Windbreaks for horticulture on the Swan Coastal Plain

Strong winds reduce horticultural yield and quality

High wind speeds cause problems on most horticultural properties on the Swan Coastal Plain. The dominant wind direction is from the east in the morning and from the south-west in the afternoon. In winter, strong winds can also come from the north-west with the approach of cold fronts.

Well-designed windbreaks will help achieve high yields and quality for most horticultural crops.

Windbreaks benefits to horticulture

Windbreaks can:

• Reduce plant damage
  Windbreaks decrease the incidence of broken stems, leaf loss and lodging of plants. The percentage of fallen and blemished fruit is reduced. Vegetable crops are protected from sandblasting.
• Increase plant performance
  Trials of various crops have shown that windbreaks can increase yield. Strong, hot winds increase evapo-transpiration, causing moisture stress so that more frequent irrigation is required. In winter cold, dry winds cool the soil and plants, slowing growth and delaying crop maturity. Wind reduces the activity of insect pollinators.
• Increase sprinkler uniformity
  Windy conditions can greatly reduce the uniformity of sprinkler irrigation. Investigations have shown that yield variation due to poor irrigation uniformity is common on many properties. Reducing wind speeds allows irrigation systems to function more efficiently.
• Decrease wind erosion
  Bare, cultivated areas — such as those during fallow periods on vegetable properties — are prone to wind erosion. Windbreaks can help alleviate this problem.
• Increase spray efficiency
  Windbreaks reduce spray drift, increasing the efficiency of pesticide applications. Reducing wind speeds also increases the number of days in the year when spraying is possible.

Windbreaks disadvantages to horticulture

The disadvantages need to be balanced against the advantages for any given horticultural area and business. Disadvantages include:

• money and time are needed to plant the trees
• the area in which the windbreak is planted is lost to production
• windbreaks require maintenance if they are to function properly
• roots and branches may need to be pruned to prevent competition with the crop
• the windbreak needs to be irrigated and possibly fertilised.

In most situations on the coastal plain, the advantages of a well-designed and maintained windbreak outweigh the disadvantages. However, on more protected areas or where land is limited, windbreaks may not be needed.

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Windbreak design

Before planting trees, considerable thought must go into planning the design of the windbreak. Poorly designed and sited windbreaks interfere with farm operations and may need to be removed later.

Orientation

To provide maximum protection, windbreaks should be at right angles to the dominant or most damaging wind direction. On the Swan Coastal Plain, windbreaks should ideally run north-north-west to south-south-east.

Windbreaks can be placed around all sides of the production area to cover all wind directions. Windbreaks running east-west may cause shading of crops to the south of the windbreak in winter, reducing production.

Other factors may affect the orientation of windbreaks. A windbreak should be compatible with the overall property plan. You may have to realign roadways and other structures to gain maximum benefit.

Windbreaks on ridges increase turbulence on the down-wind side. If possible, locate the windbreak downslope from the top of the ridge so that the top of the trees is about level with the ridge top.

Permeability

An effective windbreak should filter wind rather than present a solid barrier to it. A solid barrier diverts wind over the top, creating turbulence on the leeward side.

Ideally, windbreaks should have a permeability of about 50%. This means that thick, bushy windbreaks need to be trimmed. Foliage should extend to the ground otherwise the wind will blow under it, creating turbulence.

Access and gaps

Gaps in a windbreak will cause the wind to tunnel and can increase plant damage. Access roads through windbreaks can be protected by planting an additional short windbreak at least twice the length of the gap to the windward side of the access road. If a tree dies, plant another or turbulence will occur downwind from this area.

Height

The higher a windbreak the larger the area it will protect. However, windbreaks over 6–10 metres tall often become difficult to manage and cause excessive shading of crops.

Windbreaks offer good wind protection at ground level to a distance of 4 to 6 times their height and adequate protection to a distance of about 10 times their height. For example, a 10m high windbreak will give adequate protection at ground level for about 100m downwind. The protection offered to orchard, grapevine and other taller crops will be less because of their height above ground.

Setback from crop

To reduce shading and competition from tree roots, plant windbreaks at least 10m from the cropping area. Tall windbreak species need a greater buffer area. Leave sufficient room to allow ripping of the tree roots and 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.

Frost

Cold air flowing downhill can be trapped or dammed up behind a dense windbreak causing frost damage in susceptible crops. On sloping areas, a windbreak above the crop can reduce this problem by preventing cold air from flowing into the cropped area.

Windbreaks in the crop should be sited on hillsides with sufficient gradient to allow the cold air to flow downhill out of the crop. The lower end of the windbreak must not be blocked, so that cold air can move away freely.

Alternatively, use deciduous trees in frost-susceptible locations.

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Establishment and management

Ground preparation

Deep rip the soil along the line where the trees will be planted. Some trees, such as casuarinas, may suffer from iron chlorosis if planted on soils with shallow limestone.

