Deficit irrigation has become a California buzzword in the wake of the shortest water supplies in decades due to natural and judicial droughts.

Table grapes can withstand limited deficit irrigation late in the season. However, the usefulness of that strategy is dependent upon how much water the vines received from bloom until about four weeks after bloom, according to Jennifer Hashim-Buckey, Kern County University of California Cooperative Extension farm advisor.

“If cell division is reduced by water stress during the period from bloom to four weeks later, final berry size and yield at harvest will be reduced,” she says. “Extra water applied later will not overcome that stress imposed during that critical period of berry growth.”

However, if the vines have been adequately watered during that critical growth stage, growers can possibly save water from veraison to harvest.

“It has been demonstrated that irrigations may be cut back — approximately 75 percent of full crop evapotranspiration (ETc) — to pose a moderate stress subsequent to veraison with minimal or no effect on berry size or sugar accumulation,” she says. “If water supply is short and deficit irrigation is necessary during the ripening period, final yield and fruit quality will be less affected than if vines are stressed during the period of bloom to four weeks later.”