Considering climates

Press, 29 August 1980, Page 5

Considering climates

GARDENING

M.Lusfv

.... Almost every property present variables which influence the selection of the plants which can be grown on it, and subsequently their development. Climatic influence is vitally important and cannot be overlooked in decisionmaking on how to utilise one’s available ground to best advantage. Lack of consideration in this regard can and does lead to a lot of disappointment; this must be expected where plantings (and sowings) are unsuited or incapable of coping with the climatic conditions to which they are subjected. Rainfall, exposure (to wind), heat, sun, shade, draught, and above all cold, particularly frosts, must be contended with to a greater or lesser degree on almost every property. It is necessary to become acquainted with all these climatic vagaries as they affect one’s piece of land in order to over-ride their effects.

There are particular plants, whether of ornamental or cropping nature, to meet most situations.

Frosts can be very damaging and effects from them can vary from partial setbacks of a very temporary nature, such as leaf “burn,” to total crop or plant loss.

While due consideration must be given to the har-

diness of ornamentals, it is even more vital when growing crops — fruit or , vegetables — especially when it- is a commercial undertaking. The number and severity of frosts will not only influence, if not dictate, what can and what cannot be grown with satisfactory results, but also the' timing, development and maturity of all plantings and sowings. And then there is always the snap frost which must be contended with, something which is again more likely in some localities than in others. Such a frost can lead to total crop loss for that particular year, if, for instance, it occurs at blossom time.

It is important to appreciate that the severity of frosts can vary, not only from one section to another but from one part of the same property to another. Influencing factors which must come under consideration in this respect include elevation, surrounding shelter and/or structure and depression of the area.

It is as well to be aware that frosts occur in two different forms: there is the wind frost, and secondly the radiation frost. The wind frost, as the name implies, is caused by winds of below freezing temperature. Such frosts can be expected to occur

in exposed places such as hilltops. Their frequency is limited, particularly later in the season. The radiation frost: to understand how this . type of frost affects plants it is necessary to explain that on clear nights there occurs a loss of heat by all openly-exposed things, including vegetation — this heat is lost through radiation.

The air is thus cooled, mainly by contact with colder objects, and an extensive period of radiation can cause a very considerable drop in temperature. Unless this is dispersed by movement (wind, fan) the ground surface accumulation of cold air builds up with a corresponding intensification of frost. Now, what happens is that this cold air displaces the warmer air, the former bearing downwards and the latter being pushed upwards. Temperature readings taken at ground level and one metre above the surface will be sufficient to indicate this difference to assist in personal appreciation of the variation of air temperatures on a cold night. On sloping ground this displacement of warm air by cold air continues to the lower levels for as long as the radiation frost prevails undispersed. Where there is a sheltered

hollow at the foot of a hill or slope, even a valley or depression in flattish land, the cold air down flow or katabatic wind as it is known, accumulates and creates what is called a frost pocket, a very frost-vulnerable area of which to beware.

A frost pocket can occur at any height and is independent of altitude; it may be small or large, sometimes a natural occurrence in the topography, and at other times induced. In both cases the cold air accumulates because there is no immediate escape for it. Sometimes it is possible to overcome a frost pocket by removing the obstruction to air flow, such

as by levelling the land, or by means of removal of misplaced shelter belts, or even a fence. Frequently it is impossible to overcome this problem because the land forms part of an. extended depression or shallow valley. So you can see that the. siting of shelter belts - sometimes demands con-, siderable thought in order not only to locate them to best advantage, but also to, avoid promoting a hazard.However, discussion of, shelter takes us beyond, the intended purpose here, other than to say that sometimes it may be better to dispense with shelter if it means that it can. impede air flow.