Depending on the timing of irrigations and amount of water applied, split percentages of this year’s pistaschio harvest could vary widely, reports Bob Beede, University of California Cooperative Extension farm advisor, emeritus. As he notes in his September/October Pistachio Task List newsletter, research by U.C. Davis plant scientists shows that shell splitting is caused by the physical expansion of the kernel rather than development of an abscission zone.

Split percentages are affected by:

Time of bloom

Low levels of boron and zinc

Insufficient water from July 1 to harvest

Excessive cool weather during the growing season

Heavy big bug damage during kernel fill

“Waiting for increased split percentages at harvest after much of the crop has creamy hulls can backfire in the form of higher stain – especially on the east side of the Valley where Alternaria is a bigger problem – and insect percentages,” Beede says.

If split percentages are poor, he recommends examining your irrigation program during Stage 1 (shell development) and Stage 3 (kernel filling). University of California research shows that split percentages can be improved by inducing regulated plant stress during Stage 1.

Want the latest agricultural news each day? Click here for the Western Farm Press Daily e-mail newsletter.

If you typically have good split percentages, the gain from Stage 1 stress is primarily water savings, Beede says. You can save at least 50 percent of ETc between April 1 and June 1. In northern California, irrigation may not be necessary at all during this period.

Split percentages can also be affected by the uniformity of water application. There is no question that water stress during Stage 3 reduces split percentages, Beede notes.

He suggests comparing the amount of water you applied to average water use: July is 9.8 inches. August is 8.3 inches. For the first two weeks in September, that figure is 2.8 inches. Determining when to stop irrigating before harvest depends on weather, disease pressure, soil texture, split development and orchard access, Bede adds.

If Alternaria pressure is not a concern, he advises watering right up to within three or four days of shaking. Unlike almond trees, pistachios don’t require an extended dry-down period to avoid trunk damage by the shaker, he explains.

“In pistachio, it’s common to still be irrigating blocks awaiting harvest while shaking,” Beede says. “A little post-harvest water, 25 percent to 50 percent of ETc, is advisable for relieving shaker stress and improving nutrient uptake in the fall. I have visited several orchards with sparse canopy development. In our irrigation research, this was very characteristic of insufficient water during leaf-out. Nut size is also affected.”