Has Chill Requirement been met yet?

As I was walking around my orchard looking at swelling buds - on Dec 25th at that - I thought it might be useful to look up chill requirements and accumulated chill units.

I found this site - getchill.net. It uses the nearest WeatherUnderground station to calculate accumulated chill units. I think it’s caps sensitive - I did not get a calculation when I used noncapitalized weather station ID.

I used the dates 11/1/15 to today.

This was the result.
Below 45 Model: 911 chill hours
Between 45 and 32 Model: 786 chill hours
Utah Model: 923 chill units
Positive Utah Model: 925 chill units
Dynamic Model: 35 chill portions

I’m thinking the 32-45 calculation is most relevant, based on this info from Chicago Botanic Garden. I don’t know the meanings of the other calculations.

With 786 chill hours accumulated so far, I think we have met the chill requirement for most of my fruit species - maybe a few varieties need more, but I doubt it. One website list of chill requirement -

I guess I didn’t make myself less anxious about the buds swelling early. Maybe readers / commenters will have better calculation or more to say about chill requirement as this weird el Niño winter is affecting their orchards.

Time to settle down for a Christmas dinner of Jiaozi dumplings and red wine. Hope your dinners were equally comforting and filling.

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I’ve been wondering about the same thing here. I never keep track of chill hours or give much attention to my trees between November and March. This year, however, I’ve been taking advantage of the unusually warm weather by working on my trees over holiday break. I’ve noticed the buds getting bigger this week, especially after the long warm rains we had in the middle of the week. A few other plants have come out of dormancy and have new green growth. It feels more like mid-April. :grimacing:

Yes,that’s the calculator I use.I think the inventor,Tom,may have introduced it when we were on GW. Brady

I’m not even close. I just checked mine on getchill and I’m only at 17 (although I think that’s actually lower than it truly is).

In fact, my Blenheim and Spice Zee still have more than 50% of their leaves. If I remove the rest of them will that force the trees to go dormant?


I am not saying this happened to you but it did to me, check and make sure the info from the wunderground station you are using is correct and wasn’t experiencing problems during this time period. There is a new one that went up in my area earlier this year and I used that one for input to get chill hours since it is closer to me then the older other two. It was saying that I had 77 chill hours for Nov.-Dec. which didn’t seem correct. I went and plugged in the other two older stations and they said Between 45 and 32 Model: 440 chill hours and 414 chill hours for my area. Hope this might help in your case.

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Lance - Something is definitely wrong with either Weather Underground or GetChill. I just tried again and now it’s saying 274 hours. That sounds too high.

I don’t think it’s an issue with the station being new because one of the two near me I remember using last year. What’s really weird is that there is another station about 5 blocks away and that one is registering 101 hours.

Either way I’m going to be struggling for chill hours on a couple of my trees that I was on the border with anyway because this has been a warm December. Anjou pear is supposedly 800 hours but I got some fruit on it last year with only 450 hours. Hopefully I can at least get into the 400s again.

Just one more thing to fret about. Might as well just keep your eyes open unless you have some plan to protect your trees once they start budding out.

Some of the personal weather stations go offline for awhile and there will be missing data. Getchill.net probably should have been coded to indicate missing data. Also, I’ve seen some stations are inaccurate and will consistently be higher or lower by a several degrees. About the only thing you can do is look at all the local stations and ballpark the consensus.

What does the start date actually represent? It seems like the practice of always using November 1st would be inaccurate as the weather patterns from one year to the next have been wildly different in my area. Is the start date suppose to reflect the approximate time of entering dormancy? It seems like a very difficult factor to determine, yet important if you are on the threshold of satisfying the chill hours during freakishly warm winter temps.

I’ll start off by giving the stats for my immediate area as of Dec 31.
Below 45 Model: 163 chill hours
Between 45 and 32 Model: 144 chill hours
Utah Model: ** -208** chill units
Positive Utah Model: 315 chill units
Dynamic Model: 7 chill portions

I checked out other nearby stations and had comparable results. When I first started checking these numbers on Tuesday, I believe that I took from them what I wanted - a bit of security to bolster my desire for my fruit trees to get through the winter unharmed and to provide a decent harvest next year. I interpreted the #s as showing that we’ve had insufficient chill hours for our periods of winter warmth to propel them toward prematurely breaking dormancy.

My first fruit tree blooms have always been on my Japanese plums, followed by cherries which overlap peaches and the earliest blooming apples. I’ve been closely examining those plums and cherries each day and they seem to be sleeping peacefully. I do continue to convince myself that any slight fattening I might perceive on the cherries is only my imagination.

