Winter 2021 - 44

WINTER MANAGEMENT
Kerman pistachio showing
protracted budbreak, and many
buds failing to break, after a
warm winter. Photo: David Doll
WARMER WINTERS
What less chill means for pistachio
BY CRYSTAL NAY
WESTERN EDITOR
G
rowers tend to be more in touch with outdoor
conditions than most people, since working
with the land and keeping an eye on the weather
means growers see a lot of the annual variability in
the field. In some ways, this has led them to be more
skeptical, but within the last few years more growers have
got onboard with the idea that things are changing and
conditions aren't what they used to be.
" There's a new normal coming, " said Katherine
Jarvis-Shean, orchard systems advisor with University of
California Cooperative Extension. " I would say more and
more growers are coming around to seeing the problem that
is impacting how they farm, and will impact how they farm
in the future. "
As climate temperatures continue to increase, certain
pistachio varieties could become unsuitable for much of
the regions in which they're currently planted. Pistachio
trees require a certain amount of exposure to cold in
order to produce well, but with winters becoming warmer
in pistachio-growing regions, the trees aren't always
receiving the proper amount of chill accumulation in
order to be good producers.
There are two models that calculate chill accumulation:
the Chill Hours and the Dynamic, or Chill Portion, models.
44 WINTER 2021
The Chill Hours model counts any hour under 45˚ F as a
chill unit, but doesn't account for the warmer days, which
can make it seem that a tree has received more chill than it
actually has. The Dynamic model is an updated model that
calculates chill accumulations as portions, where different
temperatures are worth different values, and warmer days
cancel out some of the accumulated chill in order to more
accurately calculate how the tree itself may calculate chill.
The importance of chill
Looking back nearly a century, there's a warm winter
every 15-20 years. But, in the last 10 years, there have been
three winters considered to be unusually warm: 2013-14,
2014-15, and 2019-20, which had a delayed spring. The
results of trees not receiving enough chill time during the
winter appear as a variety of symptoms during harvest. Not
only does lack of chill produce blank wood, buds that never
open, and a variety in the size of the tree's leaves, but it
also creates protracted budbreak, meaning a wider bloom
window.
For instance, in a year where pistachio trees receive the
desired amount of chill, they perform ideally for farming
conditions. In spring, for example, it would take five days
from when the first bud opens on the tree to when the last
bud opens.
" But, following a warm winter, growers will instead see
10 days from when the first buds open until the last bud, or
sometimes as much as two weeks, " said Jarvis-Shean.
Winter 2021

Table of Contents for the Digital Edition of Winter 2021
