Using deficit irrigation to stretch limited water supplies for table grape vineyards requires tailoring your irrigation program to fit canopy development and timing of fruit growth.
Total water requirement of a mature vineyard can vary from 22 to 45 or more acre-inches of water per season, depending on variety, canopy size and trellis type, says Jennifer Hashim-Buckey, Kern County University of California viticulture farm advisor.
From bud break to bloom, tines require about 2 to 3 acre-inches of water to support canopy development. “A full canopy should be developed before imposing a moderate water stress,” she says. “This is particularly important for varieties that are susceptible to sunburn, such as Redglobe and Thompson Seedless.”
From bloom (early May) to veraison (late June-early July), water use ranges from about 7 acre-inches of water with small canopies to 12 acre-inches for large canopies.
The only time to avoid a water deficit is from bloom to four weeks later, she notes. That is when berry growth is most susceptible to water stress because berries are undergoing cell division and expansion.
“If cell division is reduced by water stress at this time, final berry size and yield at harvest is reduced,” Hashim-Buckey says. “Extra water applied later will not overcome a stress imposed during this critical period of berry growth.”
Water use during the ripening phase, from veraison to harvest, varies greatly by variety and harvest date. After veraison, irrigations may be cut back to about 75 percent of full crop evapotranspiration (ET) to moderately stress the vines with minimal or no effects on berry size or sugar accumulation, she notes. “Deficit irrigation during the ripening period will have less effect on final yield and fruit quality if vines are stressed from bloom to four weeks later,” she says. “Furthermore, mild water stress during this period may promote color development and reduce berry cracking and bunch rot.”
Varieties harvested earlier use more water during the post-harvest period (which ends with dormancy about mid-November) than late-season varieties. Thompson Seedless, for example, which is harvested in early-September, typically uses 4 to 7 acre-inches of water during this time. Irrigations in this period should provide enough water to maintain canopy without encouraging growth, advises Hashim-Buckey.
“Moderate water stress may be beneficial by stopping shoot growth and promoting wood maturity,” she says. “However vines should not be allowed to defoliate. Also, heavy heavy irrigation during the dormant period is important to replenish the soil-water reservoir. Failing to irrigate post-harvest generally leads to poor bud break in the spring.”
A detailed Excel table featuring water use information for wine grapes, Flame Seedless, Thompson Seedless, and Crimson Seedless on a gable trellis system grown in the southern San Joaquin Valley can be found online at http://cekern.ucdavis.edu/Irrigation%5FManagement.