Handbook for Irrigation of Wine Grapes in South Africa

Chapter 9

9.4

MEASURES TO REDUCE THE RISK OF COLD DAMAGE

9.4.1 VITICULTURAL ASPECTS Considering viticultural aspects, selection of suitable land for planting vineyards is important, particularly in localities where windborne freeze may occur (Haasbroek & Myburgh, 1998 and references therein). Cold air flows downslope and collects in depressions or troughs (Fig. 9.7). Early budding cultivars are more susceptible to cold damage (Table 9.5), and should preferably not be planted where these so- called “frost pockets” may occur (Howell et al. , 1998). Since cold damage is more severe near the soil surface, higher trained grapevines should be less susceptible to cold damage. However, this does not rule out the possibility that severe frost may damage grapevines trained onto high trellises, e.g. on a “high wire” for mechanical pruning and harvesting (Fig. 9.8). Late pruning delays budbreak, which reduces the risk of cold damage. Straw bundles can be tied around grapevine trunks as isolation against cold, i.e. if cheap straw is readily available. The efficacy of chemicals aimed at reducing the freezing point of cell sap in plant tissues, or inducing other biological changes that will protect plants against frost, is yet to be proven (Perry, 1994). Figure 9.7

Frost pocket

FIGURE 9.7. Cold air flows downslope and may collect in depressions to form “frost pockets”, thereby increasing the risk of cold damage in vineyards.

TABLE 9.5. Relative resistance of selected cultivars to cold damage in spring where, 1 = most susceptible and 10 = most resistant (Howell et al. , 1998). Cultivar Ranking Cabernet franc 10 Cabernet Sauvignon 10 Chardonnay 4 Gewürtztraminer 6 Merlot 6 Pinot noir 5 Riesling 8 Sauvignon blanc 8

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