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 [ on temperature differences between bare soil and cover crops. Therefore, an orchard or vineyard with a grass or weed cover crop is more prone to frost damage than one with bare soil between the rows (Blanc et al., 1963; Bouchet, 1965; Snyder, Paw U and Thompson, 1987). Wide variations in the temperature effects of cover crops are reported in the literature, but they all generally agree that the presence of a cover crop will increase potential for frost damage. Snyder and Connell (1993) used an infrared thermometer and found that the surface temperature of bare soils was generally 1 °C to 3 °C warmer than soils with grass and weed cover crops taller than 0.05 m during February and March. The cover crop was killed with herbicide in early December, so the orchard floor had about two months to develop canopy and temperature differences. However, during the winter, the weather was generally cloudy and foggy. On most days, they found that the orchard floor with the grass cover was colder, but an exception was found following several days of strong dry wind. The wind seemed to dry the bare soil surface layer more than the grass-covered soil, which reduced thermal conductivity and inhibited heat storage. Following this period, the bare soil was colder than the grass covered soil. Consequently, after several days of drying wind, wetting a bare soil surface is recommended to improve heat transfer and storage. Various weed control strategies were studied to determine the effect on minimum temperature at cordon height (1.2 m) in grape vineyards in the Napa Valley of California (Donaldson et al., 1993). The methods included mowing, cultivating and using post emergence glyophosate herbicide. Mowing was done just before measurements were taken and cultivation was performed depending on weather and soil conditions. Herbicides were applied before the weeds reached 0.15 m height in late February or early March. In some cases, herbicide sprays were repeated. A comparison of the number of days when the mow or spray plots had warmer, colder or about the same minimum temperature as the cultivated plots is shown in Table 6.2. The results indicate that mowing and cultivation have similar effects on the minimum temperature, with mowing being slightly colder. However, spraying with herbicide to control weeds resulted in the same or warmer minimum temperature on most days. A frequency analysis and chi-square test indicated that the minimum temperature was generally 0.25 °C to 0.5 °C higher than the other treatments. In a different experiment, Leyden and Rohrbaugh (1963) found an average 0.9 °C increase in temperature at 1.5 m height on only frost nights, when grass was killed with sprays versus having a grass cover crop. Because there are many meteorological and soil and plant factors affecting the temperature measured over cover crops, it is impossible to give universal protection figures related to cover crop management. However, removing or minimizing cover crops in 134
PASSIVE PROTECTION METHODS TABLE 6.2 Number of days when the mowing or herbicide spray treatments had warmer, about the same, or colder minimum temperature than the cultivation treatment in grape vineyards from March through May for 1987 through 1989 YEAR MOWING SPRAYING warmer same colder warmer same colder 1987 1988 1989 7 13 4 39 44 32 18 22 7 24 58 17 21 21 23 4 1 2 orchards and vineyards is definitely known to be beneficial. There are many examples of growers experiencing severe losses in crops with cover crops while there were minimal losses in the same crop without a cover crop. In the Donaldson et al. (1993) experiment, differences in minimum temperature were attributed to the fact that the mown grass remained on the vineyard floor and blocked sunlight from striking the soil surface and that reduced thermal conduction into the cultivated soil. Cultivation creates air spaces that insulate against heat transfer and increase evaporation, which lowers soil water content and reduces heat capacity. However, the soil was not compacted after cultivation and this might have improved protection. The herbicide-treated soils were cleaner and more firm and moist than the other two treatments. Tall cover crops (i.e. grasses and weeds) insulate the soil from heat transfer and may hinder cold air drainage, resulting in more frost damage. However, taller cover crops provide a greater freezing surface area for under-tree sprinkler frost protection systems and therefore could be beneficial for that method (Evans, 2000). Research in Bologna, Italy (Anconelli et al., 2002) also showed that a tall cover crop is beneficial when using under-tree sprinklers. Their hypothesis is that the temperature of the wetted surface is maintained at near 0 °C and raising the surface height by growing a cover crop will raise the 0 °C level. Although protection may be enhanced by the presence of the tall cover crop, one is also more likely to need an active protection method if there is a cover crop. Large variations in ice-nucleation active (INA) bacteria concentrations on different crops have been observed. In some cases, the concentrations are low (e.g. citrus and grapevines). However, the concentration of INA bacteria on grasses and weeds and on cereal crops is typically high. Therefore, presence of cover crops within an orchard or vineyard, or cereal crops near to a sensitive crop, increases concentrations of INA bacteria and frost potential. 135
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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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