Gardeners often talk about “microclimates”. They come to understand that part of their property is a lot warmer than another and they learn they can grow plants there that would not survive otherwise. A zone 6 plant in zone 5, for example.
But rather than just taking advantage of a naturally occurring warmer microclimate, have you ever thought of actually trying to create one, of nudging most of your yard to a slightly warmer hardiness zone? You can easily do this by using a windbreak.
A windbreak (also called a shelter belt) is a planting of trees or shrubs that serves as a barrier to slow down the wind and protect the plants growing on its far side. You see them outlining farmers’ fields and also along highways, where they’re used to prevent snow from accumulating on the road.
In an urban setting, a simple hedge can serve as a windbreak, but so can a row of trees or conifers.
Shrubs, Trees and Conifers
Three types of plants make the best windbreaks: trees, shrubs and conifers. Their woody stems, solid but flexible, allow them to withstand the worst winds without breaking. Tall perennials can also be used as windbreaks as long as you’re only seeking warmer summer conditions, as they are usually ineffective during the winter.
Windbreaks are commonly used in the countryside to reduce erosion and evaporation and to warm the soil up more quickly in the spring, but can be just as useful on a smaller scale in cities and suburbs where they help create warmer microclimates where you can grow less hardy plants or those that require hotter summer conditions.
Where to Put a Windbreak
In the Northern Hemisphere, a windbreak is usually placed to the northwest, north or west, because cold winds come mostly from the northwest. In the city and suburbs, how buildings are placed may change that somewhat. For example, wind becomes concentrated when it passes between two buildings, creating a “wind tunnel”. Planting a windbreak at the exit of such a tunnel will therefore be particularly effective.
The effect of a windbreak is remarkable: studies show it will usually protect an area at least 10 times wider than its height and sometimes up to 20 times its height, depending on its density. So even with a short windbreak only 4 feet (1.2 m) high, you can protect plants from cold and drying winds for 40 feet (12 m) or more! A taller windbreak 20 feet (6 m) high should create a 200 foot (60 m) zone of warmer conditions, enough to protect your entire lot… and probably your neighbor’s as well!
Windbreaks Absorb the Force of Wind
As wind flows through a windbreak, the trunk, branches and leaves absorb some of the momentum of the wind and wind speed is reduced. Also, as wind flows over the plant surfaces, it is slowed by its roughness of the surface and wind speed is further reduced. Together, these two processes help reduce the force of the wind.
If you think a full wall, like a solid fence, would also reduce the effect of the wind, you’d be wrong. A solid fence doesn’t absorb the wind: the wind passes over or around it, causing it to swirl, then quickly resumes blowing as strongly as ever. Fences can be used as windscreens, but they have to be fences that let the wind pass through them. Typically an effective windscreen fence is covered in fabric (often several layers of fabric) that can move in the wind.
Proof of the effectiveness of a windbreak can be best seen in snowy climates. Snow will accumulate directly behind the windbreak, dropped as the wind weakens, while snow is carried completely away on the lee side of a solid fence, a clear sign that the wind there is as intense as ever.
To be effective, a living windbreak must be relatively dense and have many branches. Very open trees such as staghorn sumac (Rhus typhina) make poor windbreaks, while those with naturally dense branches, like most shrubs and also columnar trees, are very effective.
Conifers make even better windbreaks, but in colder climates their needles tend to “burn” on the windward size, dried out by the passage of strong winds, and they then lose their effectiveness. This can be corrected by planting a row of deciduous shrubs (they better tolerate the drying effect of winter winds, because their foliage is absent) to the windward side of the conifer windbreak. This can double its effectiveness: it may remain effective over a distance equivalent to about 20 times its height!
Similarly, a windbreak made up of deciduous trees tends to let a lot of wind flow through its base because most trees have fewer branches near the ground, but this can be corrected by planting deciduous shrubs at their base to fill in that gap or by planting a second windbreak of deciduous shrubs on the windward side of the tree windbreak.
In some mining towns in northern Canada and Siberia, the entire town is surrounding by a dense windbreak maintained by the municipality in order to make life more livable for the inhabitants. They can easily maintain lawns and gardens on the leeward side, something that would be impossible on the windward one.
On a Small Scale
You don’t have to plant a long double row of trees and shrubs to profit from the effects of a windbreak. Even 3 or 4 shrubs planted in a row to reduce the dominant wind may drastically improve the gardening conditions on the other side. Try it and see: you may be surprised by the results!