[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.]

FACTORS AFFECTING RADIATION FROST DEVELOPMENT

A number of factors, both climatic and cultural, affect the development, longevity, and severity of radiation frosts.  An understanding of these factors is essential in selecting and operating various methods of preventing frost damage to crops.

Season and latitude

The cold, short days and long nights of winter prevent the sun from building up a heat store in the ground.  The longer the time from sunset to sunrise, the longer there is for frosts to develop; thus frosts are common in winter.  In summer, the sun heats up the soil during the long days, and the nights are too short to permit great heat losses, hence frosts are infrequent.  During the transition seasons of spring and autumn insufficient heat is stored to prevent frosts, or the heat is lost during relatively long nights.

Latitude plays an important role in the degree and number of frosts in any particular locality (see Table 1).

Cold air flow

Cold air is denser than warm air, hence it tends to flow from mountain ridges and hillsides to valleys and plains.  These nocturnal air flows, known as katabatic winds, seldom exceed 1-5 km/hr: given suitable topography they contribute appreciably to the build-up of a stable cold air layer (already started by the prevailing radiation conditions) on flat, low-lying land.  Snow on surrounding mountains contributes to cold air drifts.

Cold air forms pools behind barriers.  Shelterbelts placed above an orchard may lessen the frost risk by diverting the flow of cold air from susceptible crops.  Conversely, barriers downhill from a planting can contribute markedly to the frost risk by causing a pooling of cold air; removal of dense downhill shelters will improve air drainage.

When new orchards are being planted or existing blocks replanted in frost-susceptible areas it is important to choose an appropriate fruit variety for a particular area.  A temperature survey should be made, and patterns of air flow should be noted.  In places where frosts are not severe (on higher areas of land) early blossoming and frost-susceptible varieties such as apricots should be planted.  In intermediate areas later-blossoming and more frost-resistant varieties (e.g., some peaches and nectarines) should be planted.  Apples and pears which are more frost-resistant could be planted in low-lying areas of land or in the bottoms of gullies where frosts are generally most severe.  Providing soil types are suitable, such a planting plan should ensure optimum tree growth, and reduce the need for frost fighting to a minimum.

Clouds

Clouds affect frost development.  Water vapour present in clouds acts as a shield to the outgoing radiation; it absorbs and then re-radiates part of the energy back to the earth’s surface.  This back-radiation effectively reduces radiation loss.  Therefore, the presence of low cloud or thick layers of ground fog will prevent severe frosts.

Very high clouds are often composed of small ice particles, which send little radiation back to earth.  With such conditions frosts may occur on entirely overcast nights.

The effect of small banks of cloud during an otherwise clear, cold radiation night is well known to orchardists.  Air temperatures may increase by several degrees, a temperature rise seemingly out of proportion to cloud size, but unless a larger amount of cloud gathers, this increase will only be temporary.

Water vapour and humidity

Nocturnal radiant heat loss from the earth’s surface is regulated in part by the amount of moisture or water vapour in the atmosphere.  In general the greater the amount of water vapour present, the slower will be the temperature drop.

The temperature at which water vapour in the air condenses (or dew forms) is known as the dew point.  Dew point, an indirect measure of the amount of moisture in the air, is always expressed as a temperature.  If the dew point is 2ºC on a particular night, dew will be deposited on exposed surfaces when their temperature reaches that point.

When the dew point is above 0ºC, dew will form at the appropriate temperature.  If the temperature continues to drop further, the dew will turn to ice crystals.  In each of these two processes, dew formation from the water vapour and ice crystal formation from the dew, considerable amounts of heat are given out, and a marked check occurs in the rate of air temperature fall.  After these changes have been completed, the temperature will begin to fall again, often more rapidly than before.

When the dew point is below 0ºC, the water vapour is deposited on the exposed surfaces as ice crystals.  If the dew point is very low, indicating very dry conditions, then no ice crystals will be deposited until the dew point is reached, even though this may be well below freezing point, and below the damaging temperature of a particular plant organ.  In this case considerable frost damage can occur without the formation of ice crystals on the surface of the plant.  This is known as a black frost.  Fortunately, low dew point conditions do not occur very frequently in New Zealand so black frosts are rare.

Aspect

Properties with a northerly aspect have less frost risk than those with a southerly aspect, because in the former the sun builds up a greater store of heat in the soil.

Large, flat areas are usually undesirable for growing frost-susceptible crops.  Areas high on hillsides can be undesirable also, as temperature decreases with height.  Intermediate areas, on a slight slope facing north with adequate air drainage are generally best.

