Chill hours, climate change, and my orchard’s winter so far

Temperatures are warming more in winter than other seasons, according to research carried out in the Central Valley, including here in Yolo County, by UC Agricultural and Natural Resources (UCANR). That is bad news for growers of deciduous nut and fruit trees.

Tree crops will move northward (a trend that is already noticeable). Breeders are working on new varieties of pistachios that have lower chilling requirements. In the shorter run, growers are working with “other tactics to improve winter chill, such as using overhead sprinklers to cool the trees and painting them white with liquid clay to reflect sunlight.

I have used both (in my San Diego County days) sprinkling during warm spells and painting of trunks and major branches to boost chances of production on varieties that are a bit marginal for my area.

Orchard rows on Dec. 19, 2016. Trees in background just pruned. Those in front not yet, at the time. That’s the ultra-vigorous ‘Shaa Kar Pareh’ apricot in front.

Such tactics won’t be necessary for me this winter, however. We have had an exceptionally–by recent standards–chilly winter. At my location, December’s mean low and high temperatures were 36.8 and 54.9F, which is slightly below long-term averages for nearby stations (Davis and Vacaville). By comparison, our monthly means for December, 2015, were 39.4, 55.1; in December, 2014 they were 47.1, 58.0 (!).

This January, so far: 40.8, 52.2. The days have been cool, for sure! And that is much better than January, 2016: 43.9, 55.8; 2015: 39.0, 59.8 (!). Yes, 2014-2015 was pretty bad for the deciduous fruit trees.

My trees should like this winter a lot! Now, if only we can avoid really heavy rain in the bloom period. Given the trend so far this winter–on track for possibly the wettest ever recorded–that is certainly not something I can count on.

‘Spring’ already?

Normally, the first signs of fruit trees getting ready to bloom would be a time of excitement. But when several trees are showing such signs on the 26th of January, it is more a cause of concern.

It would be one thing if the trees are almonds. We just had Tu Bi-Shvat yesterday, after all, and this Jewish ‘New Year for the Trees’ is suppose to be at roughly the time that the almond trees begin blooming in the Land of Israel. And much of California, including this location, has a ‘mediterranean’ climate type, as has Israel. Yet this year we will have a second month of Adar, inserted because Tu Bi-Shvat has come “too early”; the almond trees are not supposed to be in bloom yet, and we need to adjust the calendar to keep Pesach (Passover) in its proper season–the full flowering of spring. I am told by a contact that almonds already are in bloom in Israel. Mine are merely showing the very early signs of readying to bloom. However, several other trees that should flower after the almonds are gearing up. And that is a worrisome sign of a spring coming too early. We still have winter rains to get through–expected to be heavy later this week, and probably in much of February–and the ever-present danger of wintertime frost and freeze, which can wreak havoc on blooms and young fruitlets.

This is the ‘Flavor Delight‘ aprium. Look at all those red buds. It will be blooming soon.
‘Flavor Delight’

‘Flavor Delight’ tends to bloom early, but this is more than a week ahead of last year, when it was actually a warmer winter. This is a variety with an evidently low chilling requirement, so once the weather warms enough, it will bloom early. But this early? (I might have expected that in San Diego, but not this far north.) The first bud swell for this tree in 2015 was 3 February. Its current status is more than just early bud swell. Those are about to burst! A little over a week early may not seem like much, but I would have thought last year’s was early, and yet this year it is even earlier. I suppose we might have to get used to “early” blooming as the new normal, as the climate continues its general warming trend.

All three of my pluots are also showing signs of waking up. I have ‘Flavor King‘, ‘Splash‘ and ‘Flavor Finale‘ varieties. The first two of those began to show some bud swell around 15 February last year, but the whole tree was not showing swell till another week after that. The ‘Flavor Finale’ did not begin to show its very first bud swell last year till 22 February. So these trees are really early! One good thing is that maybe they will all bloom together. My cross-pollination was not good last year, because the ‘Flavor Finale’ did not really get going till the other two were nearly done.

Two pluots shown above. The first one is the ‘Flavor Finale’ and I believe the other one is the ‘Splash’

The pluots also did not have really full blooms last year, partly due to the youth of the trees, but probably mostly because the chill hours (or “chill portions“) in the area were quite low last year (which is an ongoing trend). This winter, December was quite chilly, but January so far has been rather warm. (Data summary below.)

As for the almonds, they have their buds swelling, too. Still, it is striking that they will not be substantially earlier to bloom than some of the fruit trees. Things are just a bit whacky this “spring” so far.

Our temperatures; look how cold December was in 2013 and how warm it was for the average overnight in 2014! Also how warm January daytime highs were in 2014. This month it has been cloudy most of the time (and raining!), so the the lows have been warmer at the same time as the highs have been cool. In the two prior years, January was essentially cloudless and dry.

