At the propagation bench, Doug asks a good question: How can you lose chill hours? It is not that you lose them. As I mention in the planting to which Doug was responding (and also the subsequent one about the return of winter), nothing gained in chill is lost. If you reach 400, no matter how warm it may be between the day you got to 400 and the time the tree is ready to break dormancy, you’ve got your 400.
But the question really comes down to the following. If you have 400 on some date, and then it gets very warm, and then later there is another cold snap (like right now at Ladera Frutal), what is your total chill accumulation at the end of the cold snap?
Let’s say you have 400 hours, and then the warm spell is a net â€“50. Then the next cold snap is another +100. Then it warms up for good. How much chill did you get for the winter?
400? 500? 450?
The best answer (though one cannot say “right” because these are all just estimates anyway, as even the experts admit) would be straightforward arithmetic:
In other words, you did not lose anything. In fact, you still had a net gain. But you can’t add the 100 on top of the 400, without accounting for the week (or however long it might have been) of negative chill.
I like to think of the negative hours as marking time. Warm spells in winter put the tree closer to breaking dormancy, whether or not its chill requirement has been met. So, those 50 negative hours make it harder for the tree to meet its 500 requirement, but if the warm spell is not too long, and you get that late cold snap, there might still be time to get back on track for 500. But now you need 150 since the previous peak, not 100.
That’s why it is so hard to get 500+ chill hours in most of southern California. It is not that it is not cold enough. It is that you get a sequence of cold, warm, cold, hot, cold, etc.