[This material was written in 1971, and while some remains relevant in 2008, other sections of the text are now out-dated.  We have chosen just those sections from the book that we think are of most use to walnut growers in the present day and have left out other sections.  We will provide information from some more current sources elsewhere.]

DIRECT METHODS OF PREVENTING FROST DAMAGE

In New Zealand protection of orchards from frost damage began in the early 1930s with the use of firepots burning light fuel oil.  In recent years some water-sprinkling systems have been installed in orchards, nurseries, and small-fruit gardens.

Orchard heating

Man has been protecting his important crops from frost damage for over 2000 years.  The oldest method, and the most widely used throughout the world today [in 1971], is that of adding heat to the atmosphere by burning some form of fuel.  While wood and fossil fuels such as coal, coal tar, coke or naphthalene were used extensively in the past, by-products of petroleum, mainly diesel oil, burnt in firepots are more common today.  Originally it was thought that smoke produced by fires provided the protection, but in recent years it has been realised that the smoke has no such effect, serving only to keep the sun’s heat from penetrating to ground level in the morning following a “burn”.

Firepots

The effectiveness of firepots depends almost entirely on the presence, and height, of a layer of warm air overhead, the inversion layer.  Most of the heat given off from the firepots is in the form of a column of hot gases which rises rapidly, mixing with the surrounding cold air.  The mass of heated gases and air does not rise very far before it is surrounded by air of the same temperature.  At this point upward movement ceases; the layer of warm air above the heated area may be regarded as a roof which stops the ascent of the heated air.  The heaters continue to produce heated gases, which rise, but stop their ascent at lower and lower heights, until eventually the whole air mass, right to ground level, is heated to the desired temperature.  There is also a tendency for the warmer air in the higher layers to cool gradually and sink into the region between the firepots; in this way a continuous circulation of warm air is built up within the heated block.

The height of the inversion ceiling determines how large a mass of cold air must be heated.  The lower the inversion ceiling the less air there will be to heat and thus the greater the efficiency of firepots.  Conversely the weaker (or higher) the inversion the greater the amount of fuel required to produce a given temperature increase (Fig. 10).

A large number of small fires per unit area is more efficient than a small number of large fires having the same total heat output.  The heated gases leave the large fires at a greater velocity and a higher temperature, rising to a greater height before reaching surrounding air with the same temperature.  With small fires the heated gases leave the burner relatively slowly and at a lower temperature than from large fires.  Where there are many small fires the volume of warm gas rising is much larger than from a few large fires thus raising the temperature of the cold air mass much more rapidly and efficiently than is possible with large fires.

On most “frost” nights some lateral air movement takes place near the ground.  This drift, although only 0.5–2 km per hour, steadily carries the heat away from the heated area.  As drifts are generally consistent in direction, extra firepots should be placed along the windward border to warm incoming cold air; otherwise this cold air may penetrate three or four rows into an orchard, causing severe damage.

Orchard size has a considerable influence on heating efficiency.  In general the larger the area to be heated the smaller will be the relative length of border and the smaller the effect of lateral cold air inflow during burning.  Large areas of orchards can also reduce air drift, the trees having a braking effect.  Isolated orchards are more difficult to protect than a group of orchards, and more fuel is required per hectare to gain a given degree of protection.  The heat produced by a group of orchards tends to be shared, because of drift, and again the smaller proportion of edge reduces cold air inflow.

It is much more difficult to protect small trees than large trees, particularly if the latter tend to form a canopy between the rows.  Large trees reduce wind movement in the orchard and increase heating efficiency: they trap the rising heat beneath their canopies, giving a more rapid temperature lift than can be obtained among small trees.  The small amount of radiant heat from the firepots will impinge more on larger trees than on small ones, giving a rise in temperature to any part of the tree receiving such radiant heat.

[The frost book goes on with specific sections on “Types of firepot”, “Placement of firepots”, “Lighting firepots”, “Filling firepots”, “Lighting torches”, and “Fuel storage”.  Please contact the Editor if you would like to obtain a copy of sections not included here.]

