Handbook for Irrigation of Wine Grapes in South Africa

Chapter 7

irrigation”. Since all irrigations are applied to supplement inadequate rainfall, the term “low frequency irrigation” is preferred to “supplementary irrigation”. (ii) Medium frequency, i.e. where intervals between irrigations range from one to four weeks. (iii) High frequency irrigation, i.e. where more than one irrigation is applied per week, including daily irrigation. (iv) Pulse irrigation, i.e. where more than one irrigation pulse is applied per day. (v) Deficit irrigation (DI), where vineyards only receive a fraction of their optimum water requirements. (vi) Different irrigation frequencies in the pre- and post-véraison periods. For example, low frequency up to véraison can be followed by high frequency irrigation during berry ripening. (vii) Partial root zone drying (PRD) where consecutive irrigation is applied alternatively on each side of the grapevine row. Grapevine responses to a specific irrigation strategy under a prevailing set of climate, soil and viticultural conditions will be demonstrated at the hand of research results collected over a ca. fifty year period, as well as some case studies. IRRIGATION OF NEWLY PLANTED GRAPEVINES Mistakes made during the establishing phase can reduce the lifespan of the grapevines and/or complicate future vineyard management. Experience has shown that grapevine root systems will develop where water is most available, irrespective of rootstock cultivar. Therefore, pampering young grapevines by means of small, frequent irrigations where only the topsoil is wetted, will induce shallow root systems. Many vineyard soils tend to re-compact naturally upon irrigation or rainfall after deep soil preparation. Consequently, root development into the subsoil will be restricted if larger irrigations are applied at a later stage to encourage deep root development. This means that frequent, shallow irrigation can undo all the benefits of the deep soil preparation carried out before planting. To ensure sufficient root development, newly planted grapevines should only receive frequent, shallow irrigations until all the grapevines in the block are growing (Fig. 7.3A). When the shoots are generally 20 to 30 cm long (Fig. 7.3B), the schedule can be switched to long irrigations further apart, i.e. as for mature, full bearing grapevines. The objective is to apply irrigation over the total available root depth that was created during deep soil preparation. As the topsoil dries out, the roots will be forced to grow into the subsoil to absorb water from the deeper layers. In this way, the root system will be able to fully utilize the available soil volume. The less frequent irrigation is unlikely to reduce grapevine development, since newly planted grapevines have a longer vegetative growing season, compared to mature grapevines. Under comparable conditions, leaf functioning of young, non- bearing grapevines remains active when the leaves of mature grapevines begin to show senescence (Fig. 7.4). 7.2.1

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