Frost Protection - UTL Repository

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] F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S [ there is no known research on the topic, another possibility might be to fog or mist the air rather than use sprinklers. This could cool the air without adding water to the soil. However, this may or may not be cost effective depending on the frequency and intensity of freezing in the area. CHEMICALS TO DELAY BLOOM Cryoprotectants and antitranspirants are sold and used as protection against frost damage. However, none of these materials has been found to consistently give protection to flower buds, flowers, small fruits or small nuts. The ethylenereleasing growth regulator “Ethephon” increases bud hardiness and delays flowering 4 to 7 days if applied in the early autumn at the onset of chilling (Powell and Himelrick, 2000). It has been used on peaches and cherries. Gibberellic acid delays bloom of some crops, but multiple applications are needed and it is expensive. Gibberellin or alpha naphthaleneacetic acid applications during warm days in late winter and spring are known to delay leaf out (Nigond, 1960; Schultz and Weaver, 1977). Using growth regulators to reduce cambial activity and lengthen dormancy helps both evergreens and deciduous trees to tolerate subzero temperature. It is generally accepted that a retardation of growth reduces cell elongation. And the smaller cells have higher concentrations of solutes, which help them to avoid freezing. PLANT COVERS Plant row covers increase downward long-wave radiation at night and reduce losses of heat to the air by heat convection (and advection). Covers must have a low coefficient of conduction and ideally would be opaque to long-wave radiation. Dry soil has a lower thermal conductivity, so it is sometimes used to cover small plants (e.g. potato, tomato and coffee plants) or to protect trunks of young trees during relatively short subzero periods. In some countries with severe winters, soil is mounded up to cover the graft of young citrus to protect the trunks from frosts (Blanc et al. 1963). Removable straw coverings are used extensively in Switzerland for frost protection of the grapevines. However, because of easier application, straw is being replaced with synthetic materials. Both types of covers are left on the plants until the danger of the freezing is gone (Peyer, 1965). Mats and other insulating materials have also been used in India to protect tea (Camellia sinensis) plants from freezing (Von Legerke, 1978). In Portugal, individual plant protection methods include (1) horizontal or inclined mats for young trees; (2) shelters of diverse form for small plantings of citrus or garden shrubs; (3) wraps 130
PASSIVE PROTECTION METHODS of culm rolled around trunks for young trees; and (4) roofing tiles, adobe shelters, leaves of plants, etc., for small plants. For rows of plants, the methods include (1) larger horizontal or inclined mats for tree rows; (2) shelters forming a half hut with vertical wall facing the predominant wind direction; and (3) straw layers over horticultural nurseries, where the mats and shelters use local materials (e.g. straw, bamboo, wood, boards, hay, etc.) (Abreu, 1985). Although the materials used for coverings generally are inexpensive, the manpower needed to apply the materials can be cost prohibitive. Generally, this method is only used on small plantings or on small plants that do not require a solid frame. Sometimes, disease problems occur due to poor ventilation. Row covers are sometimes used for protection of high value crops. Woven and spun-bonded polypropylene plastics are typically used and the degree of protection varies with the thickness of the material (e.g. from 1 °C for thin sheet plastic to 5 °C for thick plastic). White plastic provides some protection and it is sometimes used for nursery stock. It is not typically used for fruit and vegetable crop protection. Schultz (1961) reported that 1.2 m wide, black polyethylene sheets were used to cover grapevine rows and it increased the air temperature next to the foliage by about 1.5 °C. Transparent plastic covers allow sunlight to pass through during the day and slow heat loss from the surface at night. The downward radiation from the sky at night depends on the apparent temperature of the sky, so when covered with plastic, the downward radiation depends mainly on the plastic cover temperature. Since the sky is much colder than air near the ground and the plastic will have a temperature closer to the air temperature, the downward radiation is enhanced by covering the plants. If condensation forms underneath the plastic, this will release latent heat, warm the plastic and provide even more protection. Under advection frost conditions, the plastic covers can also block the wind and provide some protection. Some characteristics for above-plant row covers are provided in Table 6.1. A wide variety of methods are used to cover the plants and to anchor the plastic. To keep the plants from being touched, plastic covers are sometimes mounted on hoops. Otherwise, the plastic can float on the canopy and rise up as the crops grow, but disease problems are more likely. PVC greenhouses are sometimes used to protect citrus. The plastic can be used up to three years depending on the structural design and quality of the plastic. A common problem is that the labour requirements for applying covers are high and therefore the crop value must be high. Also, the plants become less hardy against freezing and there are often problems with pollination if the covers 131
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- F A O E N A N D V I MO N I T O R
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FOREWORD Agrometeorology deals with
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ACKNOWLEDGEMENTS We want to thank D
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LIST OF PRINCIPAL SYMBOLS Roman Alp
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T n °C Observed minimum temperatur
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CHAPTER 1 INTRODUCTION OVERVIEW Whe
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INTRODUCTION Freeze and frost defin
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INTRODUCTION FIGURE 1.2 Mean air an
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INTRODUCTION FIGURE 1.4 Air (T a )
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INTRODUCTION TABLE 1.2 Categories a
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INTRODUCTION example, because bloom
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INTRODUCTION south in Florida in re
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INTRODUCTION assistance might be in
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CHAPTER 2 RECOMMENDED METHODS OF FR
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RECOMMENDED METHODS OF FROST PROTEC
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CHAPTER 4 FROST DAMAGE: PHYSIOLOGY
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FROST DAMAGE: PHYSIOLOGY AND CRITIC
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CHAPTER 8 APPROPRIATE TECHNOLOGIES
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APPROPRIATE TECHNOLOGIES frost prot
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APPROPRIATE TECHNOLOGIES fashion. O
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES other crop
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APPENDIX 2 CONSTANTS Specific heat
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- FAO ENVIRONMENT AND NATURAL RESOU
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