Irrigating during hull-split and prior to harvest can be tricky. Irrigating too much can increase diseases, the risk of shaker damage, and delay shaking. Too little can increase stick-tights, increase mites, and decrease kernel weights.
During this time, consider maintaining the same frequency of irrigation, but making the adjustments in the duration of the irrigation sets. Reductions to irrigation (i.e. 50% at the onset of hullsplit) to apply a stress can be made by reducing the duration. Trees should be monitored by either a pressure chamber or observations (i.e. wilting) to identify stress levels. If the trees are over/under-stressed at the end of the cycle, adjust the duration.
A basic strategy from hull-split through post harvest could be:
Hull split initiation, week 1: 50% reduction to reduce the incidence of hull rot and even up hull-split (-15 bars with the pressure chamber)
Hull split, week 2: 30-50% reduction to reduce hull rot (-15 bars on the pressure chamber)
Hull split, week 3: 0-30% reduction to reduce hull rot, based upon tree stress the past two weeks (2 bars more negative than baseline). A mild deficit at this point should cause minimal loss in kernel weights, but will save some water. It will not increase the process of hull-split – this is a physiological process. A severe deficit may impact kernel weights.
Post Hull split, week 1: 100% ET – irrigation reductions at this time can decrease kernel weights until the final abscission layer between the peduncle and nut is formed (2 bars more negative than baseline).
Post Hull Split, week 2: Depending upon timing of harvest – 100%ET if more than two weeks from harvest.
Pre-Harvest week: 20%-50% reduction to help begin a slight dry down of the orchard.
Harvest week: 30-50% ET – some water should be applied, but with enough time to allow orchard floor drying for shaker movement.
Post Harvest: 100% ET – stress should be minimized at this stage as the tree prepares for next year.
The recommendations above are made for micro, drip, or solid-set irrigation systems, but the recommendations would be very difficult to achieve using flood irrigation. With flood irrigation, growers will want to adjust their irrigations so that some trees show signs of water stress between irrigations during hull split, and harvest will be delayed until the wettest part of the field is dry. Increasing distribution uniformity by taking into account soil differences with different checks may help with a uniform harvest.