E. W. Hewett
NZ Department of Scientific and Industrial Research Information Series No. 86
Printed: Wellington, 1971

[This is the fourth and final part of our series on frost protection for fruit trees taken from the 1971 book.]

OPERATION OF FROST-PREVENTION SYSTEMS

Starting temperatures for protection

Factors discussed in the section on frost damage to plants [published in Health in a Shell, Issue No. 69] make it very difficult, even dangerous, to make definite rulings on damaging temperatures for different fruits.  Because of plant variability, many of the temperatures listed as dangerous to various crops have been somewhat conservative; in some years this conservatism may be expensive, as firepots may be lit on occasions when the crop was hardy enough to withstand the cold.  However, frost fighting is an insurance, and growers who have orchard heating equipment cannot afford to allow frost to develop without lighting.

Uneven development of floral buds in deciduous fruit trees up to the full bloom stage makes the possibility of losing the entire crop remote.  Depending on the season, full bloom may occupy a few days to 2 weeks, and a moderate frost during this period is unlikely to destroy all the flowers.  However, a series of heavy frosts will sequentially damage susceptible stages and can drastically reduce or destroy the crop.

The post-bloom period is the most critical.  From petal fall onwards the entire crop, particularly apricots, tends to be very susceptible to low temperatures, and the whole crop can be destroyed by one severe frost.  At this time frost fighting must be conducted with maximum efficiency.  The duration of the frost becomes almost as important as its severity: for example, apples at the small green fruit stage will stand -1.5ºC for up to 30 minutes; however, if the temperature has been at -1ºC for more than 1 hour, substantial damage is likely to occur.  This may not result in the death of the fruits, but their marketability will be reduced because of russetting and distortion.

Once the small green fruit stage has been reached, many growers ignore thermometers and determine when to begin frost fighting by the condition of the fruit on exposed surfaces high on the tree.  With cherries and apricots, exposed fruit shows a dark green mottling on the surface of the fruit at temperatures between -1 and 0ºC.  As soon as this appears firepots are lit, and the temperature within the block is maintained above 0ºC.  However, it is not desirable to let mottling occur; it is preferable to have a thermometer in an open space away from the trees or in an exposed position near the top of the trees, and to begin protective measures before the temperature drops to the critical level.

Where sprinklers are used for frost protection, the system should be started at a higher temperature than that for lighting firepots.  For sprinklers to be effective, water must be freezing evenly and continuously on the exposed surfaces to be protected; 30 minutes of sprinkling is necessary to wet the trees completely, and until a continuous film of ice is formed over each flower, fruit and leaf protection is incomplete.

The information presented in Tables 5 to 8 giving temperatures for tree fruits at which to light firepots or to start sprinkling is intended as a guide only [parts of these tables have been reproduced in this article].  Firepots are lit according to the temperature recorded on a covered thermometer, whereas sprinklers should be started according to the temperature recorded on an exposed thermometer.  This means that the sprinklers will be started at an air temperature 1–2ºC above that for firepot lighting.  The tables tend to be conservative, allowing for those years when susceptibility is likely to be greatest because of favourable growth conditions.  All figures are given for temperatures that the fruits named will withstand for 30 minutes.

As sprinklers are turned on at higher temperatures than those at which firepots are lit, they operate longer than firepots and are used on some nights when firepots would not be lit.  It has been found that over a whole season sprinklers have to be operated two to three times as long as firepots in Central Otago.
Firepots protect trees as soon as they are lit, and much more flexibility is possible in their lighting than with sprinklers.  The decision to light firepots depends on the duration of critical temperatures, the time left until sunrise, the presence or absence of air movement, the direction of drift, the temporary presence of clouds in the sky, and the grower’s observation of the condition of the trees and fruit (for example, dew formation or fruit mottling).  Sprinklers on the other hand, should be started at a predetermined temperature, which depends on the average stage of development of the crop on a given night.

Wet fruit

Temperatures listed in Tables 5 to 8 apply only to dry fruits.  When frost develops while trees are wet from rain the danger points are 0.5–1ºC higher than the values indicated in the tables, and protection measures should be started earlier.

When to cease protective measures

Frosts generally reach greatest intensity just before or at sunrise, and the temperature tends to drop suddenly, by up to 1ºC, at this time.  Before sunrise it is essential to ensure that all firepots are burning adequately and that there is enough fuel remaining to combat this sudden drop in temperature.

Once the sun has risen firepots can be extinguished safely when the temperature recorded on a covered thermometer outside the protected block and away from the influence of the firepots has reached 0.5ºC above the critical temperature.  In certain areas large masses of smoke tend to drift back over the property after sunrise and the temperature is liable to drop quickly.  Careful observation of the direction of smoke drift at this time should be made by growers in difficult areas.
Fluctuations in air temperature frequently occur during the night as small masses of warmer air drift onto a property or where small clouds appear in the sky.

