[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.]

INTRODUCTION

Frost in spring is a climatic phenomenon that imposes geographical limits on many horticultural crops in the temperate zones of the world.

Notwithstanding the risk of damage from frost, many orchards have been planted in areas that are otherwise favourable for fruitgrowing.  Unseasonal spring frosts can, in a few short hours during the night, completely destroy the year’s production, if some form of protection is not undertaken.  Consequently much effort has been devoted to developing efficient methods of protecting horticultural crops from frost damage; no one method has yet been perfected, and work is continuing.

Parallel to this effort, many people are working to understand the fundamental effects of ice formation in plant tissue, and attempts are being made all over the world to breed varieties with inherent resistance to cold.  Unsuccessful efforts have been made to find methods that might either increase the resistance of plant tissues to cold or prolong the dormant period enough to allow the blossom to develop under less unfavourable conditions in spring.

Crops may be protected from frost damage by indirect or direct procedures.  In some cases the best protective measures are indirect, that is by planting crops where there is no frost risk or by planting varieties that can withstand freezing or low temperatures.  A thorough knowledge of the frequency and severity of frosts is necessary in such cases, and this information can often be obtained from local experienced farmers or the Meteorological Service.  Other indirect methods of preventing frost damage include careful and skilled manipulation of cultural practices and plant management.

Direct protective measures are applied when damaging frosts are imminent.  Such methods include conserving heat, adding heat, or stirring the atmosphere surrounding the crop.

Central Otago is the most frost-susceptible orchard area in New Zealand, although damaging frosts do occur in some years in Canterbury, Nelson and Hawke’s Bay.  A substantial proportion of New Zealand’s stone fruit, including the bulk of apricots, are produced in Central Otago.  As apricots, cherries, plums, nectarines and peaches blossom earlier in spring than apples and pears, they are more susceptible to frost damage; consequently more care and effort is taken in protecting them from frost damage.  Strawberries, raspberries and boysenberries, all grown to a lesser extent, must also be protected from frost.

Before 1930 very little was known of frost protection methods, with the result that fruit crops (and orchardists’ incomes) fluctuated markedly from year to year depending on the vagaries of the spring climate.  With the advent of the lard-pail frost pot, growers were more or less assured of regular crops, although initial damaging temperatures for different fruits were known only empirically.  At present [1971] use of the lard-pail type of frost pot is still the most widespread method of preventing frost damage.

Since the setting up of the Department of Scientific and Industrial Research experimental orchard at Earnscleugh in 1946, many different methods of protecting fruit trees from frost have been investigated.  Many different types of fire-pots have been evaluated; a wind machine has been tested; a fog-producing machine has been tried and found wanting; sprinklers have been under test for many years; the new concept of a centralised oil distribution system is currently being evaluated; and any new ideas that arise either in New Zealand or overseas are thoroughly investigated, and tested if considered practical for New Zealand conditions.  In conjunction with this work, detailed meteorological records have been taken in an attempt to understand the various climatic parameters associated with frosts in Central Otago.  Investigations into damaging temperatures have been continued, and the seasonal variations in cold hardiness have been followed, with particular attention to the marked increase in frost susceptibility of apricot flower buds in the late winter–early spring period.

THE NATURE OF FROST

Frost is generally associated with the sight of feathery ice crystals on exposed surfaces, the temperatures of which have fallen below the freezing point of water.  In simple terms, and for the purposes of this bulletin, frost is defined as a condition in which the air temperature falls below 0ºC.

Types of frost

There are essentially two types of frost:

Advection frosts are primarily due to the influx of large air masses with sub-freezing temperatures which are of polar origin and are accompanied by strong winds and low humidity.  Cold conditions usually persist for several days.  These “freezes” or “wind frosts” occur in continental areas of Europe and North America, causing widespread damage to many plants.  Protection of plants from these severe conditions is extremely difficult, and often impossible, especially in large orchards.  Some protection may be afforded by covering or enclosing, but such measures cannot at present be undertaken on a large scale.  Fortunately advection frosts occur very rarely in New Zealand; they will not be dealt with in this bulletin.