Transplanting

You can propagate trees from seed or cuttings or buy them from a nursery. If buying transplants, check that the seedlings are not root-bound. This can cause strangulation which restricts the growth of new roots into the soil and may cause the trees to blow over in strong winds.

Seedlings must not be allowed to dry out or they will suffer from transplanting shock and subsequent growth will be reduced. The best time of year for planting is following the first autumn rains but, with irrigation, trees can be planted at any time of the year.

Tree spacing

Most trees should be planted in a line about 1.5–2m apart. Eucalypts however should be planted at 3–4m. If sufficient ground is available, plant multiple-row windbreaks; a second parallel, but offset, row of trees can be planted about 2.5m from the first row. Alternatively, a small tree or shrub can be planted on the outside and a tall tree on the inside of the windbreak.

Weed control

Before planting the trees, kill weeds in the 1 or 2m wide planting area by cultivation and/or with herbicides. Use non-residual herbicides, such as paraquat, diquat or glyphosate. For the most effective weed control, use a residual herbicide as well as a knockdown herbicide. If using a residual herbicide wait at least 2 weeks before planting the trees. Black plastic or organic mulches can be laid before planting to control weeds.

Weeds in established windbreaks should be controlled as necessary by herbicides or cultivation. Grass weeds can be controlled with selective herbicides. Other weeds need to be controlled by a directed spray of a knockdown herbicide. Take care as spray drift can damage or kill trees in the windbreak.

The registration and availability of chemicals for disease, pest and weed control change regularly. Consult a trained and experienced horticultural agronomist or the Australian Pesticides and Veterinary Medicines Authority for chemicals which are currently registered or have a permit for use in your situation. The information on the label or permit for a chemical must be followed, including the directions for use, critical use comments, withholding period and maximum residue limit. Quality assurance schemes for horticultural crop production require producers to have current information on chemical registrations and permits at their fingertips. The information to be found at this website allows this requirement to be met.

Pruning and trimming branches

Almost all tree species require pruning and trimming to maintain a suitable windbreak shape. In some trees, corrective pruning in the first year or two may be necessary to maintain a single dominant leader. After the first 2–3 years, side trimming of branches may be needed every second year.

Side trimming is necessary to maintain about 50% porosity and prevent the tree from becoming too wide. This can be done either by hand or with a pruning machine. Trees should not be lopped until they reach the final desired height.

Managing roots

A major problem with some species is invasion of tree roots into the production area. At Medina Research Centre, roots from a casuarina windbreak were detected in the crop about 20m from the windbreak. For species with extensive, shallow root systems, root pruning should be done at an early stage to encourage deeper rooting. Roots should be ripped to a depth of at least 1m at a distance of about half the tree’s height.

On deep sandy soils, it is likely that the tree roots will eventually grow below the rip line and resurface. On these soils, the roots of established trees should be ripped at 3, 6 and 10m from the windbreak to help prevent them invading the crop area. Irrigation of the windbreak will reduce the extent of root invasion into the cropped area.

Irrigation

Irrigation is essential for establishment. Once established, irrigation ensures sustained tree growth and reduces root invasion into the production area. Drip or sprinkler irrigation can be used. Water at about 50% of pan evaporation.

Fertiliser

Trees respond to fertiliser when soil nutrient levels are low. On newly developed, sandy soils, a light basal dressing of macro and trace elements can be applied before transplanting. About 100g per tree of a fertiliser containing nitrogen, phosphorus, potassium (NPK) plus micronutrients is sufficient.

Alternatively, use a slow release fertiliser with a 3–4 month release period. To increase growth, apply a light dressing of nitrogen annually. Apply fertiliser over the whole plant root zone rather than just at the base of the tree.

Pest and disease control

Some windbreak species may be damaged by specific insects and diseases. Spraying pesticides on a mature windbreak may be costly. Choose species with low risk of being attacked and, if pest or disease problems occur, spray the trees as soon as possible. Stock should not have access to windbreaks at any time because they will graze the lower foliage.

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Suitable windbreaks

Many species have been evaluated for their potential as windbreaks. No single species has all of the following beneficial criteria:

• cheap
• fast growing
• upright habit
• retention of lower branches
• immunity to pests and diseases
• 50% porosity to wind
• non-competitive or non-intrusive root system
• branches which don’t break in strong winds
• minimal suckering
• easily trimmed or hedged.

The following species are most suitable for use as horticultural windbreaks on the Swan Coastal Plain.

Casuarinas

Demonstrations have shown casuarinas to be one of the best species for windbreaks on horticultural properties on the Swan Coastal Plain. The best performing casuarina species in most cases is Casuarina cunninghamiana (river sheoak).

