Fruit Harvest Depends on Winter Cold

peaches in Woodland Hills

If you have a fruit tree or two, there’s one branch of science, despite its unfamiliar name, which you probably know something about.  The science to which I refer is phenology.  Phenology is the study of recurring biological phenomena, such as bird migrations and blossoming of plants, and how these phenomena are impacted by climate and seasonal weather.  These phenomena may also serve to predict the weather in the days and weeks ahead or, by the same token,  the weather during a discrete period of time may predict the extent to which these recurring phenomena are expressed in a given year.  When lilacs bloom, for instance, there is generally no more danger of winter frost. And when jacarandas begin to flower, intense summer heat is usually just around the corner.

A familiar example of phenology, although of the bogus variety, is that which associates the behavior of a rodent with the arrival of spring.  On February 2 or Groundhog Day, a coddled groundhog known as Punxsutawney Phil emerges at dawn from his burrow in  Punxsutawney, Pennsylvania, and informs his primary handler in Groundhogese as to whether or not he has seen his shadow.  If he has seen his shadow, winter is supposed to continue for another six weeks.  If he has not seen it, spring will come early.  For the record, Phil’s predictions — or his handler’s translations of them — have been accurate only 39% of the time.
With deciduous fruit trees, however, you can comfortably rely on the extent of winter cold to be predictive of spring blossoming.  The reason you cannot grow cherries south of the Antelope Valley is that winters are just not cold enough down here for cherry trees to bloom.  And in the Antelope Valley itself, a cold winter will produce more cherry blossoms and fruit than a warm one. To take another example:  if you live in Van Nuys and did not see many flowers or fruit on your Santa Rosa plum tree this year, it’s because your winter was much warmer than usual.
This reality is difficult for beginning fruit growers or novice gardeners in general to grasp.  The reason for this is that the primary limiting factor to the growth of plants in general is winter cold.  You will not be able to grow citrus or avocadoes in the Antelope Valley because it is simply too cold for these tropical trees which, if planted in Palmdale or Lancaster, will probably die the first time it freezes.
Climate zones, whether they are the 10 conventional USDA zones over the entire fifty states or the 24 zones into which the Sunset Western Garden Book divides the Western states, are determined by coldest average winter temperature.  So hardiness — which is the horticultural term for cold tolerance — kind of turns climate zones on their head where deciduous fruit trees are concerned and, suddenly, a colder climate, as long as its in the West, expands the selection of deciduous fruit trees you can plant.
Each deciduous fruit tree variety has a number of winter chill hours that it requires in order to flower and fruit.  Those are the total number of hours that temperatures fall between 32 and 45 degrees between November 1st and February 28th.
The above discussion was prompted by an email from Pete Torrez.  Torrez, who gardens in Sylmar, wondered why his peach tree is not bearing fruit and he thought it might be associated with watering, pruning, or fertilization practices.  However, when he told me he did not get any flowers this year, and that he has not seen much fruit for several years, even though a nectarine next to is is covered with fruit, my best guess is that it is simply not cold enough in Sylmar for his peach variety to flower and fruit.  He says he bought his peach tree in the Central Valley which could explain his lack of fruit since the Central Valley — from Sacramento to Fresno — experiences considerably more winter chill than the San Fernando Valley.
To see how many hours of winter chill were recorded in your area each winter since 2011, go to ucanr.edu/chillcalc/ and scroll down the page.  Clcik on “Station List” and you will be taken to a list of weather tracking areas such as Los Angeles, San Bernardino, and so on.  Since you live in Sylmar, click on Los Angeles.  The weather reporting station closest to Sylmar is Arleta.  There you will see a number, 184, which was the number of chilling hours recorded beween November 1st of last year and February 28th of this year.  Under “reporting dates,” you can click on “historical” and “view data” and there you will find that there were 204 chilling hours last year and 302 chilling hours the year before.  By looking at all the data since 2011, you can calculate the average number of winter chill hours for your area.
Now that you have this information, you can visit Otto & Sons Nursery (ottoandsons-nursery.com) and click on the “Fruit Tree Guide” to find deciduous fruit tree varieties that match your winter chill hours.  Each variety listed is accompanied by the number of winter chill hours required for flower and fruit production.
Tip of the Week:  Let’s say that the average number of annual chill hours in your area is 200 hours.  If you were looking for suitable peach varieties, you could select from Eve’s Pride (100-200 hours) and May Pride (175 hours).  Other fruit trees for low chill areas such as yours would include persimmon (200 hours), pomegranate (150-200 hours), ‘Ana,’ ‘Dorsett Golden,’ and ‘Ein Shemer’ apples (150-200 hours), fig (100 hours), as well as Chinese date (Zizyphus jujube), a drought tolerant tree that does not require any winter chill whatsoever to flower and fruit.  Low chill blueberry varieties include ‘Sunshine Blue’ and ‘Misty’ (both 150 hours). ‘Concord Seedless,’ ‘Flame Seedless,’ and’Thompson Seedless’ grapes thrive in mild winters, too, requiring only a 100 hours of winter chill to produce fruit.
All of the varieties just mentioned would grow well anywhere in the San Fernando Valley.

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