Over the past four decades, the Agricultural Research Council (ARC) has focused on developing irrigation strategies for table grapes. As parts of South Africa experiences a drought, water resources are extremely limited. Producers will therefore have to use water resources judiciously to produce grapes.
Under drought conditions, where possible, the planting of young grapevines must be postponed until the drought ceases. However, young grapevines that have already been planted should be irrigated at 70 to 80% depletion of plant available water (PAW), i.e. longer, heavier irrigations further apart, from when the shoots are 20 cm long. The growth points should be monitored to ensure that they remain active.
Taking previous irrigation research results into consideration, table grapes should be irrigated at approximately 40 to 50% PAW depletion from bud break to harvest. This will ensure an optimum balance between grape yield and quality. Bear in mind that grapevines are sensitive to water stress from flowering to véraison, and water shortages during this period will have a negative impact on yield so should be avoided. During ripening, i.e. from the onset of véraison, water shortages can retard the sugar accumulation of white table grape cultivars and, where possible, should be avoided. In the case of red grape cultivars such as the ARC Sunred Seedless, it has been shown that irrigation can be terminated from 13 to 15°B. Water can therefore be saved in this period. The termination of the irrigation will promote colour development of the red grapes.
Soil water content in winter does not affect grape quality but can affect production. Therefore, production can be affected after extremely dry winters in the Western Cape. Consequently, it is recommended that grapevines are irrigated at 70 to 80% PAW depletion during dry winters.
Alternative water sources can be used to irrigate grapevines. However, water of poorer quality should only be used from véraison. If water of a poorer quality is used, its’ quality should be monitored. Good quality water can also be mixed with poorer quality water.
There are several soil and viticultural management practices that can be applied to vineyards in times of drought. Mulches can be applied on grapevine rows to reduce evaporation from the soil surface. All weeds growing in the vineyard should be removed regularly. The soil must not be disked as this can increase evaporation. Grapevine vegetative growth should be contained as it has been shown that a large amount of water is lost to the atmosphere through transpiration from the leaves. At this stage, it is not necessary to remove bunches. Irrigation scheduling should be based on soil water content monitoring so that water is only applied when necessary. Preferably, techniques that combine plant and soil responses are highly recommended.
Recently, it was demonstrated that scheduling irrigation at the hand of grapevine water status could save a substantial amount of irrigation water. Pressure bombs are used to measure midday stem water potential. For table grapes, irrigation should be applied when the readings are between -0.8 and – 1.1 MPa.
Although there are many challenges facing producers during a drought, if water resources are managed properly, table grapes of good quality could still be produced.
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