Windbreak and shelterbelt benefits and the best tree species to use
First of all, what is a windbreak?
The purpose of a windbreak is to form a semi-permeable barrier to filter the wind rather than to stop it entirely (as a fence or wall would do). Solid boundaries create eddies of wind on both sides of the structure causing air turbulence instead of providing protection, whereas a windbreak allows the wind to filter through the trees whilst reducing its force, and providing a better protected area for crop production whilst allowing some airflow to help deal with pests. A windbreak can be any row of hedging plants or trees but our top recommended windbreak species is Alder which has some specific advantages and should either be included as hedgerow trees within a mixed hedge around an orchard or ideally used as a single species planted about 60cm cm apart so that it grows fairly densely without being solid. For really effective windbreaks in particularly windy areas, you can plant in double rows (at least 1m between the rows). Alternatively plant a row of Alder and then on the leeward side, plant a mixed native hedge.
What is a shelterbelt?
A shelterbelt is a planted area, containing trees and smaller shrubs, planting in 3 to 5 staggered rows.
What is the difference between a windbreak and a shelterbelt?
A windbreak is just one or two rows of plants whereas a shelterbelt is 3 to 5. A shelterbelt takes up more space and is really more suited to farmland or a large estate whereas a windbreak can be used on farmland/estate land but also in a garden setting. Shelterbelts create a larger, more sheltered area than windbreaks but the purpose of them both is the same – to deflect and slow the wind and slower wind is warmer wind, so they both also mitigate from the effect of cold winds.
All of the following text talks about windbreaks but can apply to shelterbelts too if you have the space.
Reasons to have a windbreak
All arable crops but especially valuable vegetable crops benefit from wind protection. There will be a small crop loss near to the hedge (unless field margins have been established) but for a considerable distance from the hedge there will be a gain in yield and potentially also in quality.
All animals benefit from wind protection and the shade offered by a tall windbreak – new lambs particularly.
All wildlife benefit from taller hedges – the bigger the better from a wildlife food, shelter and nesting site point of view.
Windbreaks are particularly important for orchards. It’s not just newly planted fruit trees that can suffer from damaging wind rock but also older trees. Wind also causes damage to stems, branches, flowers and fruit and it reduces the air temperature.
With orchards specifically in mind, windbreaks of trees or very tall hedging provide a natural shelter for pollinating insects, enabling fruit set.
Windbreaks are also recognised to reduce moisture loss from soil and foliage.
Windbreaks aid the establishment of early crops like daffodils in Cornwall and early potatoes from all parts of the south west.
Factors to take into account when planning a windbreak
The height of the windbreak is an important factor. The protected area on the leeward side is generally expected to be up to 20 x the height of the tallest trees in the windbreak (so to protect an orchard or field area of 40m width you’d need a windbreak of 4m height).
Ideally the length of the windbreak should be in the region of 10 x the height of the windbreak so if the height is 4m, the length would ideally be not more than 40m. The purpose of this 10 to 1 ratio is to reduce turbulence at the ends of the hedge.
For extra protection, windbreaks should be longer than the area to be protected or extended down the sides to prevent gusts of wind.
Obviously the prevailing wind direction (normally south west but can be from other directions) is a crucial factor but you also need to consider whether planting a windbreak will create shade and prevent sunshine reaching crops to ripen them.
Which hedging plant species are best for windbreaks
Any native species which will grow to the full height that a flail can reach can be used as a windbreak. For example we have seen a 12ft hawthorn hedge acting as a windbreak in a hop field – the hop bines are susceptible to wind damage when the crop has grown to the top of the wires in mid-summer and are heavy with the weight of the crop.
Hawthorn is not recommended for apple orchards because it is susceptible to “fireblight” that can also seriously affect fruit trees. Blackthorn is susceptible to “silverleaf”, a bacterial disease affecting Prunus species so should be avoided by plum growers.
Poplars are used extensively for windbreaks but care needs to be taken because although their fast growth may be really useful in the first 10 years, they continue to grow into large timber trees, with large and invasive roots competing for nutrients and water with nearby crops.
Alder is the species of choice for many windbreaks, particularly orchards but it’s assets help in many other situations
- It’s not susceptible to the fungal pests which can affect orchards so it can provide a safe haven
- It grows very fast (1m pa) until it is approaching maturity (when its growth rate will slow) at a height of about 6m so if it is planted when the orchard is new or young, it will very quickly provide an effective windbreak
- All varieties of Alder are wind tolerant so will not suffer excessive foliage damage or dieback whilst doing their job
- Alder is a species with dense foliage and it comes into leaf early in the year and keeps its leaves until very late in the year (sometimes December) so it can do its job for a very long period throughout the year
- Alder can be trimmed or coppiced if it’s getting too tall. Trimming side shoots to encourage bushier growth can be helpful in early years
- Alder is one of the earliest trees to produce pollen providing a great start for bees and other pollinating insects
- They have a compact non-invasive root system that does not have a detrimental affect on nearby crops
- They are able to fix their own nitrogen supply so do not draw nitrogen from the soil
Which variety of Alder to use?
There are many varieties of Alder but for the purposes of growing a windbreak the main varieties are Alnus glutinosa and Alnus cordata and it’s very easy to choose the right one for your soil conditions
- Alnus glutinosa is the native or common Alder and is great on wetter soils, even really quite wet sites
- Alnus cordata (Italian Alder) is better suited to drier sites and as native of southern Europe it actually will do very well even on very dry sites
In an orchard setting Alnus cordata would be the most usual choice. You can start with small bare root whips (40/60cm or 60/80cm) and due to their fast growth rate when young, they will outstrip the growth of fruit trees and grow into a very effective windbreak before the fruit trees are in full production. Always protect young alder trees with spiral guards, supported by canes and it is highly recommended that you keep all grass and weeds away from the newly planted trees until they are fully established.