Year Dec. mean low Dec. mean high Jan. mean low Jan. mean high Feb. mean low Feb. mean high
2013-14 31.7 59.1 35.9 66.8 44 63.2
2014-15 47.1 58 39 60 44.2 66.5
2015-16 39.5 55.1 43.7 55.1
January 2016 data through the 26th


The forecast says the snow level will drop to as low as 2000 feet Saturday evening, locally 1000. We are at 1500.

It has been a really cold week, by local standards, especially for late February.

Too bad all this chill is basically useless for the deciduous fruit trees, coming this late. We surpassed 500 chill hours earlier this month. However, we really won’t get more useful chill, as almost everything is leafing out or blooming by now. And getting snow, or freezing temperatures on the buds and blooms is not a good thing.

But some snow would certainly be interesting…

It rained a bit

Thursday, early in the morning, one of the most incredible storms this region has seen finally moved out. We had 6.25 inches in just over six days, 2.76 of which fell on Wednesday; many areas to the north had a good deal more.

It rained heavily enough for a time on Wednesday that we had a little river running through the property, not to be deterred by fresh prunings off one of the apple trees (which has several varieties that were grafted on to it last spring).


The main event of the rain lasted about 72.5 hours. During that time, only near the end were there as many as five straight half-hour increments (the archive time on my weather data-logger) in which no rain was recorded. At one time on 20-21 December, rain was recorded in 42 consecutive half-hour periods. That was part of a run of 123 of 132 half-hour periods in which rain was recorded. So, it rained rather persistently.

I can recall some phases of rain over a week or so long in the past that were impressive. As recently as January of 2010, for example. And no one who lived in Southern California at the time will forget “epic” rainy periods in 1983 and 1969. But usually these involve a series of discrete heavy storms, punctuated by several hours of some sunshine and no rain. This time, as the stats above reveal, it just kept raining. And raining. There were not even any breaks in the clouds, at least during daylight hours (and the record suggests not at night, either) from Saturday afternoon till Wednesday afternoon.

Today it was sunny and relatively warm (first time over 60 since 14 Dec.). But more rain is forecast for Saturday night and at some point during the coming week.

It’s a bit saturated around here.


We won’t have to irrigate for a while. And, thanks to that cold snap in late November, and more than a week of cool days (albeit fairly warm nights for the time of year) during the rain, we are almost to 250 chill hours already, which is good for the stone fruits.

fruittrees in the rain

Enough chill after all?

On this Tu Bi-Shvat, there is much to celebrate as the fruit trees begin a new year. With the very hot January (or perhaps I should say most of Tevet and of Shvat’s first half) I had feared we would be too far behind in chilling accumulation for many of the deciduous fruit trees to bloom adequately.

It is still early spring (and today it feels rather wintry), but the signs are already very encouraging. The Mesch Mesch Amrah plumcot, which is always one of the harbingers of the bloom season, is now covered in blooms. A few blooms are now open on the Flavor Delight Aprium and Minnie Royal cheery. These all have quite low chilling requirements (300 or less?), especially the Minnie Royal, so their blooms are perhaps not especially indicative of significant chilling having been received. However, several moderate-chill (over 400 hours?) varieties are also showing bud swell, including the Autumn Royal apricot (usually a later bloomer), and the Moorpark apricot. Others that have chilling requirements most likely in the 300-400 range are also showing significant bud swell, including Newcastle, Katy, Royal Blenheim, Royal Rosa, and Shaa Kar Pareh (!) apricots and the Flavor King, Flavorosa, and Dapple Dandy pluots, as well as the Flavorella plumcot.

That concentrated chill in the second half of December/Kislev may have produced more chilling hours than I had dared to believe.

All in all, a good way for the trees (and their keeper) to start the new year!

As for Tu Bi-Shvat, with rain having washed out the trail, our shul’s annual ‘walking seder’ for the day was canceled, but the rabbi suggested we all hug a tree and eat dried fruits and nuts for the occasion. And my thought was, for that we need an occasion? Baruch ata Adonai, bo-rei p’ri ha-eitz! Now pardon me while I complete the blessing by enjoying some dried Royal Rosa or Shaa Kar Pareh from last season…

Cold December, Hot January

We are in the midst of a very unusual hot streak. For the seventh straight day here at Ladera Frutal, the high temperature passed 80 degrees (on the quaint Fahrenheit scale). This is quite a contrast with December, which featured seven straight days with a high below 60. I am not sure which 7-day streak would be rarer, but neither could be counted on to occur most winters. To have one such hot and one such “cold” streak in the same winter might just be unprecedented. (The December streak even featured a day when the high was only 49; that’s the only 24-hour period in the six-plus-year history of Ladera Frutal’s weather station to feature a high of less than 50!)

So the weather has been weird. And all this weirdness greatly confuses the fruit trees. There have been a few blooms sporadically on the Earlitreat peach since late December. This is not usually one of our first bloomers–it would usually start in mid-February after a few other peaches–but it always is the first ripener. One year it gave us fruit at the end of April. Maybe this year we’ll have peaches in March!