Terraces along river banks or in gorges are ideal for growing fruit, provided they receive adequate sunlight.  Here there is almost invariably a fairly strong air drift during the night as air from higher altitudes flows first down to the river and then unimpeded away downstream.  In such areas turbulence is set up, no cold air strata are built up at ground level, and frosts seldom develop.  The Cromwell Gorge in Central Otago, famed for its apricot production, provides good examples of such a terrace effect: orchards are on narrow terraces about 30m above the Clutha River and have a low frost risk.

Heat flow in the soil

Any cultural practice that can reduce the nocturnal temperature drop of the ground surface will reduce frost development.

Heat loss from the soil at night is very much affected by the condition of the soil.  Because of its higher heat conductivity, a compacted soil allows more heat to escape from its surface than does a loosely cultivated soil.  The upper layer of cultivated soil has many air spaces; trapped air, being a good insulator, prevents heat loss.  Compaction of soil by rolling can make the air immediately above the surface 1–2ºC warmer than above a loosely compacted soil.

Water can store considerably more heat than air.  When the air spaces in the soil are filled by water, from either rain or irrigation, the conductivity is increased, more heat flows to the soil surface, and the air temperature does not drop as rapidly or as low as it does over dry soil.  Irrigating the soil can result in an increase of 0.5–1.5ºC in air temperature at the soil surface.  Although this practice of early-season irrigation is used by some orchardists in New Zealand, it is not recommended where a series of frosts is likely.  Evaporation during the day following irrigation removes heat from the soil and prevents daytime warming; this results in wet soil being colder than dry soil the following night and therefore more subject to frost.  Consequently surface irrigation for frost protection is only useful for low-growing crops in localities where frosts are infrequent.

The slight temperature increase obtained at the soil surface by compaction and wetting can be important in low-growing crops, but the increase does not extend very far upwards and is likely to be lost at the height of mature, bearing fruit trees, through dissipation by air currents.

Effect of vegetation cover

Plant cover has definite effect on the severity of frosts.  Vegetation, such as weeds or a cover crop, forms an insulating layer at the ground surface, preventing both heat inflow to the soil during the day (the amount of heat stored is less than in bare soil) and heat outflow during the night (Fig. 4).  A cover crop also has a much greater radiating surface and a lower heat capacity than bare soil, so that, by night, temperatures drop more rapidly over cover crops than over bare ground.  Temperatures as much as 3ºC lower have been recorded over grass than over bare soil.

A high cover crop also raises the level of the radiating surface and brings the cold air layers closer to fruit-bearing branches in an orchard.  Therefore it is essential to keep grass short and any cover crop low during the frost season.

Organic mulches have the same insulating effect as vegetation cover, and while they may protect low-growing crops, such as strawberries, from frost damage, particularly if completely covered, they can increase the severity of frosts in orchards.

Damaging frosts that occur in November may appear unusual to the orchardist, as the coldest temperatures are more often found at tree-top level than on the ground.  At this time of year fruit trees are in full leaf, and the dense layer of foliage at the tree top forms a canopy which prevents efficient penetration of the sun’s rays during the day.  At night this canopy effectively screens fruits and leaves on the lower parts of the tree from the sky.  Only those fruits and leaves on the exposed upper branches radiate to the sky, and these become cold very quickly, as their heat capacity is low.  Radiation from the ground and the lower parts of the tree is absorbed and re-radiated by the canopy back onto the objects from which it came.  This results in what is commonly called a frost which “comes down”: the upper parts of the tree are the first to reach damaging temperatures, while the lower parts of the tree (and any thermometer at 1.4m under the tree) may be 1–2ºC warmer.  The upper layers of leaves and branches are acting as an “artificial ground level”, the height of which depends on the density and height of the trees, also on the wind speed and net radiation on a particular night.  In effect, the ground level is moved up to the canopy layer, and it is here that the coldest temperatures are recorded (Fig. 5).

At the time of year that these conditions are likely to occur a thermometer should be place either in a completely open space, away from the influence of trees, or in the upper canopy of the trees.  Frost fighting should be started according to the measurement of this thermometer, and not delayed until the fruit on high branches shows signs of “mottling”.  The appearance of mottling indicates that some ice has formed within the fruit, and damage to the cortical tissue will already have occurred; this should be avoided if at all possible.

