I know there are 2 or three standard methods for estimating the amount of chilling hours for a fruit tree variety.
One of them says that you can add up the number of all hours below 7 ºC (45ºF), another one says that you can take into account the hours between 0 and 7 ºC (32 and 45 ºC) only,
both of them between November 1st and february 28/29th. And there are other a bit more complicated ones to apply like the Utah method.
The question is how do I chose which one to use, because each of the two methods explained produce different amounts of chilling hours, which in some cases can be quite wide, especially if in a specific location there have been many hours of below zero temps.
Also somewhere I have read that the chilling hours that are important are the ones among 0 and 7 ºC, negative temps apparently don't really affect the dormancy of fruit buds, but I am not sure if this is true.
Anyone has any experience with this?
I use the under 45 degrees f method, for our area it seems to be the right one.
I am of the opinion that the south of the USA is where the <45 degree method works best for calculating chill hours.
Which means that the further north you are in the USA the more you should use the >32 but <45 degree method.
Down south you aren't going to get many "chill' days until at least the Middle of December and this has been true for the last 10 years and will probably only get closer to January year by year now.
My biggest concern with our orchard is spring bud out, I lost a lot of growth this spring due to a late frost so next spring I will be covering the trees to prevent that from happening again.
I am not yet 100% clear though about the logics of this 2 methods. You say
Down south you aren't going to get many "chill' days until at least the Middle of December
Are you meaning that in the south, the chill hours accumulate mainly from late autumn (up until early spring) so using the other criteria (>32 and <7) would not allow to catch as many chill hours
On the other hand in colder climate areas, using the second criteria allows for discarding a number of negative temps that would probably not accrue so much to the whole count?
hau Antonio, yes and no to your question. Yes our chill hours should accumulate starting in late autumn, and colder climate areas would start earlier perhaps. The problem is that we are in the changing time, the earth mother is shifting the timing of the seasons. I will try to explain what I mean.
Where I live (Arkansas) the past 20 years the winter temps have changed drastically. Where we used to get temperatures for weeks that were either near or below freezing (1940's thru 1980's), we now are lucky to see one day at 32 f. at a time.
If I was to try and calculate chill days any where near 0 c or 32 f, I would not be counting many days. Interestingly, our orchard trees (Pear, Plum, Apple, Mulberry and Fig) seem to be adjusting their bloom to the new temperature range.
Plus, the method of calculating chill hours is also dependent on the variety needs, some fruit tree varieties need colder weather than others (hence the zone system I think).
Our chill hours used to start in November and now we don't see them start until mid December. This year many of the commercial peach orchards here didn't get the needed chill numbers for the varieties they were growing and there is no fruit set on those particular varieties.
Fortunately all of these orchards have several varieties growing so they did get fruit set on the ones more tolerant of low chill hours.
The way I set up my calculations is from the first registerable day below 45 f, this day is marked as day one and I keep track from there.
Unfortunately, the temp fluctuates quite a lot beyond the "norm" now so even if we get that first day, the temps might go back up to near 80 soon after, if that happens I start over, since it was a fluke low temp.
The changing weather patterns are creating havoc for everyone that depends on chill days for fruit crops and even planting times for vegetables.
I am of the opinion that what we used to be able to depend on for calculating chill hours can no longer be considered how to do it, since we are not yet to the middle of the cycle, fluctuations are going to continue and the start periods will continue to shift.
For example; Arkansas now gets holding cold in January, 10 years ago it started in December, 20 years ago it started in November and prior to that it was the end of October the first good cold snap would occur.
I now keep recorded temps and don't start counting chill until there have been three consecutive days with low enough temps.
The varieties I grow all have fewer chill hours needed so I have a better chance to have the trees produce well.
Even with doing that, this past winter/spring we had a severe cold snap of one day that killed the buds on our Brown Turkey Figs, resulting in tree die back to the roots. They are recovering but the old branches and trunks are dead.
While fluctuations are going to happen, I feel that we need to be able to have at least a few consecutive days in for achieving an accurate and meaningful chill count.
This changing time of the earth mother will have all of us guessing if we don't have good records to guide us along while the cycles shift to their new places.
What we used to be able to depend on, is now in flux.