoyed up to at least 10 times the height and up to 20 or 30 times the height.. Planning is generally based on 20 foot tall evergreens so the calmest area would be 40 feet up to 100 feet from the planting line. Of course many large evergreen trees eventually get much taller than 20 feet and you will be able to create a calm zone of 200 feet or more with the tallest trees.
How many rows of trees
The depth of the windbreak (how many rows deep) depends on the severity of the wind. In open rural areas you may need to plant as many as 3 to 5 staggered rows of evergreens to do a really good job of blocking winds. In typical suburban areas even one line of evergreens can do a pretty good job, but two staggered rows may be necessary to block strong northerly winds. The second line of trees should be planted behind but staggered between the first line. Wind will funnel through any open areas and actually accelerate, and the actual density of the windbreak will also affect the distance protected from wind. So to achieve the calm zones calculated above you must have adequate density (or number of rows). Again, in typical suburban areas two rows in a staggered line is often plenty. But do consider your specific property, I happen to have a property fairly open to northerly winds so three rows or two rows very closely planted works best.
How long should the row of trees be
Offering a calculation for the length of a windbreak for a suburban property is not quite possible. It is, however, important to know that wind will not only be redirected up, but also around the ends of the windbreak. So when planning be sure to consider that the wind will flow around and to the front of the end trees. You will want to be sure that does not happen to be at a point that will disturb plants that are susceptible to wind dessication or summer relaxation spots. Snow will also drift around the end plants.
Where to plant the windbreak
In a typical suburban setting in the North Central regions a homeowner often experiences cold winds coming from the North and Northwest. Since the line of your windbreak should be perpendicular to the winds you are trying to block, a U shape, curved line or angled line will do the best job. The line should run primarily Northeast to Southwest or in an angle oriented east to west on the north side and north to south on the west side. More simply put, along your north and west property line. Wherever you live, knowing exactly the direction of the wind you need to block is critical in the planning. If your concern is strong westerly winds in summer, your windbreak planting line should run north-south.
How far apart to plant
The final step before planting is determining how far apart to plant the trees. There is no hard and fast rule, it depends on the ultimate width of your selection, how dense you want the screen to be, how many rows of trees you are planting, and how long you are willing to wait for the trees to fill in. Remember that mature width stated on the plant tag is typically only 10 years, although with large evergreen trees maturity is more like 20 years. Your plants will still grow after they have matured.
The most efficient planting for a windbreak is two (or more) rows of trees planted in a staggered pattern, like footprints you leave in the snow. Plant your first row, then the next row should be planted between the first line of trees but of course some distance away. A somewhat haphazard planting of different varieties looks most natural, but may not be as effective if you have very strong winds to deal with.
How far apart you should plant depends a lot on how large the trees are. If you are planting large arborvitae you should plan for the branches to meet or slightly overlap at their greatest width. Probably about 8 to 12 feet apart would be a good range to consider. The rows of trees should be 10 or 12 feet apart. If you are planting a single line of trees you should plant them closer together so that eventually you have a more “solid” wall of foliage. Planting evergreens quite close together could create dead zones where branches are unable to get light and air circulation. Closely planted evergreens also creates a lot of competition for water and nutrients, so you must make sure you deep soak the soil consistently.
Although arborvitae are commonly used because they have dense foliage and many can easily reach 40 to 60 feet tall, they are by no means the largest of the evergreen trees. Larger trees will have to be spaced further apart. Black Hills Spruce and Colorado Blue Spruce are commonly used for screening in the North. They can easily reach 90 feet tall or more with an ultimate spread of 20 or more feet. Evergreens of such size should be planted at least 18 or 20 feet apart. Competition for water with these enormous trees will be fierce.
One final consideration is the creation of new micro-climates by the windbreak. A densely planted windbreak may cause higher temperatures and humidity levels in the calm zone. You may lose cooling breezes in summer but perhaps you will gain shade protection from the afternoon heat. There are so many variables for each property situation that again, I just can’t give you a formula for calculation, only things to consider. Typically though, a suburban windbreak will not be so high and dense and long that the effects will be hugely dramatic. But definitely worth thinking through.