- Author: Mark Bolda

So the question now is, so what Mark, we're around 68 chill units accumulated as of September 22 in Macdoel, but what does that actually mean to me as a grower? How does it compare to years past?

Fortunately, we can reference past year's chill accumulation and make just that comparison.

Again, this year up to September 23 (I'm adding in last night's numbers right now), we have 32 hours below 32^{o }F (0.0 chill units/hour x 32 hours), 153 hours between 33^{o }F and 45^{o }F (153 hours 1.0 chill units/hour), 170 hours between 46^{o }F and 60^{o }F (170 hours x 0.5 chill units/hour), and 188 hours over 61^{o }F (140 from 61^{o }F to 80^{o }F and another 48 over 81^{o }F). Negative chill units will be from the 188 hour (which we multiply by a quarter to obtain an approximate range for temperatures from 61^{o }F and 65^{o }F), which gives us (188 x 0.25) 47 multiplied by -0.5 chill units/hour = -24 plus the balance of 141 multiplied by -1 chill units per hour = -141. Adding -141 + -24 + 153 + 85= + 73 chill units accumulated so far.

Looking at the numbers for 2014, which we consider a pretty hot fall in Macdoel, the same calculations as above from 3 below 32^{o }F, 137 hours between 33^{o }F and 45^{o }F, 147 hours between 46^{o }F and 60^{o }F, and 256 hours over 61^{o }F (202 from 61^{o }F to 80^{o }F and another 54 over 81^{o }F) brings us to -13 chill units accumulated by September 23. This squares with observations from the field in September 2014 that the plants were barely chilled at all.

Next, let's have a look at 2013, which has been described to me as a "normal" chill year. Referring to the Lassen historical chill chart, by September 23, we experienced 10 hours below 32^{o }F, 127 hours between 33^{o }F and 45^{o }F, 208 hours between 46^{o }F and 60^{o }F, and 226 hours over 61^{o }F (181 from 61^{o }F to 80^{o }F and another 45 over 81^{o }F) which by using my calculations brings us to +34 chill units accumulated by September 23, 2013.

Conclusion; from the looks of it we are tracking right along and having a normal accumulation of chill in the autumn of 2015.

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