This tree is native to eastern and northern Australia and grows to about 15m. River sheoaks are fast growing and have a good shape and form to reduce wind speed.

On heavier soil types, Casuarina glauca (swamp oak) and Casuarina obesa (also called swamp oak) have also performed well, though they are more prone to splitting in high winds.

On white and pale grey sands, Casuarina equisetifolia (coastal sheoak) performs well, though tree shape is fairly open, possibly allowing too much wind to pass through.

All casuarina species are resistant to almost all pests and diseases (including weevils). Rabbits like to eat young casuarina seedlings and should be controlled before transplanting.

Like all natural windbreaks, casuarina windbreaks need to be maintained. Lack of pruning, ripping, irrigation and weed control will usually result in a poor windbreak.

Take particular care when using casuarina windbreaks for drip irrigated perennial crops. If the tree’s roots are not adequately pruned they will grow into the production area, seriously competing with the crop and even entering into the drippers.

The problem is less severe with annual crops as shallow tree roots in the cropped area are destroyed by cultivation before each crop. Increase the setback distance from the windbreak when growing drip irrigated, perennial crops.

Eucalypts

A number of eucalyptus species have been used for windbreaks on horticultural properties. Limb breakage on mature trees during high winds can be a problem and, with many taller species, an additional, lower growing species is needed to fill in holes in the base. Eucalypts are prone to attack by a range of insects. Spacing between trees should be 3–4m. Species that may be suitable are listed in Table 1.

Large trees are generally only suitable for external windbreaks and need to be placed further from the crop area to prevent competition. Smaller trees can be used for both internal and external windbreaks.

Exotic deciduous trees

The three main deciduous groups suitable for boundary and internal orchard windbreaks are Populus (poplars), Salix (willows) and Alnus (alders). All are moderate to fast growing and respond well to intensive management and side trimming.

Deciduous trees provide less wind protection during the winter. This is a disadvantage for annual vegetable cropping and evergreen perennial crops.

Poplar rust affects poplars, though some species are resistant. The Tasman poplar (Populus x euramericana hybrid) and Alnus cordata (Italian alder) have performed well in windbreak trials at Manjimup Horticultural Research Institute. Willows are a host for the carrot aphid, which is a pest of carrots, parsnips and celery.

Radiata pine

Advantages of Pinus radiata pines:

• They are fast growing and reach a height of 15–20m.
• They are cheap and relatively easy to establish.

Disadvantages of Pinus radiata pines:

• The canopy can be too dense when young (leads to turbulence near the windbreak).
• Older trees:
  • become too open at the base and often require an underplanting of a shade tolerant, lower growing species; the lower growing species should be planted at the same time as the pines
  • become too wide unless regularly pruned, and do not form good hedges; limited side pruning just before the spring flush is useful in younger trees.
• Heavy pollen loads may discolour adjacent crops (causes a blue-green appearance on citrus).

Pinus pinaster (Monterey pine) is more suited for planting on white sands. It is slower growing when young and has much better resistance to sun-scorching of the trunk.

Cypress

Cupressocyparis leylandii (clone Naylor’s Blue) has been trialled in the south-west of Western Australia with encouraging results. It is a hardy, fast-growing species that has a narrow, columnar shape. It will grow at about 1.5m a year and reach a maximum height of about 15m. Leighton’s Green may perform well, but it is slower growing than Naylor’s Blue.

Cypress species can be killed by the fungal disease Seiridium. Take care that the disease is not brought in on the trees from the nursery. Seiridium is spread by rain splash from infected plants. If present, it should be immediately controlled by fungicide.

Bana grass

Bana grass grows rapidly to about 3m within 6 months and will reach a mature height of 4–5m. It is usually planted from cuttings in spring at a spacing of 60cm. The plant is erect growing and there is little lodging or lateral spreading.

Negative characteristics include lateral root growth, shedding of leaves, fire hazard and poor wind porosity creating turbulence. As bana grass is very fast growing, it is useful as a temporary windbreak. It can be removed by slashing and spraying regrowth with glyphosate, although some people have reported difficulties in killing the plant.

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Artificial windbreaks

Artificial windbreaks have advantages and disadvantages:

• they take very little space
• they do not have any competition for moisture or nutrients (but may shade adjacent crops)
• they work immediately after erection
• they can be part of a fully-netted protection program
• they are relatively costly to establish, especially if very tall or fully enclosed.

Nursery crops

Nursery crops of cereal rye or other cereals can be planted between the rows of direct-seeded vegetables, such as carrots and onions, to reduce sandblasting of emerging seedlings. Cereal rye is tough and withstands sandblasting much better than other cereals. It is sown at a rate of about 50kg/ha. The cereal rye is killed with a selective grass herbicide once the crop is established and before it seriously competes with the vegetable crop. Nursery crops only protect the emerging seedling and do not offer many of the other advantages of windbreaks that are listed above.