California Pistachios: Assessing Chilling Hours And Potential Freeze Damage

    Chilling and Cold Weather Update: December 5-12 got pretty cold in the San Joaquin Valley. Chris Wylie, Agri-World Ranch Manager, said it reached 19-200F around Highway 145. Goyo Jacobo, West Hills Farm Management, recorded 180F in Coalinga, and Ron and Carl Fanucchi reported temperatures as low as 130F in Buttonwillow.

    So, what does this mean for pistachio trees?

    Thus far, comments from the field indicate that most of the trees are symptom-free of cold damage, but it is on the early side to make any firm declarations. There have been scattered reports of first year trees with blackened tips, and shriveled green bark. These orchards were supposedly irrigated well past the recommended cutoff date of early to mid-September, depending upon soil water holding capacity.

    I have heard of one young orchard receiving water through October. The danger of doing that may soon become evident! When surveying your trees for cold damage, look first at the most tender growth near the shoot tip. Freeze injury to green tissue is obvious, given the fact that it turns a dark green and wilts. Cold damage to slightly older tissue shows black spotting on the bark, and brown discoloration beneath the surface when scraped with a knife or fingernail.

    Young branches with developed bark tissue take two or more weeks to show damage. Look for wrinkles developing in the bark the length of the shoot, and poke your fingernail into the bark to test it for dryness. Cutting into the bark also allows for dryness assessment, as well as examination of the color of the tissue beneath it (phloem).

    The phloem should be moist and lemon in color. Injured tissue acquires a gray, “smoky” color, which can turn dark brown with time. Two or three weeks after the freeze event, injury to two and three year old wood can be suspected if the lenticels (gas exchange sites on the bark surface) become more pronounced (swollen and corky), and examination of the phloem with a hand lens reveals dry pockets.

    Limbs suffering significant injury will also begin to acquire a fermented odor when cut into. The extent of any possible freeze damage is really never entirely known until spring, when bud break begins. Injured wood emerges from dormancy much more slowly, or not at all. After such cold as this one, affected orchards often have considerable loss of the one­year-old wood in the tree tops, but the rest of the tree grows extremely vigorously (and with more lateral branching) from the “natural pruning”.

    One comforting aspect of this event is the occurrence of cold weather prior to the freeze, which helped “condition” trees. I believe freeze events are much more damaging if they occur after a warm period.

    The issue of chilling hour accumulation is not yet one to be concerned about, since we still have half of December (at the time of this writing) and January to meet the estimated requirement of 800 “effective” hours. December and January are the most critical months for hours below 450F, since the tree is now in a state of “full rest”.

    It is more important to have 750 hours of consistent cold, rather than 1,500 hours below 450F which includes lots of warm days, followed by cool nights. You can think of this like poking the hibernating bear with a stick while he slumbers! Disruption of whatever physiological processes occur during dormancy with warm weather results in erratic leaf out and blooming. This translates into reduced fruit set, and uneven maturity at harvest. This is why I keep beating on you to install your own weather station at the ranch!

    If you do not become a student of the quality of chill hour accumulation, you have no basis to determine the need for oil treatment, or what effect chill hours had on leaf out and fruit set.

    Personally, I just do NOT get why most of you neglect this critical aspect of basic horticulture, when there is so much money on the line. Your defense might be that you have no control over this, but without site specific weather data it is much more difficult to intelligently discuss production concerns pertinent to your property. It all becomes just rhetoric and conjecture! So….install a SIMPLE weather station!

    To check on your local chilling, refer to the Weather-Related Models and Services section of the UC Fruits and Nuts Center.

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