Winter chill: As mild as winters can be alongside the bay — perfect for subtropicals that grow so successfully here, e.g., Feijoas, Persimmons, Figs and Citrus — locals still love to grow trees, shrubs, and bulbs that need winter chill in order to perform. Tulips and other bulbs from northern latitudes, for example, can stay in the soil like perennials in the Netherlands; here they’re treated as annuals, bought in the fall and refrigerated for a good three months before being planted out, then chucked in the green bin by end of May.

Winter chill protects deciduous trees and shrubs by allowing them to go into a state of dormancy, where sap retreats into the roots. The sap rises again the following spring via hormonal trigger only once the amount of winter chill that tree or shrub has evolved to require has been reached. Having once gotten too close to a bucket of sap on a late winter day in New Hampshire, I learned at age four that sugar maples are too busy breaking dormancy via sap rise to play nice (Mom had to take scissors to my hair).

During our occasional January false springs, if the tree has not gotten its customary amount of chill, it is protected against early bloom and leafing out. But if the tree doesn’t get the accumulated chill it needs by springtime, blossoming and leafing out is sporadic and too drawn-out. Fruit production drops, and the tree itself is weakened.

Nearly 30 years ago when planning my own garden, I paid a visit to a local U.C. Cooperative Extension office on Bay Farm Island, where I learned that the winter chill data for Alameda was, like everywhere else stateside, calculated by the number of hours the outside temperature hovers between 33ºF and 45ºF, and this accumulation is collected between November 1 and February 28 of every year.

I learned that Alameda was expected to reach 500 “chill hours” 50% of the time. Otherwise, a lower number of chill hours should be expected. Over time, I found this to be correct, though some years Alameda received as many as 700 chill hours. I knew this because early on I was lucky enough to come across a handy online winter chill calculator set up by home fruit grower Tom Economous of San Diego (http://getchill.net/). It allowed me to type in the station numbers for home (personal) weather stations connected to Weather Underground (www.wunderground.com/) in my neighborhood and check their accumulating chill data. Sadly, the Get Chill site does not seem to be connected to local home weather stations anymore, and Economous, whom I had talked with some years ago, did not reply to my recent email. I may just have to break down and buy my own weather station, because once fixated on this kind of data, one never goes back.

Some years I celebrated the whopping number of chill hours because of a sour cherry tree we had that required 700 chill hours. But as time went on, I began to see my neighbors’ chill hours decline and my cherry tree decline with them. It died after three consecutively warm winters, too confused to blossom and dying branch by branch (which may have been a bacterial disease). Had I bought that tree from one of the nurseries here in town, where the tree varieties are chosen specifically for our soil and climate needs, I would not have worried every winter whether I’d be making cherry pies for Fourth of July. Then again, no sour cherry varieties seem to be a good fit for 94501 chill-wise.

But there are always a few fudge factors! Throw a little caution to the wind by planting a marginal variety (say, a 700-chill-hour apple like Braeburn, which seems to do well in several Alameda yards I know of). Start by planting it in deep winter shade; save those sunny winter spots for citrus and other subtropicals. There actually are a few higher-chill apple trees thriving in Farm2Market’s orchard on Alameda Point. The Belle de Boskoop apple (pictured) requires 800 to 1,000 chill hours and yet surprises me every year by pumping out the fruit regardless.

As for future winters and the chance for normal winter chilling in California, Jeannette E. Warnert, in her February 19, 2020 blog entry titled “Warm winter renews concerns about orchard chill,” quoted Lauren Summer on NPR’s Morning Edition: “Fruit and nut trees that require the most winter chill will run into trouble by mid-century, when experts predict consistently warmer weather.” Plan accordingly, but don’t be afraid to garden with insight and a little devil-may-care.

Alameda Backyard Growers is dedicated to teaching our neighbors how to grow food. We offer online and in-person educational programming. Visit us at https://www.alamedabackyardgrowers.org to join our mailing list to receive our educational newsletters and information on classes and events, locate the Free Seed Library nearest you, or join Project Pick as a fruit picker or fruit donor. Contact ABG at info@alamedabackyardgrowers.org.