California Pistachios: So, When Will We Harvest?

    Depending on your location, pistachio harvest should be about as early as last year, which was about 10 ahead of average. Kernel filling began as early as June 15 this year, which is another strong index for early maturity.

    Initiation of kernel filling varies greatly this season due to the effect of insufficient chilling. Orchards on the valley floor more likely received sufficient rest to leaf out and bloom normally. However, orchards at even slightly higher elevations were reported as having significant delays.

    Past articles have discussed the major impact that high ambient temperatures in January had on negating effective chill hours below 450F. Bud temperature monitoring by Dr. Bruce Lampinen at UC Davis showed a 200F increase over ambient temperatures.

    So, if the mid-day temperature was 70F, the bud temperature was as high as 90F. Because of this, I began using the Dynamic Chill Portion Model to report chill hours rather than the traditional “below 45” method, since the Dynamic Model takes into account the negating effect of fog-less, warm winter days.

    The 2014 winter one was of the best in the past five years for temperatures below 450F, but the worst chilling year in the past five based on the Dynamic Model. You have also heard me rant about growers never having any temperature monitoring equipment in their orchards to help diagnose their problems, so a big shout out goes to Corky Anderson and Jeremy Blackwell, who sent me their hourly records from November 1 to February 15.

    • One of the two orchards represented was significantly delayed in leaf out, and the other was pretty much on schedule. Analysis of their records revealed that the “normal” orchard received 55 chill portions, just shy of the 58 estimated by Dr. Katherine Pope, newly hired Yolo County Farm Advisor, who studied chilling in walnut and pistachio for her PhD dissertation.
    • The other orchard received only 50 chill portions, which provides evidence supporting the belief that its erratic leaf out was largely the result of inadequate rest. I can only hope that your great example motivates others to install inexpensive weather recorders, Corky and Jeremy.

    I also looked at struggling orchards, which are located in areas having received little or no post harvest irrigation or winter rainfall. They leafed out and bloomed extremely erratically, and the Peters male trees still have barren upper shoots with flowers that have yet to dehisce. One wonders if winter irrigation in January would prove helpful in hydrating the tree in preparation for the spring. My inquiry to Dr. David Goldhamer about this indicated he was in favor of winter irrigation under such severe conditions.

    Kernel filling requires lots of water, nitrogen, potassium and boron. Average water use in July is 9.8 inches (55 gal/tree/day, 150 trees/ac). In August, it is 8.2 inches (50 gal/tree/day). Keep an eye on the temperatures and adjust your schedule accordingly. If you do not have any soil moisture monitoring equipment in the orchard, be sure to auger occasionally to check for moisture below two feet. The surface can look mossy and wet, but the lower depths can be dry as chalk.

    Believe me, deficit irrigation sneaks up on you, and before you know it, your trees are stressed and limited in kernel filling rate. The amount of water applied must be greater than the tree’s water requirement because of application inefficiency (70¬80% efficient in basin or furrow systems, 85-90% in low volume). Deficit irrigation, zinc or boron deficiency and cool weather during kernel filling will dramatically reduce split nut percentages.

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