It had been a very mild chilling season to start, but suddenly the chill mostly has caught up with last year. In all my years monitoring winter chill for my deciduous orchards–in San Diego County and now since mid-winter 2012-13 in Yolo County, pictured above–I have experienced very few 24+ hour periods like the current one. Below is a capture from my temperature station from Saturday evening. The high for the day was 44F. The low was 33F, but the temperature was below about 36 for only around five hours. And since then it has been even more remarkable: all night long and until a little after eight on Sunday morning the temperature has been a steady 40. As I type this, a little after 10:30 a.m., it is only 42.
This range is prime chilling. An hour between about 38 and 45 is a full “chill unit.” Hours below about 38 but above freezing count for somewhat less than a full chill unit. Above 45 it also tapers, with some chill models saying you need to subtract hours above 65 or so from your running count (I have some reservations about that, based only on my own monitoring, but it matters little in my current climate–at least for now). Stone fruits and other deciduous fruiting trees have a chilling requirement, varying by fruit variety. Many varieties I grow here do their best with over 500 hours (or “units”), and a few would like 600. The hours/units need to be consecutive, but extended warm spells in the winter can accelerate the process of breaking dormancy. If that happens before the variety’s chilling requirement is met, fruit set will be reduced or nonexistent.
At the moment of my writing, we have had 24 chill units in the past 24 hours, and probably 20 in the preceding 24 hours. According to the UCANR station nearest me, the season total stands at around 234 chill units. At this point last year, it was 264. Last winter was a very good one. As of early December, I was a little concerned about the current winter chill season, as it had been so mild. The UCANR station, for example, showed only 122 chill units as of 9 December, compared to 211 on the same date the previous year–and 3 vs. 39 back on Nov. 17! But starting on the 10th of December, we have been enjoying overnight lows anywhere from 30 (which means some hours of no chill) to mid-40s (the prime range) with only one early morning low above that. And our daytime highs during this spell have not broken 60 and generally have been in the low 50s, till the unusual 44 yesterday. A couple good weeks really can make up for a slow start!