Amid reports of nut growers severely hedging trees, shaking off blooms, and killing fruit at set to reduce water needs this season, David Doll, University of California Cooperative Extension farm advisor for Merced County, is urging caution.

“Lack of water is the fear on every grower’s mind,” he says. “But some are overreacting.”

The amount of water expected to be available in the county this year varies by irrigation district. Those on federal water projects in the have seen their allotments slashed to zero. Elsewhere, though, some irrigation districts have announced allocations of as much as 2.5 to 3 acre-feet of water. That compares to the 3.5 to 4.5 acre-feet districts have typically allocated in the past, along with the option for growers to buy more.

“Recent rains, which have been pretty consistent, have filled the soil profile,” Doll says. “This is providing at least a month before trees would begin to show any signs of severe drought stress. It makes sense to let the fruit set and do a little wait-and-see before taking any drastic action.”

One alternative for some walnut growers, as well as some almond and pistachio producers, is regulated deficit irrigation (RDI). The practice intentionally stresses trees by reducing irrigation water applied, which forces trees to pull water from deep soil moisture. Irrigation rates are lowered during certain growth periods of the season when tree yields won’t be severely impacted.

University of California studies involving almonds, walnuts, and pistachios show that RDI can reduce a tree’s water used by as much as 25 percent to 30 percent, with minimal impact on current and future yields. The specific details for implementing the practice vary by tree species and require the use of a pressure chamber.

“A few walnut growers are beginning to do this, and we'd like to see more adopt the practice in dealing with the drought,” Doll says. “The idea is to give walnut trees water through the spring when they are growing rapidly and the seed and kernel are developing.

“By early June, the fruit has reached its full external size and much of the shoot growth that develops into fruit-bearing wood in the subsequent seasons has been completed. By mid-June, shell hardening, nut-fill, summer bud differentiation, and further shoot growth begin. Research has shown that growers can withhold water during these periods of late season tree and nut growth stages. Reductions at these times will save the most water with the least negative impact on tree health and productivity.”

More information is available at http://ucmanagedrought.ucdavis.edu.