And I was out picking the sumptuous new crop of Page mandarins and suddenly my nose detected one of the most delightful of all fragrances. Could it be? Yes, there are a few blossoms on the Page! (Citrus usually start blooming after mid-March.)


There is still no sign of bud break on the usual first-bloomers: the Mesch Mesch Amrah plumcot, Newcstle apricot, Flavor Delight aprium, or Tropic Snow peach. (There have been blooms on the Anna apple, but that doesn’t count; that crazy no-chill apple always blooms in December.) But with such warm weather, I’ll be surprised if one of these is not beginning to bloom by the end of next week.

Of course a limiting factor in triggering blooms will be whether chilling requirements have been met. In fact, it is precisely to guard against too-early a bloom, with possible later freeze or frost damage, that deciduous fruit trees evolved their chilling requirements. If they have not had their chill needs met, they will hold off at least a little bit longer. But at some point, if the warmth continues, they’ll break dormancy anyway, but may not flower or fruit well.

Despite the warm spell, the chill count is pretty good, thanks to two factors: (1) that extraordinary cool week in December, and (2) the dry air. When the air is dry and there is no cloud cover, the nights can be chilly even when the days get quite hot. And it is with dry and cloudless nights that the full flowering, so to speak, of Ladera Frutal’s microclimates become apparent.

Up here at LF HQ, at one of the highest locations on the finca, the hottest day reached 89. That night it cooled to 56. Down the slope, at the coldest part, where all but the lowest-chill deciduous fruits are planted, the high was a bit lower, at 87. But the following night it got to 45. Yes, a 42-degree difference from high to low, and an 11-degree difference in low temperatures between the two locations! What a difference 100 or so feet of vertical change over 200 or so feet of horizontal can make! And in the protection of the big old grapefruit trees, the hedgerow (where I cheat on chill) stays cooler still: the hottest day was 83, rather than 87 or 89, and it is almost always 1-3 degrees colder at night. At times, even just shortly after sundown, we experience a 9- to 12-degree difference in temperature between the locations. Following Madison’s “scientific farming” principles, we have planted varieties in locations intended to maximize their microclimatic adaptation.

As a result of these microclimate effects and the dryness, the chill count is not too bad. Of course, it is not as good as the December cold seemed to promise, but it’s decent. By estimate it seems to have peaked around 310-320 at LF HQ, although we have been subtracting 15-22 hours a day during the hottest phase of the warm spell and now are probably under 250. By my understanding of chill models (and they are just models, not empirical descriptions) that means anything that needs 300 hours to bloom well would be OK, but anything requiring 400 would now need an additional 150, rather than 80-90 before it would be satisfied. Of course, anticipating that this part of the finca might often get under 300 chill hours, I have planted only low-chill varieties up here. (As well as tender subtropicals here and even higher, though that did not work out so well.)

Down in the corralito, at the lowest part of the finca, the chill count is much more impressive. We probably already had 265- 280 by the end of December, and with the impressive cold air drainage down the slope on these dry and cloudless nights, we have had very few significantly negative days. (In fact, at the coldest hedgerow location, none, unless you consider one night of an estimated -.25 chill hours to be “significantly negative.”) Thus down there the trees that are most exposed (to the air mixing of wind and to daytime sunlight) may have had no less than around 350 hours at their peak, while the more protected ones may have had as much as 375 even now (and counting!).

So as long as the heat wave breaks soon and we get even “normal” temperatures for a change, there remain grounds for optimism about the fruit season to come. The forecast calls for only moderate cooling for the next few days, but then a “pattern shift” by the middle of next week. If we are lucky, maybe the rains will return, too.

A blanket of chill


The fog was dense this morning, and is unlikely to break completely before sundown. Viewed from LF HQ, and looking out over our house below, one can just make out a little bit of our driveway and the grapefruit grove. Somewhere just to the right of the palm you can see the corralito. Oh, you can’t? Well, it helps when you know it’s there.

Down at corralito level, the fog is not quite as visible, but the deciduous fruit trees are being treated to a nice chill-containing blanket.


Thanks to the fog, the temperature stayed chilly. It did not even reach 45 till after 10:00 a.m., nor 50 till after noon. That meant many hours of chill accumulation, which should be great for the Shaa Kar Pareh apricot (foreground), and the cherries (back by the fence).

It has been a chilly winter so far, with more than 225 chilling hours accumulated at almost all levels of the finca, thanks in part to an unusually high number of days with lows in the 50s (and one that did not even get that high). And, while normally the chill is significantly less up the hill (due to cold air drainage down the steep slope) this year there is little difference, in part due to some near- or sub-freezing nights at the corralito. When the temperature is in the mid 30s and below, there’s little or no chill accumulation. (Prime temperatures for chill are about 38-45, and anything up to the mid 50s is still weakly positive.)

Late next week it may get rather warm. That will slow down the chill accumulation. But 225 hours is a good total as of the first of January, especially as some warm days at the start of December meant we did not really get started till well into the month. If we get another cold snap or two later in January or early February, 2009 might be another good year for the deciduous fruits.