I am anxious to see what this year’s split percentages are. Depending on water availability and time of application, we could see a wide range in split percentages. Research by Vito Polito, U.C. Davis Plant Sciences Department, indicates shell splitting is caused by the physical expansion of the kernel rather than development of an abscission zone.
Split nut percentages are affected by all of the following: low boron and zinc, insufficient water from July 1 to harvest, excessive cool weather during the growing season, time of bloom, and heavy big bug damage during kernel filling when nuts show no symptoms.
Waiting for increased split percentages at harvest after much of the crop has creamy hulls can backfire from higher stain (especially on the east side of the Valley where Alternaria is a bigger problem) and insect percentages. So, do NOT wait! Growers with poor split percentages need to examine their irrigation program during stages 1 (shell development) and 3 (kernel filling).
Research by Dr. David Goldhamer shows that split percentages can be improved by inducing regulated plant stress during Stage 1. If you typically have good split percentages, the gain from Stage 1 stress is primarily water savings. Growers can save a minimum of 50% of Etc between April 1 and June 1, and 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 water stress during Stage 3 reduces split percentages. Compare your applied water to the following average water use: July is 9.8 inches, August is 8.3 and the first two weeks in September is 2.8 inches. Deciding when to stop irrigating before harvest is dependent upon weather, disease pressure, soil texture, split development and orchard access. If Alternaria pressure is not a factor, water right up to within three or four days of shaking.
Unlike almond, pistachio does NOT require an extended “dry down” period to avoid trunk damage by the shaker. In pistachio, it is common to still be irrigating blocks awaiting harvest while shaking. A little post harvest water (25-50% of ETc) is advisable for relieving shaker stress and improving nutrient uptake in the fall. I have visited several orchards with sparse canopy development. This was very characteristic of insufficient water during leafout in our irrigation research. Nut size is also affected.
In addition to inadequate nutrition (zinc and boron), it is my professional opinion that the time of bloom and pollination affect split percentages at harvest. In high chill years, pistachio trees have the potential of pushing and blooming early, PROVIDING the weather is favorable.
When spring temperatures are warm, bloom occurs early and sharply. This, in my opinion, allows for more uniform nut development and size since they all begin at about the same time. But when spring temperatures are cool and erratic, I believe nut size and expansion reflects this. Consequently, some nuts pollinate late and experience different developmental weather than those setting earlier.
These subtle differences may affect the AMOUNT of cell division and the RATE of cell expansion during shell development. The result is that some nuts have thinner or smaller shells, which are more prone to premature splitting. We really need to research this, so I can quit giving you my coffee shop opinion.