Results of one season’s experience at a single trial plot in Kern County indicates that pistachio growers may want to proceed with caution before using Randy trees instead of Peters (male) as a way to boost performance of Kerman (female) trees by trying to compensate for low winter-chilling hours.
Craig Kallsen, University of California Cooperative Extension pistachio advisor for the county, provides details in the February, 2015, issue of his Kern Pistachio Notes newsletter.
Deciduous fruit trees, like pistachios, require adequate winter chilling. Otherwise, for reasons researchers don’t fully understand, yields can suffer due to poor synchronization of bloom with a resulting drop in pollination rates.
This is likely to be the second straight winter of severely deficient chill for pistachio orchards in Kern County, Kallsen reports.
Sensing that lack of chill may be becoming a trend in the southern San Joaquin Valley and based on the timing of the bloom last year in their Golden Hills (female) and Kerman orchards, some growers are planning to graft some Randy budwood into their Kerman orchards, either into existing young orchards or in the future, Kallsen notes.
Last year, in a University of California advanced-selection variety trail in an isolated orchard near Buttonwood, Calif., which included Randy, this pollinizer variety may have improved the performance of Kerman, Kallsen notes.
The Randy trees are in an extreme corner of this particular orchard, Kallsen said. Pollination of the Kerman trees in the rest of the orchard depends on Peters. As a result of the low chill, bloom was very sparse and late in 2014.
Typically, Randy trees are used to pollinate Golden Hills, while Peters are the pollinizers in Kerman orchards. In general, Randy and Golden Hills bloom about six days earlier than Peters and Kerman trees.
“During bloom evaluation within the trial area in 2014, it was observed that full-bloom of Peters and Randy was six days later than their female counterparts, Kerman and Golden Hills, respectively,” Kallsen reports.
“As a result, full-bloom of Randy matched full-bloom of Kerman almost exactly.”
Yield of the orchard outside of the trial area and Kerman yield averaged less than 1,000 pounds per acre. Meanwhile, the yield of the individually-harvested Kerman plots, all adjacent to Randy males, was close to 2,400 pounds per acre, he notes.
However, Kallsen points out, these results are not scientific proof that Randy was responsible for the yield increase. They are merely an association.
“Keep in mind that in a normal chill year, whatever that might be, or at least in years like 2003, Randy was through blooming, before Kerman started blooming,” he writes. “We have also observed in pollen germination testing that pollen produced late in the bloom period, that is, a few days following full bloom, often demonstrates poor or poorer germination. For this reason,
Randy should not replace Peters as a Kerman pollinizer, but should be considered an adjunct pollinizer.”