Damage from radiation frosts may be reduced by applying orchard heating, by sprinkler irrigation, by use of wind machines or by use of fogging systems during the frosts.
- The objective of all of these measures is to alter what is called the thermal regime of the air layer near the ground and to reduce the loss of long wave radiation from the ground and from the trees.
- Scientists at Pennsylvania State University in the USA have attempted to develop a Decision Support System for frost protection.
Orchard heating
In the late 1960s and during most of the 1970s frost protection of UK orchards using heating systems was quite popular, but their popularity declined as fuel prices rose. Three types of orchard heater were used: paraffin wax candles (the most popular), propane gas burners and ‘stack heaters’.
- Usually approximately 35 ‘stack heaters’ per acre were recommended, whilst wax candles were positioned between trees in the row at about 8-10m spacings.
- They were able to provide temperature lifts of 1 to 2oC on nights experiencing radiation type frosts. The life of the propane gas and stack heaters was estimated to be 5-10 years.
- The heaters provided radiant and/or convection heat which increases the temperature of the air and the crop surfaces.
- Although these systems operated best under low wind conditions, they were one of the few methods of frost protection that will help under wind frost conditions. This is explained by the radiant energy they produced.
- A current problem concerning their use is the pollution they cause and in many areas their use is proscribed.
Research has shown that combinations of heaters with wind machines provide more benefits than would be expected from their additive effects. Experiments combining orchard heaters with porous covers have also been carried out with the aim of providing some protection under wind frost conditions but the benefits were not as great as expected.
- Orchard heating during frosty nights can provide some reduction in damage from radiation frosts, by raising temperatures 1-2oC.
- However, the fuel costs for such systems are high and the pollution of the aerial environment caused by the candles, or propane/paraffin burners is usually unacceptable.
Tractor-mounted warm air blowers can provide frost protection but need to be driven round the orchard constantly during a frost event and work better when the air is still.
- The number of hectares that can be protected in this way depends on the orchard layout, the degree of frost, local topography and wind movement.
Sprinkler irrigation
Sprinklers used for frost protection in apple orchards may be high level (overhead) or low level (under tree) in type.
Under tree sprinklers
Under tree micro-sprinklers or spray jets are the most common form of frost protection used in California and Florida, in the USA.
- The aim is to transfer the heat contained in the irrigation water to the surrounding air, whilst maintaining the soil temperature at around 0 degrees C by release of fusion heat when water and ice co-exist on the soil surface.
- Under tree sprinklers are most appropriate where only small lifts in temperature are required and the system is flexible in that it can also be used for irrigation and nutrition at other times of the year.
- Sprinklers applying approximately 2mm/hour can raise temperatures at 2m above the surface significantly, if conditions are calm with no wind.
- The treatment is most effective if applied in humid conditions to soils, which are already moist.
- It is most useful with tree crops that are in full leaf at the time of application, but may also be of value with deciduous trees such as apples with less canopy cover.
- As with over tree sprinkling, ‘pulsing’ of the under tree micro-sprinklers has also been investigated, as a means of saving on water use.
- One particular advantage of the technique is that limb breakage, occasionally a problem with over tree sprinklers, is not a problem with these low-level systems.
- When using over tree sprinkling this is often a problem due to excessive ice build up on nights of extended freezing.
- Low level, under tree micro sprinklers can reduce frost damage.
- In calm (no wind) conditions applications of 2mm water/hour to compact and previously moist soils can raise orchard temperatures by 1 or 2 degrees 2 m above the soil surface.
- Micro sprinklers cause no limb breakage, which is common following extended use of over-the-tree sprinkler systems.
Over-the-tree sprinklers
This technique works by the release of the latent heat of fusion as the water lands on the tree and is turned into ice.
- It is essential that this is a continuous process and that the surface of the ice layer is not allowed to freeze completely.
- The tree tissues are, in efficient systems, maintained at approximately 0 degees C.
- There is an additional advantage in that some of the heat in the water droplets applied also serves to lift the temperature of the air in the tree canopy slightly.
- Over-the-tree sprinkler systems, using impact type nozzles applying 2-3mm of water per hour during frosts, can provide useful protection to the flowers.
The advantages of overhead sprinkler systems are that:
- Once installed the running costs are low.
- The heat release is directly to the plant surface and not indirect via the canopy air.
- They cause no air pollution.
- The equipment can also be utilised for other management operations in the orchard, such as irrigation application and in some parts of the world it is also used for inducing bloom delay.
