What are chill hours? There are different ways to calculate chill hours, but basically they’re the cumulative hours under 45°F from the beginning of November through the end of February. And why do we care? The cold is required to break down the hormone that causes deciduous trees to be dormant, allowing for new growth, bud break, blossoms and eventually fruit.
There are lots of low-chill varieties of fruit trees and bushes available to grow in San Diego. Most of the recommendations I’ve heard are to look for varieties that require fewer than 300 chill hours. For blueberries – that’s the southern highbush types. For apples, we found a triple graft tree with Anna, Dorsett Golden and Einshemer on a single rootstock. (It’s been a few years now, but I’m guessing that must have come from Walter Andersen nursery.)
I just found a link for weather services on the UC-Davis Fruit and Nut site that shows the chill hours to date based on a weather station on the Balboa Park Golf Course. So far this year – we’re at 30 hours, and all of that was in the last 5 days. That’s more than twice what we’ve had in any of the last 8 years for the same period. Time to start thinking about the dormant spray and winter pruning.