WCN April 2020 Draft E

Winter Chill Wrap Up 2020
By KATHERINE JARVIS-SHEAN | UCCE Orchard Systems Advisor, Sacramento, Solano and Yolo Counties
A
few warm spells and a record-breaking
dry February with
little-to-no fog have a lot of people
in the tree crop industry wondering
about chill accumulation this past
winter, and if it would impact bloom in
the spring. As I write this article in late
February, almonds and a few early stone
fruit are the only trees to have bloomed.
But, despite some warm conditions, the
chill accumulation numbers indicate
that we should have enough chill for
a decent bloom in our later blooming
crops. You can be the judge, reading
this in April, as to whether those
accumulation numbers match what the
trees felt and needed when it comes to
winter chill accumulation. The story of
this winter varies depending on how
you count winter chill, so in addition to
reviewing how chill accumulation went
this past winter, now is a good opportunity
to discuss tools to help you count
chill in the future.
Why Do We Care About
Winter Chill?
Deciduous trees have evolved a
mechanism to essentially count the
passing of winter, to know when cold
conditions are in the rearview mirror,
warm conditions are stable, and
the outside world will be a safe place
for tender flower blossoms and young
leaves and shoots. Crops and cultivars
vary in how much winter cold they
need to meet this cold accumulation
threshold. Some spring warmth can
compensate for trees not getting all the
chill they want, but there is a minimum
chill requirement that needs to be met
for buds to open in the spring. As the
warm winters of 2013-14 and 2014-15
reminded us, when deciduous trees
don’t experience adequate winter chill,
they will have straggled, prolonged
bloom, which can lead to problems
protecting flowers from spring diseases
and a variety of sizes and harvest readiness
timings at the end of the season.
Even worse, inadequate chill can result
in delayed bloom, which can mean
pollinizers don’t overlap with the target
variety, leading to poor fruit or nut set.
So, How Did Chill Accumulation
Stack up This Year?
When I ran the numbers on Febru-
Figure 1. Chill accumulation counted as chill portions at the Firebaugh CIMIS
station, November 1-February 19.
Continued on Page 16
14 West Coast Nut April 2020