Planting a windbreak

Wind is the most under-rated constraint in Australian gardens. It dries soil and foliage faster than heat alone, it physically damages soft growth and staked young trees, and on the coast it carries salt that burns leaf margins on anything not adapted to it. Shelter is usually the first planting a windy site needs, and it is generally the last one people think of.

Why permeable beats solid

The instinct is to block wind. A solid barrier does stop air at the barrier, but the air has to go somewhere, and it goes over the top and then drops back down a short distance behind, often with turbulence that is more damaging than the open wind was. The sheltered zone behind a solid fence is short and unstable.

A planting that lets roughly 40 to 50 per cent of air through does something different: it slows the whole body of air passing through it rather than deflecting it. The result is a longer, calmer sheltered zone extending ten to twenty times the height of the planting downwind. This is why a mixed, slightly open row of shrubs and small trees outperforms a tight clipped hedge, and why filling every gap in a windbreak is counterproductive.

The nine steps

1. Work out where the damaging wind comes from

   Note the direction of the wind that actually causes problems, which is often not the prevailing wind. In much of southern Australia it is the hot dry northerly in summer and the cold south-westerly in winter. On the coast it is the salt-laden onshore wind. In the tropics it is the wet season squall.

   Watch the garden over a season, or read it from the plants: existing shrubs lean away from the wind that shapes them, and foliage burn appears on the windward side first.

2. Decide how much shelter you need and set the height

   A windbreak shelters a distance downwind of roughly ten to twenty times its own height, with the greatest reduction in the first five to ten heights. A three-metre windbreak gives useful shelter over about 30 to 60 m; a six-metre one roughly doubles that.

   Work backwards from the area you want protected. Sheltering a vegetable bed 10 m away needs far less height than sheltering the far end of a paddock, and an over-tall windbreak on a small block simply takes light and root space from everything else.

3. Aim for a permeable barrier, not a solid one

   This is the principle that matters most, and it is counter-intuitive. A planting that lets roughly 40 to 50 per cent of the air through slows wind and spreads the shelter over a long distance downwind. A solid barrier — a paling fence, a dense clipped hedge — deflects air over the top, which then drops back sharply a short distance behind and creates turbulence that can be worse than no barrier.

   In practice that means mixed planting with varied foliage density rather than a single tight row, and it means resisting the urge to fill every gap.

4. Plan it in layers

   A windbreak works best with three layers: a low outer row of shrubs on the windward side, a taller middle row, and small trees behind. The low outer layer lifts air gradually rather than presenting a wall to it, and it also protects the base of the taller planting.

   Two rows is workable on a domestic block. One row is better than nothing but recovers poorly from any gap or loss.

5. Plan the length

   A windbreak should extend well past the area being sheltered at each end. Wind accelerates around the ends of a barrier, so a short windbreak concentrates the problem at the very edges of what you were trying to protect.

   As a working guide, extend the planting at least the height of the windbreak beyond each end of the sheltered area, and more if you can.

6. Leave no gaps

   A gap in a windbreak funnels air through it at higher velocity than the open wind, which is why a gateway or a lost plant in a shelter row causes visible damage in a narrow band downwind. This is the single most common failure in established windbreaks.

   Where an access opening is unavoidable, overlap two sections of planting with a staggered path between them rather than cutting a straight gap through.

7. Choose species that suit the zone and the exposure

   Coastal and temperate: Banksia integrifolia at 8 m by 5 m is the most wind-hardy tree on this site, with Westringia fruticosa and Correa alba as the low outer layer and Agonis flexuosa behind in frost-free coastal positions. Cool districts: Leptospermum scoparium, Grevillea lanigera and Poa labillardierei, with Acacia cognata for height.

   Arid districts: Atriplex nummularia at 2 m by 3 m for the outer layer, Rhagodia spinescens lower, Hakea laurina and Eucalyptus caesia behind, with the latter in a sheltered position because it is brittle. Tropical and subtropical: Syzygium australe, Melaleuca linariifolia and Callistemon salignus, with Lomandra longifolia at ground level.

8. Space the planting

   Space shrubs at roughly two-thirds of their mature width within a row and set rows 1.5 to 2 m apart. Space small trees at about two-thirds of mature canopy width, which for most of the trees listed above means 3 to 4 m.

   Stagger plants between rows rather than lining them up, so that gaps in one row are covered by the row behind. Resist planting closer for faster effect; crowded windbreaks thin at the base, which is exactly where the shelter is needed.

9. Establish it properly, then leave it alone

   Plant in autumn, or the early dry season in the tropics. Stake each plant low with two stakes and a broad flexible tie, because a windbreak is by definition in a windy position. Mulch to 50 to 75 mm, clear of stems, and water deeply twice a week through the first two summers.

   Check ties every six months and remove stakes after eighteen months to two years. Once established, prune only to remove dead wood and to keep the base furnished — a windbreak that is shaped and tightened loses the permeability that makes it work.

Windbreaks on small blocks

Most suburban gardens cannot fit a three-row windbreak, and do not need one. On a small block the same principles apply at a smaller scale: a single staggered row of shrubs at 1.5 to 2 m, sheltering a vegetable bed 10 m downwind, does real work. Even a 1.5 m planting of Westringia gives useful shelter over 15 to 30 m.

Where a fence is already there, planting a permeable row a metre in front of it on the windward side improves things considerably, because the planting slows the air before it reaches the fence and reduces the turbulence behind it.

What a windbreak costs you

Be honest about the trade-offs before planting. A windbreak takes light, and on the southern side of a small block it will shade the garden through winter when light is already short. It takes root space and water from a strip considerably wider than the planting itself. And it takes several years to do its job, which means the plants it is meant to protect need interim shelter.

Site it on the windward side, not simply along the nearest boundary. Check where the shadow falls in June rather than in December. And plan the planting behind it knowing that the strip immediately downwind will be drier and more root-competitive than open ground.

Interim shelter while it establishes

Shade cloth or windbreak mesh on a temporary frame gives immediate shelter and protects the windbreak plants themselves through their first two years, which is when they are most vulnerable. Use a mesh with roughly 50 per cent permeability rather than solid sheeting, for the same reason the finished planting should be permeable.

Remove it once the planting has grown through. Left in place, it becomes a solid barrier as it collects debris and undermines the thing it was protecting.