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 –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:
-
400-50+100=450.
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.
It’s worth noting that warm weather does not necessarily equal negative chill. It depends on which model you use (and what plant species), but I consider the 45-60 range to be pretty neutral as far as chill is concerned. It’s only in the 60+ range that loss of chill is really an issue.
Here in North Florida, we have similar issues of meeting chill requirements…and while by no means “cold”, we do have enough cold nights we can’t really grow anything really tropical, so we’re sort of in a no-man’s land with sort of marginal chill for temperate crops, but unable to grow tropical crops without covering them occasionally in the winter (not that we don’t manage pretty well with a lot of things.) There’s definitely a niche for low chill fruit breeding out there. (I’m hoping to come up with some nice low chill blackberry options…made a few crosses this year.)
Seed planted by Evil Fruit Lord — 25 March 2006 @ 13:05
Yes, the chill model I use has the 45-60 range (and especially 50-60) fairly neutral. In fact, while my calculation program is more complex, my quick mental estimates always look at a day in terms of how many degress the high was above 60 and how many degrees the low was below 60. When I know those two, I have a pretty good idea how much chill was gained or “lost” on that day.
And the more complex calculations that I do on the computer seem pretty reasonable in terms of what blooms when, given the listed chilling requirements of most varieties. For others, however, there is no match, meaning the variety’s listed chilling requirement is inaccurate or else the chilling model is not valid for that type of fruit.
More low-chill fruit breeding would be most welcome, of course, though even in the decade or so that I have been growing a wide range of fruits, the selection (and quality) of cultivars has really taken off.
Thanks for stopping by, Evil Fruit Lord! And do so often.
Seed planted by Professor Matthew Søberg Shugart — 25 March 2006 @ 15:37