Centralised Distribution Systems

Specifications for an ideal orchard-heating system are known, and such systems are technically feasible.  Unfortunately, capital costs are high and in many situations still prohibitive.  Such a system should be permanently installed in an orchard, so that normal orchard operations are not disrupted.  It should have a central fuel supply for the burners, eliminating the need for manual refuelling during and after operation.  The burners should be simple yet robust in design, burn the fuel efficiently without producing smoke and soot, produce a maximum of radiant heat, and be easy to light.  The system should be fully automatic, igniting at predetermined temperatures, and regulating the burning rate during operation to compensate for variations in temperature.

[Further information is also provided on this topic in the frost book.]

Water sprinkling

Water sprinkling has been recognised for over 40 years as a useful means of preventing frost damage to plants.  Water has been used successfully to prevent the freezing of fruit trees, grapevines, berry fruits, vegetables, ornamentals and flowers, and the practice is spreading in horticultural crops throughout the world.  Sprinklers used for frost fighting can also be used for irrigation, for preventing high temperature damage, and with some modification, for application of spray materials and fertilisers.  Despite the initial high capital costs, the low running costs and the multi-purpose nature of the system make sprinklers economically feasible.

Greater risks are still involved in the use of sprinklers for preventing frost damage than with heating methods.  With heaters, improper use will result in no more than partial crop loss, but, with sprinklers, faulty operation can result in the complete loss of a crop.

Theory of water sprinkling

The principle involved in frost protection of plants by sprinkling is relatively simple.  As water freezes and changes from the liquid to the solid state it gives up a finite quantity of heat, the latent heat of fusion.  A total of 334 kJ is produced for every litre of water frozen.  As long as water is being frozen continuously and latent heat being given off, the temperature of the surface over which the ice is forming will remain at or about 0ºC.  Heat is not transmitted from the ice-water surface to the plant regions, but because this ice-water surface remains at 0ºC, anything inside is prevented from falling below this temperature.  As this is above the critical killing temperature of blossoms and fruits, continuous application of a sufficient quantity of water to these organs will protect them from freezing damage.  Table 3 shows the amount of heat released at different precipitation rates.

TABLE 3.  Heat released by water sprinkled at different precipitation rates

Not all of the sprinkled water freezes on the plants, and the effectiveness of sprinkling depends on the amount of water retained on the vegetation.  The efficiency of water retention depends on the crop and the type and amount of foliage present.  Much of the water falls on the ground, freezes, and gives off heat, and with continuous water application the heat released in this way can itself increase the air temperature by 0.5 to 1.5ºC.

The temperature of the water used for sprinkling has little effect on the protective capacity of the system.  When a litre of water decreases in temperature by 1ºC (say from 10ºC to 9ºC), 4.2 kJ of heat is liberated; so in cooling from 10ºC to 0ºC only 42 kJ is given off.  This is only a small fraction (13%) of the 334 kJ given off when the same amount of water changes to ice.  Better protection will be afforded with cool rather than warm water, as less runoff from the plant is likely, more ice will form, and more heat will be released where it is most useful.

[The frost book continues with sections on “Crops that can be protected” (most fruit trees, vegetable and flower crops), “Water supply”, “Design of the sprinkler system” (the normal height of the riser pipes for overhead sprinkling of fruit trees is 4-5m, which would only protect very young walnut trees; the section also describes sprinkler spacing and design of the sprinkler heads), “Precipitation rates”, “Installation and maintenance”, and “Side effects of sprinkling”.]

Other methods

Firepots and sprinklers are the only two methods used to protect orchards from frost damage in New Zealand [in 1971].  Several other methods are being used in other countries, and two of these have been tested in Central Otago but were found unsuitable there.  However a brief summary of the results obtained are presented here.

Wind machines

Wind machines have been used for many years on a large scale in certain parts of North America to protect fruit trees, particularly citrus trees, from frosts [and are now commonly used in New Zealand to protect, for example, vineyards].  However, they have been successful only in limited areas where definite climatic and topographical features exist, e.g., in Florida and certain parts of California.  Where conditions are suitable wind machines offer considerable advantages when compared to orchard heating: they reduce labour requirements, require less refuelling and less fuel storage, are permanently located in the orchard, have low running costs, and operate cleanly with no air pollution.  The disadvantages include the high capital costs and the occasional failure to protect the crop.