Radiation frosts are caused by loss of heat from the earth’s surface by radiation.  Such frosts occur frequently during autumn, winter and early spring in temperate zones.  Conditions favouring radiation frosts are a clear, dry atmosphere with calm conditions or light winds.  In the Southern Hemisphere they are associated with slow-moving anticyclones.

Most of New Zealand’s weather is affected by a series of low-pressure cyclones or depressions followed by high-pressure anticyclones, moving from the west.  The depressions bring northerly winds with warm and moist air.  As a depression moves toward the east, wind direction changes to the south, and cold southerly air floods the country.  The wet and cloudy weather associated with the depression disappears, and is replaced by clear, calm, cold weather, especially over inland areas removed from the influences of the sea.  Frosts will occur in autumn, winter, and spring, when the barometer is high and rising.  Chances for the occurrence of frost are accentuated if snow has fallen on the surrounding mountains or country lying to windward.  Fig. 1 illustrates the weather conditions over New Zealand that are likely to result in frosts.

Radiation frosts are common in New Zealand, and measures can be taken to reduce or prevent frost damage to plants where necessary.

Daily heat balance

All objects continuously radiate energy.  This energy is called radiant energy and is in the form of waves which travel with the velocity of light.  When they fall on an object that is not transparent to them (such as the ground or a plant) they are absorbed and their energy is converted to heat, with the result that the temperatures of these objects rise.

The sun provides a tremendous supply of radiant energy which during the day heats the ground and all objects on it.  These in turn heat the overlying air.  Although the ground and other bodies still continue to radiate heat from their surfaces, this is far less than the heat received from the sun; therefore, by day net radiation is positive.  As a result, air temperatures will be greatest nearer the ground and decrease with altitude (Fig. 2), the temperature decreasing approximately 2.8ºC per 300 m of altitude.  Warm air is lighter than cold air, hence convection currents are created.  As the warmed air rises cool air sinks, continually stirring the lower layers.

At night, however, the situation changes.  Very little radiation is received by the earth from the atmosphere and clouds, but all objects on the earth continue to radiate energy (or heat) in the same manner and in the same magnitude as they did during the day.  The ground surfaces and exposed objects become colder than the air, and in turn cool the air in contact with them.  Thus, at night, the net radiation is negative.  The air near the ground cools gradually during the night.  This cooler air remains in contact with the ground forming a stable layer giving a reversal of the daytime temperature gradient.  Some hours after sunset a temperature inversion is established, the lowest temperature being at ground level, and the temperature increasing with height up to the inversion ceiling (Fig. 3).  Such temperature inversions may have a ceiling as low as one metre or may be a hundred metres in height.  Above the inversion ceiling the temperature becomes colder with height, as it does by day.

The height of the temperature inversion, which varies greatly from locality to locality and from night to night, is largely determined by the conditions of the preceding day.  If the day was clear and calm with high temperatures, there will be a gradual fall in temperature to freezing the following morning, with a low inversion ceiling.  If, on the other hand, the preceding afternoon was cold with strong winds, effective stratification of the air layers is prevented and the cold air mass will extend to a considerable height with a weak or high inversion ceiling.  Such conditions often occur before severe frosts in Central Otago.  Strong, cold southerly winds blow during the day under anticyclonic conditions; in early evening the winds drop, but the atmosphere is very cold; the sky clears, and a radiation frost with a high inversion ceiling develops early in the night.

If appreciable ground winds occur (greater than 8 kph) at night, the air layer near the ground becomes turbulent and is mixed with the warmer air above.  This destroys the inversion temperature gradient, and the temperatures near the ground increase.  Such winds either prevent a radiation frost from developing or terminate existing frost conditions.