  Again, careful observations must be made to assess such effects before burning rates are reduced or firepots extinguished.  A rising wind is usually a sign of a large-scale change in the weather, and putting out the pots may be justified.
Once sprinklers have been started they should be left on all night, except where a major change in the weather is obvious.  If a strong, warm wind begins and continues to blow steadily, the sprinklers could be turned off when the temperature rises to 2ºC on an exposed thermometer.  Should the wind drop again before sunrise, sprinkling would have to be started again, and adjustment of the starting temperature would then be necessary to allow for the trees being wet.
Sprinklers may safely be turned off once the sun is up and the air temperature outside the protected block has risen to 0ºC, with the thermometer continuing to rise steadily.  It is not necessary to leave the sprinklers on until all the ice has melted from the trees, unless daytime temperatures remain just above freezing for a long time.  Under these conditions sprinkling should be continued until the warmer water melts the ice.

Thermometers

Accurate temperature readings are the basis of successful prevention of frost damage.  Without accurate and reliable thermometers no grower can consistently protect a crop from frost damage.

Thermometers should be tested carefully before each season.  The simplest way to do this is to immerse the bulb in a mixture of clear ice chips and either distilled or rain water.  Leave for 10 minutes, stirring the mixture occasionally.  An accurate thermometer will indicate 0ºC.  If there is a small error it should be marked clearly on the metal frame of the thermometer, so that readings can be corrected throughout the season.  If the error is greater than 0.5ºC, the thermometer should be discarded.  Frost alarms should also be carefully tested in the ice-water mixture before the season, and the batteries and connections should be inspected.

Thermometers must be properly distributed within a block.  At least one thermometer is required for every 1–1.5 ha, preferably one in each different variety or type of fruit to be protected.  Each should be located in the coldest spot in its block.  A reference thermometer outside and not influenced by the protected area is essential to show when temperatures start rising at the end of a frost.

Where firepots are used thermometers should be placed in shelters, where they will record the ambient air temperature, not the temperature of the exposed buds, blossoms or leaves.  These shelters should be placed within the block, in most cases near a tree, so that thoroughfare is not interrupted.  Shelters should be painted white; this reduces cooling by radiation at night, and also makes them easier to locate.  No firepot should be placed in the tree space next to the thermometer, as direct radiation from the flames would result in misleading temperature readings.

Shelters can be made very simply, from two boards 25 × 45 × 2.5cm, fastened together at right angles at their longest sides to form a top and a back.  The shelter is fastened to a post, which is hammered into the ground so that the thermometer is 1.4m above the ground.  The thermometer itself is fastened to the back, 10cm down from the roof in a slightly sloping position with the bulb being 1cm lower than the upper end; this discourages alcohol separation [for alcohol thermometers, the main type available in 1971] and also helps reduce the friction drag on the minimum recording index, which is likely to penetrate the meniscus if the thermometer is mounted in a horizontal position.

An electric torch should always be used when making readings of thermometers.  Breathing on the thermometer should be avoided, as should the use of a naked flame, as temperature readings could be raised 0.5–1ºC.

Where a sprinkler system is installed, a thermometer exposed to the sky should be used for determining when to start frost fighting.  The exposed thermometer will record 1–2ºC lower than a covered thermometer because it is radiating directly to the sky, but it gives a close indication of the actual temperature of the exposed buds, blossoms and fruits on the trees.  These thermometers may be placed on the top of a shelter or exposed at an easily accessible position in a tree or on top of a ladder.

Frost alarms

All methods of frost protection rely on the use of an alarm to give warning when temperatures drop near to the danger point.  Obviously such an alarm must be accurate and reliable.  Several different types of alarms are available commercially at varying prices, and most are satisfactory.  It is desirable that the alarm has a variable temperature setting.  Models are available that indicate the condition of several sensors, which can be vitally important on large properties, as a grower can locate the coldest block by moving a manual switch, and take protective measures accordingly.  The best alarm is one that breaks a circuit at a predetermined setting as the temperature is lowered.  This enables the alarm to be tested each night with a simple switch, and means that any fault in the system will operate the alarm, giving warning of the fault.

The sensing thermometer of an alarm should be placed at crop height.  It should be placed within the crop to be protected and well away from buildings and windbreaks.

Where sprinklers are used it is possible to have the system automated.  Variable setting thermostats are available that can switch on the system at a predetermined temperature and that will shut off automatically at a safe temperature above the critical temperature of the crop to be protected.  It is advisable to have such an installation wired in with an alarm so that a quick check can be made by the operator to ensure that all sprinklers are operating properly.  Automatic starting of sprinklers can be achieved only with self-priming or submerged pumps.

Keeping records

An essential part of successfully preventing frost damage is the keeping of accurate records.  Information recorded during a frost is extremely valuable for future guidance and should be readily available for quick reference.  Memory has its shortcomings and should not be relied upon.

Records should include the following: date; weather conditions of the preceding day and during the frost; minimum temperatures recorded within the protected blocks and from reference thermometers; occasional checks of temperatures throughout the night; the times and temperatures at which protection starts and finishes; the time involved in protection; the number of firepots lit and in what sequence; the amount of fuel burnt; the average stage of development for different varieties; and any unusual occurrence.  After each frost a note should be made of frost damage in different varieties.

If a frost protection system fails, it should be possible to check back through such records and determine the reasons for failure.  Steps can then be taken to ensure that a similar failure does not occur again.

[A section on the costs of sprinklers and firepots has been omitted as these costs are out of date.]