For wind machines to be effective a relatively strong temperature inversion with a low ceiling must exist.  The propellers mix the warmer air from above the trees with the colder air near ground level.  They produce little or no heat themselves, but depend entirely on the layer of warm air above the orchard to provide protection.  The net effect of their operation is to slow the rate of temperature fall and to maintain a safe temperature in the fruits to be protected.  This system is not effective with winds greater than 8 km/hr or when the inversion layer is weak.  Their main use has been against the relatively mild winter frosts that occur in citrus-growing areas of the United States.

Preliminary investigations in Central Otago indicated that a wind machine would not be effective as a permanent single means of frost control because of the large variations in the height and strength of the inversion ceiling.  On several nights during most seasons, conditions exist where the air temperature at a height of 15m, although considerably warmer than the air at tree level, is below 0ºC.  On such nights there is no warm air within reach of wind machines, and no amount of mixing will prevent severe damage to flowers and fruit.  [However, in the present day in New Zealand, it is common to use helicopters hovering over the vineyards for mixing the air layers and driving the warm air downwards; clearly helicopters can be placed at whatever the height of the warm air on that particular night].

At Earnscleugh, Central Otago, a wind machine driven by a 44hp tractor raised the temperature by 1ºC over an area of approximately 1 hectare, and by 0.5ºC over a further 1.5 hectares.  Temperature inversion during these tests ranged from 2.8–4.4ºC warmer at 15m than at 1.2m above ground level, but this variation of inversion had little effect on the area protected.  The maximum increase in temperature recorded at 1.2m above ground level was 1.7ºC, but this was over only approximately 0.1ha.

From these results it was concluded that neither the particular wind machine tested nor the general technique would reliably and economically prevent frost damage in Central Otago.

Artificial fog production

Natural clouds act as blankets to outgoing radiation.  The water vapour present in the cloud re-radiates the outgoing radiation back towards the earth and often prevents the temperature from falling to danger point.

Many attempts have been made to produce artificial cloud or fog locally as a means of preventing frost damage.  While it is easy to produce fog opaque to visible radiation, it is difficult to produce one with particle sufficiently large to prevent passage of the long-wave, infra-red, terrestrial radiation.  For artificial fog to be effective in preventing frost development, the water droplets in the fog have to be from 10 to 20 microns in diameter.  The production of a sufficient quantity of particles in this size range could prevent frost.

Oil and chemical fogs produced by special generators have been ineffective, because either the particle produced are too small or the components do not have the necessary properties to reflect infrared radiation back to earth.

Experiments have been carried out in New Zealand with a machine designed to produce a diesel oil–water fog reputed to be effective in preventing frost damage.  Although impressive amounts of fog were produced, no temperature increase was recorded.

Although a suitable water fog produced economically could have some practical use, the problem of reduced visibility on highways, railways and airports may well prevent widespread acceptance of such a system.  Most frost nights are accompanied with slight air movement, and it would probably be difficult to keep the fog over a given area.  A forecasting system would also be necessary if such a system were adopted, for the fog would have to be in place over the area to be protected before the air temperatures reached a critical level.

Solid fuel

Recently, in the United States, the use of solid fuels in orchard heating has increased.  These fuels are made of a range of materials, including wood, compressed sawdust, compressed paper, petroleum solids, and wax.  Capital outlay is less for solid fuels than for oil.  It is impossible, in most cases, to control the burning rate or to put out the fires easily.  Solid petroleum fuels burn without smoke, but they are not yet available in New Zealand.  Production costs for the small amount that would be used prevent their manufacture locally, and to date the cost of importation has been uneconomic.

Chemicals and frost resistance

Recently attempts have been made to use a range of plant growth regulating chemicals either to increase frost resistance of fruit trees or to delay the development of blossom until the danger of spring frosts is past, or at least reduced.

Reports have been published on a chemical (decenyl succinic acid) which increases the frost resistance of apple and plum blossoms.  When sprayed on immediately before frost, it apparently reduced the percentage of blossoms killed.  This material has been tested for two seasons in Central Otago, but although a slight degree of resistance appears to be conferred sometimes (up to 0.5ºC), the frosts usually experienced there require an increased resistance of 2.5–3.5ºC.

A delay in blossoming of 2 to 3 weeks could reduce or eliminate risk of frost damage in some varieties.  Various chemicals have been used with success in this field, although in general a delay in blossoming has been accompanied by damage to the rest of the plant, with a reduction in total crop.  An understanding of the nature of the plant’s dormancy and blossoming and the development of new chemicals based on this knowledge seems to be a promising avenue for future work.