Pistachio must be exposed to cool temperatures during late fall and winter to produce adequate bloom and pollination the following spring. This exposure to cool temperatures is called ‘chilling’ and the threshold for an adequate bloom and yield response is called the chilling requirement.

However, although it is known that the chilling requirement for pistachio is relatively high, compared to almond or citrus for example, the chilling requirement is not known precisely. How a tree ‘measures’ chilling physiologically is not known. Sunlight may interact with temperature to alter the chilling requirement. Bright, sunny days during the winter appear to interfere with tree dormancy and the accumulation of chilling. As of this writing, in early January, it looks as if chilling may be inadequate to promote an even bloom in the spring of 2006 in many areas of the San Joaquin Valley.

Inadequate chilling is associated with the following: an extended bloom period, uneven bloom with even the cool side of a tree blooming days ahead of the warm side, and poor synchrony of bloom between male and female trees. Orchards with elevational differences may show differences in bloom timing of days between trees on the hilltops versus those in the swales. These differences can result in poor pollination that reduces yield, and can adversely affect the uniformity of nut maturation, which will require a grower to make two or three harvest passes through the orchard.

Measurement methods

The amount of chilling that a tree accumulates is estimated by various methods. One method simply involves adding up the number of hours less than 45 degrees that occur from Nov. 1 through the end of February. Using this method it can be demonstrated that compared to the past six years, the southern San Joaquin Valley is in a deficit chilling situation. The winter of 2005-06 in the southern San Joaquin Valley has also been characterized by frequent bright, sunny days.

Although, admittedly, the chilling requirement is not well understood, we do have an approximate feel for knowing when we do not have enough. In general, work by U.C. Farm Advisor Bob Beede in Kings County and UC Extension Specialist Louise Ferguson located at the Kearney Research and Extension Center in Parlier, determined that the Kerman female variety and the Peters male variety required about 550 accumulated hours of chilling below 45 degrees to achieve a minimum of 50 percent bloom of male and female flowers. At 800 hours, 80 percent of the female and male flowers bloomed.

Unfortunately, the number of blooming flowers required to achieve adequate pistachio yield are not known. Large yields of pistachio (over 5,000 pounds per acre) in large orchards have been achieved when as little as 600 to 650 hours have been accumulated during the period from Nov. 1 to March 1. However, this season, it appears that many areas of the San Joaquin Valley will not accumulate 600 hours of chilling.

Even more alarming, in the past there appears to have been an interaction between accumulated chilling and whether the tree was in its ‘on’ versus ‘off’ year in the alternate bearing cycle. Inadequate chilling going into an off year can change the disappointment in yield that is normally associated with an off year anyway, into a disaster.

‘Off’ year expected

Most orchards in the San Joaquin Valley have experienced two relatively on years in a row, so 2006 is expected to be an off year for many orchards. Low chilling can also keep 6th, 7th and 8th leaf orchards that would normally be expected to begin or continue producing economic levels of yield, almost entirely vegetative.

Short of hauling snow from the Sierras, there is not much a grower can do to lower orchard temperatures or shade the trees from winter sunlight. Interestingly, in the 1990s, UC Farm Advisor, Bob Beede noticed that applications of insecticidal petroleum oils, applied to dormant pistachio trees in January and February, resulted in an early bloom of pistachio trees and a more uniform bloom. In some years, the application of the oil increased yields. Petroleum oil was applied at the rate of 6 gallons of oil in a spray volume of 200 gallons per acre.

Oil-treated trees bloomed even earlier than if they had had sufficient chilling. Generally trees treated in the period of Jan. 15 to Feb. 15 performed the best. Apparently, pistachio growers on some of the Greek islands have been applying oil to improve flowering for decades. Unfortunately, applying dormant oils to pistachios for improving bloom and pollination in years of inadequate chilling is not on the label of any petroleum oil. In fact, with the apparent disappearance of Volck Supreme oil from the marketplace at least for this year, only one 470-weight oil, Britz Supreme Spray Oil, appears to be currently registered for use on pistachios as an insecticide.

Britz supreme oil is registered for use to control certain scale insects in pistachio. Coincidentally, an excellent time to control pests like the European fruit lecanium scale is in the January and February time period. Pistachio trees have been damaged by petroleum oil sprays, and this damage has generally been associated with a breakdown of the oil emulsifier as a result of dirty spray tanks. Petroleum oils are best applied from clean tanks and free from mixes with other materials.

Hopefully, temperatures will cool in the coming weeks, and additional needed chill hours will be accumulated in January and February. If not, the pistachio industry may be in for a potentially disappointing season.