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Chilling in my greenhouse, messed up again!!

Posted by fruitnut z7b-8a,4500ft SW TX (My Page) on
Sat, Mar 2, 13 at 12:47

Ran chilling cycle 56 days this year and wrote down high and low temperature each day. The extremes in 56 days were 63 and 34F. Average high and low 54/37F. These are almost exactly the averages in the best fruit growing areas of CA for Dec and Jan in say Modesto where Zaiger resides. What more could you want, right?

According to Texas A&M the Utah model allocates 1.0 units chilling per hour to any temperature between 37 and 48F. Based on that over the years I've been estimating 14-18 units, hours, chilling per day. 56 days x 16 hours/day = 896 hours chilling.

But the left side of figure 4 in the link below shows a very sharp peak in chilling effectiveness at 40F. At 37 and 48F chilling is only about 60% as effective as at 40F. Using this I'm maybe looking at 10-12 hours chilling per day at best. That's 560 to 672 hrs chilling this year.

Based on my trees reaction 600 hrs is probably more accurate. The sweet cherries and higher chilling apricots are not going to set more than 5-10% crop again this year.

Next years it's 90 days all out chilling or bust!!!!

Here is a link that might be useful: Check left side fig 4: utah hr vs temperature

Follow-Up Postings:

RE: Chilling in my greenhouse, messed up again!!

Oh my. Welcome to my world, fruitnut. I got about 600 ch as well. So, secretly, I am sort of excited to see how things do for you, as that will tell me exactly what will work in my yard, lol! Seriously, though, sorry about the miscalculations :-( Vista's not such a bad place for fruit growing, and you've replicated Vista in the hills, where I am surrounded by more commercial growing grounds than anywhere else in the entire country :-) Just not so much stone fruits, lol!

Patty S.

RE: Chilling in my greenhouse, messed up again!!

  • Posted by fruitnut z7b-8a,4500ft SW TX (My Page) on
    Sat, Mar 2, 13 at 18:20


Thanks for the reply!!

I know pretty much what the climate is like in Vista and I'd have bet a bundle that my greenhouse got a lot more chilling than you did this year or any other. I'm replicating Modesto not Vista or at least that's what I thought.

Your average high/low in Dec and Jan is 66/46F. My greenhouse averaged 54/37F. Modesto runs 56/40F. Maybe I was too cold at night.

Next year I'm setting the heater on 39F at night. That will cause the nights to cycle 39-41F as the heater goes off and on. I can average at least 10 hrs a night near optimum of 40F that way. In the past I've set the heater at 34 to 37F, mostly 37 since A&M had that at optimum.

Here is a link that might be useful: Vista vs Modesto climatic averages

RE: Chilling in my greenhouse, messed up again!!

Fruitnut, a few basic questions, because your greenhouse is so wonderfully unique. Do you change 'shade' cloth gauges by the month? or season? Pollination? Your greenhouse is superb, sorry about your calculations. Mrs. G

RE: Chilling in my greenhouse, messed up again!!

  • Posted by fruitnut z7b-8a,4500ft SW TX (My Page) on
    Sat, Mar 2, 13 at 20:39

Mrs G:

I do add extra shading during chill cycle. That helps keep it cooler during the day. Also I run the evaporative cooler. These together allow me to keep it 5-12F cooler inside than outside during the day. Pollination is by hand or mostly bumblebees.


There is another relationship published by A&M that relates chill hrs to mean temp during the two coldest months. Using that gives estimates of:

Vista 66/46 = 56F mean = ~500 chill hours
Modesto 54/40 = 47F mean = ~1050 chill hours
greenhouse 55/37 = 46F = ~1100 chill hours

Greenhouse was only 56 days rather than 60 but that should still equate to ~1,000 chill hrs more like I was thinking. Maybe I'm not short on chilling but I've tried working through other ideas and keep coming up short.

Here is a link that might be useful: A&M temperature vs chilling in figure 1

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