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30 January 2011
A trivia question to our readers: Chill hours are (a) the amount of time a recipe calls for congealing Jell-O (b) the duration of your fridge's warranty (c) the number of hours between the temperatures of 32-45 degrees Fahrenheit that aid in an important biological process. Drum roll please...the correct answer is (c)! I am sorry to disappoint those of you who were breathless at the thought of a Jell-O molding program at the farm.
Our Freeway Food Forest depends on chill hours. Without them, our fruit trees would not bear, you guessed it, fruit. Due to this, chill hours were one of the farm's main criteria when it selected fruit trees last winter.
Let's go over the basics. Trees fall dormant during the winter to protect new growth from frost. Longer nights initiate dormancy, and chill hours are like Sleeping Beauty's kiss--- they cue the trees to wake up in the spring. A chill hour is a winter hour between 32 and 45 degrees, but if the temperature rises above 60 degrees, a chill hour must be subtracted from the overall total.
You may be thinking, this is San Francisco, what do we have, 10 chill hours? While I've witnessed a handful of days where cars are glazed with frost, I was wondering the same thing, so I asked farm director Jay Rosenberg about it.
"In San Francisco," Jay said, "We're generally looking for 450 hours or less. Chill hours vary in each neighborhood due to its unique microclimate. So, rather than holding up a thermometer and recording the temperature every hour, we can work backwards and gauge the chill hours by observing when a fruit tree variety thrives in a neighborhood."
Jay also confirmed that our fruit trees are doing very well, especially the ones were planted in the ground outside our gates. Low-chill varieties were selected last year, including the Pink Lady Apple, Santa Rosa Plums, and Katy Apricots. When they joined the farm, these low-chill trees had already been grafted on rootstock that favors our climate.
Our trees will fruit next year and serve as a living demonstration of chill hours.
Discover more about our trees and how you can purchase one for your garden.
"Chillin' with the Trees" by Angela Goebel, January 30, 2011
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Over a thousand volunteers, neighbors, and community members have helped coordinate the farm efforts.