Land owners situated on Western Australia's Swan Coastal Plain are predominantly faced with strong easterly winds on summer mornings, which swing around to the south-west in the afternoon. During winter, damaging winds can prevail from the north-west.
The risk of damage can be greatly reduced by establishing windbreaks to provide protection. Windbreaks work by reducing wind speed and turbulence downwind of the break. They should be permeable and aim to filter the wind, not block it out completely.
The most cost-effective windbreaks are trees, which can also offer many additional benefits to the property, including:
- prevention of soil erosion
- improvement to plant growth by reducing moisture stress
- protection of plants from wind damage
- provision of shade and shelter for livestock
- lowering the watertable in ‘boggy’ areas
- increased biodiversity
- reduction of evaporation from farm dams.
The benefits of correctly designed windbreaks generally outweigh the possible disadvantages.
The drawbacks of planting windbreaks are a perceived loss of land as well as the money and time taken to plant the trees and ongoing maintenance (pruning, irrigation, fencing and fertilising).
Windbreak design
To ensure a windbreak achieves the desired purpose, it needs to be carefully designed. The following key factors should be considered:
- orientation
- tree spacing
- height and length
- set back distance
- choosing suitable species.
Orientation
Windbreaks should be at right angles to the prevailing wind direction. For properties on the Swan Coastal Plain, a windbreak running north-north-west to south-south-east will give the best protection without causing excessive winter shade.
It’s important when planning to be aware of access roads or other structures that may be impacted.
Tree spacing
An effective windbreak will filter the wind, not stop it from passing through. Permeability should be about 50% with gaps minimised in the windbreak to prevent wind tunnels, which may cause direct damage to plants on the other side.
Windbreaks work best when they consist of several rows of staggered trees and shrubs. This allows the trunks of one row to be aligned with gaps in the next row.
Planting at least three rows offers some insurance against gaps from seedling or mature plant deaths. If a tree dies another should be planted in its place. Shrubs are best planted on the outside (windward), with taller trees on the inside of the windbreak.
The rows should be planted about three to four metres apart. Most trees/shrubs should be planted in a line about three to five metres apart (wider gap between trees, lesser gap between shrubs).
If planting only one or two-row windbreaks is unavoidable, reduce the spacing between trees to achieve faster closure of the canopy and to insure against gaps.
Where an access track or gate is needed, plant some shrubs right up to the gate/track to help prevent the wind from accelerating through the space. An additional short windbreak at least twice the length of the gap to the windward side of the access gap can also help.
Height and length
The higher a windbreak the larger the area it will protect. However, height is best limited to 10m to prevent excessive shading of the adjacent area.
Windbreaks offer wind protection to a distance of about 10 times their height. For example, a five metre windbreak will give protection for about 50m downwind. Protection is greatest at ground level so orchards, grapevines and other taller crops will be less protected because of their height.
The length of the windbreak needs to be a minimum of 15-20 times the height of the trees, otherwise wind will be deflected around the ends, reducing its effectiveness. Therefore a five metre high windbreak should be 75-100m long.
Set back distance
To reduce shading and competition from tree roots, windbreaks should be planted at least 10m from any crop. Allow a greater buffer area if tall windbreak species are being used.
Sufficient room should be left to allow ripping of the tree roots, vehicle access and turning space. A greater distance between crop and windbreak is required when trees are planted to the north of the cropping area to minimise shading in winter.