Summary

Many climatic, topographical, and soil conditions influence the number and severity of frosts in different localities; the following list summarises the factors that favour high or low surface temperatures at night:

Factors favouring warm surface temperatures    Factors favouring cold surface temperatures

THE NATURE OF FROST DAMAGE TO FRUIT TREES

Fruit trees show marked seasonal variation in their resistance to cold.  Before leaf fall in autumn, longer nights and colder temperatures induce dormancy, when the flower and vegetative buds become cold hardy.  Maximum cold hardiness is attained in June, and although minor fluctuations occur, depending on the season, it takes temperatures of below -15ºC to damage flower buds on apricot trees during this period.  Peaches, apples and pears are more resistant than apricots at this time and require even lower temperatures to cause damage.

A common practice among orchardists in Central Otago is to “harden off” fruit trees in autumn by withholding water after mid March.  Growth ceases, and the trees rapidly acquire considerable cold hardiness.  Autumn frost damage to actively growing shoots is avoided and the trees become less susceptible to damage from the bacterium (Pseudomonas syringae) causing stone fruit blast.

Chilling requirements and late winter damage

A definite period of cold temperatures during winter is required by deciduous fruit trees before they can blossom and grow vigorously in the spring.  This chilling requirement, which is different for different fruits and varieties, ranges from 650 to 1,400 hours below 7ºC.  Where insufficient winter chilling occurs trees will blossom erratically over long periods, buds and blossoms may drop, and growth is reduced.

In Central Otago the chilling requirements for trees are met by the end of June in most years, but the trees do not come into blossom at this time, as dormancy is maintained by the prevailing low temperatures.  However, in some years short periods of relatively warm temperatures in late July or August cause a rapid loss of hardiness.  Some growth may occur in the tissues of the tree, although this is not visible externally.  If a period of cold conditions follows these warm temperatures, flower buds can be severely damaged.  Apricots are particularly responsive to these temperature fluctuations, and in some years up to 90% of the floral buds have been lost by the end of August.

Bud susceptibility in spring

As development occurs, from the first movement of buds through the bloom period to the stage of small green fruit, there is a progressive and marked increase in susceptibility to frost damage with each stage.

Damaging temperatures for different fruits at various stages of growth have been defined over the years by artificial bud-freezing experiments and from the accumulation of practical knowledge by orchardists.  These damaging temperatures fall into a relatively narrow range for any particular stage of development.

Climatic conditions, mainly temperature, can change bud susceptibility from season to season.  Cold overcast weather causes a long-drawn-out blossom period.  In such seasons blossoms will withstand colder conditions than in a warm and sunny season with rapid growth and short blossom period.  The increase in hardiness occurring in a spring of adverse temperature conditions can amount to 2.2–2.8ºC, although it may change rapidly if favourable weather follows.

Many factors play a part in determining the amount of damage to fruit trees in any particular season: e.g., the minimum temperature, the length of time low temperatures persist, the weather preceding a frost, the rate at which temperatures fall, and the vigour of the trees.  Blossom and fruit on a weak, poorly nourished, or diseased tree will often sustain more damage than those on a vigorous healthy tree.  For this reason an adequate fertiliser programme is necessary to maintain optimum tree vigour; over-fertilising must be avoided, as very succulent growth is more susceptible to frost than normal growth.

Damage to plant tissues is not due to the formation of frost as such, but to the effect of low temperatures on the water solution in the cells of buds and fruits.  The more dilute this internal solution the more readily freezing occurs.  As the trees break dormancy in the spring the internal cell solution becomes diluted, markedly decreasing cold hardiness.

As the temperature is lowered the water between cells freezes first.  Ice crystals grow in extra-cellular spaces, causing gradual dehydration of the surrounding cells and tissues.  If only a few cells are affected the buds may recover without serious consequences.  At some further point in the number of cells damaged the bud may recover, but the resulting fruit may be marked by cracks or russet, and probably deformed.  If the cell damage is extensive enough, the entire bud or fruit will be killed.

Not all tissues in trees are equally susceptible to cold damage.  The reproductive tissues are the most likely to sustain damage, the female parts of the flower (the ovary, style and stigma) being more susceptible than the pollen-carrying parts (the stamens).  Frost damage to buds is easily recognised by external browning of the ovary and style.  Once fertilisation has occurred, the seed becomes the most frost-susceptible tissue.  A viable seed produces the growth-promoting substances necessary for the continued growth of the fruit, so in most fruit crops death of the seeds usually causes death of the fruits, which eventually drop from the tree.  Stone fruits, such as apricot and peach, have no more than two seeds, and the slightest frost damage, indicated by a browning on the seed coat, can cause fruit drop.  If only one seed is damaged, the fruit may develop, albeit distorted.

Pip fruits, on the other hand, have numerous seeds, and loss of a proportion of the normal complement can occur without abscission, although distorted fruits may result.