Handbook for Irrigation of Wine Grapes in South Africa

Chapter 9

Preventing cold damage in vineyards

9.1 INTRODUCTION Adverse weather conditions such as hail, heat waves or excessively low air temperatures may have negative impacts on the sustainability of wine grape production. Protection against hail is costly, and not economically viable in terms of South African wine production. Fortunately, severe hail damage does not occur extensively in most of the grape growing regions. On the other hand, damage caused by severe heat spells is more common. However, there is very little growers can do under such conditions, particularly where irrigation water is limited and vineyards are planted in shallow or sandy soils. When air temperatures fall below 0°C, it could cause severe yield losses if freezing or frost occurs in vineyards (Fig. 9.1). With the exception of the Coastal region, cold damage may occur in the higher, inland areas of the Western Cape, e.g. the Breede River valley (Haasbroek & Myburgh, 1998). However, in the more inland grape growing regions such as the Lower Orange River the incidence of cold damage is higher and more severe. Cold damage occurs when the air temperature in the vicinity of grapevines falls below a critical value, which is just below freezing point. The critical temperature goes hand in hand with wind speed. The duration of the cold period also determines the critical temperature. Furthermore, the occurrence of cold damage depends on the phenological stage of the grapevine, e.g. sprouting buds are more susceptible than mature leaves (Haasbroek & Myburgh, 1998 and references therein). Apparently, the frost risk is highest in spring when new buds can be damaged if the air temperature falls below -0.6°C. Therefore, the objective of cold damage prevention is to maintain the temperature of plant tissues above the critical temperature. The dew point temperature is an important consideration when it comes to cold damage. The T dp is defined as the temperature at which the atmosphere becomes saturated with moisture, and condensation occurs, in other words, when dew is formed. In practice, a higher T dp means that the air contains more moisture, i.e. the air contains more latent, or “stored”, energy which becomes available when freezing sets in. Consequently, the probability of sub-zero air temperatures is less if the T dp is high. In contrast, the risk of cold damage due to sub-zero air temperatures increases if the air is dry and the T dp is low.

284 CHAPTER 9 – PREVENTING COLD DAMAGE IN VINEYARDS