Most farm shelter belts are planted with exotic species, usually radiata pine (Pinus radiata) or macrocarpa (Cupressus macrocarpa). Native species such as lemonwood (Pittosporum eugenioides) or ngaio (Myoporum laetum) are fast growing and also provide very effective shelter. It is important to select species that are adapted to the local conditions. Shelter may also be provided by pampas grass (now considered an invasive pest plant) or other scrub species.
The most commonly used trees for orchard shelter are alders, eucalyptus, poplar, willow and casuarinas.
Generally shelter belts do not produce good timber, and trees pruned for timber do not provide good shelter. However, trimming a shelter belt will improve its effectiveness and help it live longer.
A hedge is a narrow band of low, dense, shrubby vegetation that separates fields or paddocks. Hedges are usually a single row, and in New Zealand are often gorse or barberry. A shelter belt is a line of trees planted as a windbreak, and may be single- or multi-rowed. They are usually radiata pine or macrocarpa.
The ideal shelter belt reduces wind speed by 50–80% out to a distance of up to five times its height, and by 30–50% for a distance of up to 10 times its height. To do this, it should have about 40% porosity or air passage space, so some of the wind goes through. This is more effective than an impermeable wall, which creates wind turbulence on the downwind side. To provide an effective buffer zone from snow, shelter should reach from ground level to at least 15 metres in height.
Shelter belts may be a single row of trees, a double row or more. Wide shelter belts usually provide more effective and rapid shelter, but occupy more land. Single-row shelter belts are normally of one tall species that will grow to the final effective height. Double-row shelter belts may have a low-growing, bushy species as the second row. This provides early, effective shelter, and if planted on the windward side, will shelter the taller species while it gets established.
Length and height
The shelter belt should be as long as possible to minimise ‘end’ effects where the wind comes around the ends of the belt. The length should be at least 12 times, and preferably up to 24 times, the height.
In orchards or gardens, the shelter belt should be at least twice as high as the plants it is protecting.
To be most effective against wind, the shelter belt should lie across the direction of the prevailing wind. But to minimise shading of pasture, it should be oriented north–south. North–south-oriented shelter protects against dry north-westerly winds, and east–west shelter protects against cold southerlies. Shelter belts are usually planted along existing fence lines.
Newly planted shelter belts must be fenced to exclude grazing stock, so the trees can grow to maturity undisturbed.