The disadvantages of over-the-tree sprinkler systems are:
- The high installation costs
- The potential for limb breakage with excessive ice build up
- The possibility for excessive evaporative cooling and hence enhanced frost damage.
- The fact that it is essential that sufficient water is applied during the frost event; if too little is applied then frost damage may be worse than if none at all is applied.
- In addition, if the frost persists for many hours very large quantities of water are applied.
Sufficient water supplies must be available to meet this demand and the soils beneath the trees must be capable of draining the applied water away.
- The rates of water application needed vary to some extent in relation to wind speeds and to dew points.
- Uniformity of water distribution is of paramount importance for success with these systems.
Most conventional systems utilise medium pressure impact-drive sprinklers.
- Average application rates of 3.8 or 3.0mm/hour are recommended to achieve minimum application rates of 3 or 2mm/hour respectively.
- New Zealand research has shown that the very high water volumes used can be reduced by approximately 30% by using a targeted mini sprinkler system suspended above the trees.
- This saves water by not spraying between the rows and the headlands.
- Also, it is argued that when water application rates are inadequate to account for a severe frost, the damage resulting will be spread more evenly throughout the orchard using this targeted approach in comparison with the damage in an orchard protected with the medium pressure impact sprinklers.
Another method of saving on water used is to ‘pulse’ the sprinkler applications (Hamer, 1980) and 18-70% savings can be made using this technique.
- This uses a temperature sensor in the orchard, which is designed to respond the same way as a flower or bud.
- It causes the system to switch on when the temperature of the sensor falls to below -1 degree C. Nevertheless, this pulsed system does have its problems and advice should be sought from experts before adopting it.
Wind machines
In the most commonly experienced radiation frosts, ground based temperature inversions occur. This means that the temperatures above the tree canopy are higher than the temperatures below and within the canopy of the orchard.
- The aim of the wind machine is to mix these two air layers and to pull some of the higher and warmer air down into the orchard.
- The machines comprise large two-bladed fans (5m in diameter) mounted on towers approximately 10m in height.
- The fan should rotate around the top of the tower once every 4 to 5 minutes.
- The fan blades are mounted with a small downward tilt to facilitate the vertical movements of air.
- Although expensive to install, the wind machines have a 10-20 year life and cause almost no gaseous pollution (compared with orchard heaters) although noise pollution can be a serious problem.
- Nevertheless, they can be expensive to run and are only effective in conditions where strong inversions occur.
- Where nocturnal winds are more frequent their effectiveness will be less.
- Given strong inversion conditions a modern machine of approximately 75 kW can provide a 1 degree C lift in temperature over an area of 3.5ha.
- Use of wind machines for frost protection in UK orchards is not popular.
- The reasons are the high costs of installation and the unproven effectiveness of such machines in UK frost conditions.
Fogging systems
Clouds and fog can modify night temperatures because water droplets of a specific size (10‑20μ) can intercept a proportion of the long wave radiation loss from the soil and trees and bounce it back towards the surface. The fog also precipitates on the crop surface and, as it freezes, releases latent heat of fusion.
- Successful systems, which generate fog, have been used on vines in France and citrus crops in the USA.
- Fog generating lines placed 6-8 metres above the crop canopies produced a 1-3 m layer of fog, which spread over 300 to 500m in the slight wind.
- A line of 100m length gave some protection to 1-3ha.
- It is essential for success that wind speeds are no more than 0.6‑1.0m per second.
- The fog generating lines must deliver at least 25 grams/metre/second for any chance of success.
A mobile fog generating system has also been developed: the Gill saturated vapour gun.
- The system relies on a jet burner using diesel fuel at a rate of 150 litres/hour to generate a high-speed jet of very hot gases.
- Water is injected into this jet of gases via nozzles at a rate of 2 cubic metres per hour.
- This water is partly vapourised and part is dispersed as small droplets.
- The mixture of hot gases, water droplets and water vapour is then cooled with compressed air and by the ambient air.
- The gun is capable of generating a wide range of droplet sizes and also a range of vapour to droplet ratios, so modifying the type and buoyancy of the fog.
- The 60m jet on the gun creates a fog over approximately 20ha.
Fogging systems, used for frost protection of grapes in some parts of the world, have not been tested in apple orchards in the UK.
Decision Support System for frost protection
In the early 1990s, scientists in the USA produced a prototype decision support computer programme for the protection of crops from frost (Heinemann et al., 1992). The aim was to assist managers with the development, implementation and management of frost protection systems.