Written by Drew Hubbard, OSU-MCAREC November 27, 2016

Questions regarding the status of cherry dormancy have been abundant this season. With a lack of frost,
except in outlying areas, the status of winter acclimation and dormancy is largely unknown. Since I’ve
begun sampling in early October, specifically mature ‘Sweetheart’ on Mazzard, I’ve found that trees are
well ahead of last year with regard to acclimation. In 2015, it was late October before I could detect a
change in the hardiness status of flower buds and shoots using the technique of differential thermal
analysis (DTA). In 2016, I’ve been detecting increasing hardiness since the first sample on Oct 5th. Since
then, trees have continued to acclimate regardless of the lack of freeze events. Because cold acclimation
of temperate-zone trees is triggered by shortening day lengths and progressively lower temperatures,
trees have developed acclimation and hardiness despite the lack of ‘hard’ frosts. Recall that in 2014 we
did not have nighttime temperatures below 40° F prior to the freeze events between Nov 15-18. In that
event, temperatures throughout much of The Dalles and the Hood River Valley were typically between 0
and 7° F and the lethal temperature at which 50% of flowers were killed (LT50) was ~6.5° F.
So far, the LT50 of Sweetheart flowers is approximately 5oF (as of 11/24/2016). In last year’s study, full
dormancy was largely reached within several weeks of the first freeze events without nearly the same
acclimation levels reached as this season. Acclimation is a dynamic process that responds to weather
events; therefore, it’s difficult to give an exact figure for all cases. So far using data for ‘Sweetheart’, we
appear to be in good shape. I will provide another update as more data are collected.