The following is a precís of an article by Bernie McMullen, printed in the magazine 'Good Fruit and Vegetables, Sept 1997'. Part 2 appears in the October edition.

There is a range of detrimental effects of wind on crops - an extensive list appears in the article - and they can be calculated in dollars and cents, so attention to windbreaks is advantageous.

Wind disrupts the micro-climate around crops and trees, reduces bee activity and therefore pollination, causes soil-related damage in sandy soils and causes root damage as young newly-planted crops move in the wind.

Young fruit may suffer damage from abrasion and russetting from wind chill. Mature plants suffer limb breakage and reduced yield.

Wind interferes with spraying and spray irrigation systems, causing poor distribution.

Loss of heat and structural damage may be a major economic loss to glass and plastic greenhouses.

What is a horticultural windbreak?

A horticultural windbreak is a shelter belt of trees or an artificial structure which shelters a crop from the wind.

The type chosen and continuing care can vary dramatically according to use. There are 2 types, boundary and internal.

In both cases the trees are managed with irrigation, fertilisers and weed control. Depending on the value of the crop and the potential damage, artificial windbreaks can either be boundary or internal also.

Many of the principles of windbreaks are included in the Farm Trees Booklet No 5, "Designing Windbreaks for Farms", available from the NSW Dept of Agriculture. (One would assume the QLD DPI or Forestry could help also.)

Formula: An increase in windspeed causes damage to increase by the power of 3. i.e. double the wind velocity and damage increases 2x2x2 or 8 times.
N.B. A poorly designed windbreak may actually cause wind damage due to gaps or excessive thickness.

The following are useful points to be considered for perennial horticultural crops.

• Windbreaks should be planted several years before the crop whenever possible.

• the distance for maximum protection is usually accepted as six times the height of the windbreak above the crop. That is, with the same windbreak height, a vegetable crop with a height of less than half a metre will have a protection distance greater than a tree crop with a height of 5-6m.

• avoid gaps, such as access roads, that encourage the "funnelling" of wind. Gaps for air drainage or for access roads can be overcome by staggering or angling the wind break.

• do not clear under the break. A turbulent air flow can be created.

• narrowed-leaved species such as Casuarinas are more effective at higher wind speeds than are some of the broader-leaved poplars.

• slope can increase or decrease the effectiveness of a windbreak.

• the minimum length of a windbreak is 12 times greater than the mature height. A short windbreak has little effect.

Local knowledge is valuable in deciding where to place windbreaks.

In fruit orchards, the space between tree rows provides a natural funnelling of wind. This can cause an increase in wind speed so wind protection across the end of rows is recommended.

On the other hand, orchard rows themselves can provide wind protection for winds at right angles to the row. Vertical trees in the orchard rows will provide less protection within a planting and may cause some additional turbulence.

Locating windbreaks

There are several practical considerations concerning the location of windbreaks.

• access around both sides of the windbreak is important for general maintenance, including weed control and mechanical trimming.

• access to the planting requires gaps in the windbreaks. Care is needed to prevent these gaps creating wind funnels.

• occasionally overhead electrical supply lines run either along the border of a planting or across the planting. The local county council should be contacted to ensure that tree heights and distance from the lines will be suitable. At the same time, clarification should be sought regarding where responsibility lies for the trimming of windbreaks under electrical lines.

Artificial Windbreaks

You would need to read this section in full to grasp the context of these principles and includes some information on horizontal breaks using hail netting.

Advantages and Disadvantages
Comparison of the advantages and disadvantages of natural and artificial windbreaks.

AdvantagesInstant protection
No root competition
Minimal light competition
No pests or disease
Minimal maintenance
Minimal wasted
Low initial cost
Aesthetically pleasing
Habitat for beneficial insects
DisadvantagesHigh initial cost
Limited in height
Slower to protect crop
Continuing maintenance costs
Bernie McMullen,
Good Fruit and Vegetables, Sept, 1997

DATE: May 1998

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