I’ve been checking the apples now and then out of curiosity over when and if some of them were ever going to go into total dormancy. If it’s true that chill hours don’t count until all the leaves have fallen, then those that hadn’t completed, or in some cases even begun, the process of shedding them would still have zero hours.

I do not enjoy cold weather. To me “cold” is when nights are in the 30’s or below and days are less than 65F for highs. However, since my understanding is that the fruit trees need that weather, I’ll welcome it for their sake.

That’s the way I’ve been thinking up until Wednesday when I went to check the apples to see how far along the established ones were toward losing their leaves. Now I’ve got a glimmer of one reason people keep creating different ways of calculating chill. I’m also disheartened over an increased likelihood that I may once again harvest zero apples this year.

My earliest blooming apple is a Double Red Delicious. So far, I haven’t had any other varieties overlap its bloom. So, of course, I’ve never gotten any fruit from it. My understanding is that Red Delicious varieties require between 700 and 800 chill hours. Wednesday I noticed that some of what I’ve been sure are fruit spurs had wee tips of green showing. Today those tips have become mouse ears and I can see that, yes, they are bud clusters. Those branches have leaves popping out all over them now. Some other branches are a few days behind.

My other youngish established apples appear to be following behind that one at about the same rate of development as last year. However, we are about to leave this protracted warm “fall” behind and enter a period of days in the 50’s and 60’s and nights in the 30’s and 40’s. There are still two months of winter to go. I suspect that at least this one tree may wind up damaged during that time.

As much as I’m apprehensive about the apple trees’ performance over the year, it’s also been interesting observing the performance of all the trees and food producing plants during this year of extremes and anomalies. I suspect that chill hours may be less of a factor in determining when apples and berries push out new growth and blooms than it is with stone fruits. We had a wet spring, followed by a long, hot summer with few rains, and a very warm autumn interspersed with short periods of chill on approximately a bi-weekly basis for the past 6 weeks while the ground has been alternating between flooded and just plain wet since the last week of September. Fruit trees here start losing leaves during the summer heat. They don’t really put on growth in summer, but the apples do put out new growth in fall. For the established apple trees that still retain some leaves, it’s that autumn growth that is still holding on to them.

In answer to Bear’s question of whether chill requirements have been met yet - I don’t know. I suspect that chill hours don’t stand alone, that the determination of whether they’ve been met operates in conjunction with other factors such as day length and whether it is increasing or decreasing, moisture, nutrition, past and current stresses and relief from those stresses, and the number of growing degree days since the last period of growth. I also suspect that the significance of any combination of factors differs from one variety of fruiting plant to another.

The thoughts I’ve shared are intuitive based on observation and extrapolating from others anecdotes, not on controlled studies. I’m just surmising from my very limited knowledge and experience. There are others here who have much much more of both than I do. I hope they speak up and right the course of my thinking if it is off track.

For the past 7 days, or one week, 164 of the 168 hours for Portland Oregon have been chill hours. Our 30-year-based average temperatures for this time of year is 45high and 35low–all chill hours. This goes on for weeks.

California gardeners might want to check out the UCDavis website for chill hour calculators. Here. If that doesn’t work, you might have to do it by hand.

For the SouthEast USA, here is a chill hour calculator.

Here is a focused discussion related to peaches from TAMU related to NE TExas: "The dormant buds of many plants require a period of cold weather to grow, flower, and develop properly, but requirements vary widely by species. For dormant buds of fruit trees, this is commonly referred to as the chilling requirement. Chilling hours are calculated as a tool for fruit producers to gauge whether their crop has been exposed to cold temperatures for a long enough time period. This table shows chilling hours as recorded at the Overton Center and should provide you with an estimate of chilling hours if you live in northeast Texas.
Temperatures can fluctuate from one location to another, however, so the number of chilling hours at your peach orchard may be different than what we have recorded here. For chilling hour data from past years, see the table below. Several methods exist for calculating chilling hours; two are presented here.

Also on peaches, and also SE USA, here is some info from Clemson, with some nice photos of what happens when chill hours are less than optimal.

So far I have not found a different chill hour calculator focused on Pacific Northwest. It would need some specificity by weather station. Ocean, mountains, and the Columbia River Gorge have major effects on local climates. The one that I linked seems pretty good. I can look at data from several nearby weather stations and make my best estimate from that.

@MuddyMess_8a, I think you have the ideal insight and approach to this topic. The intent is to provide information or tools or experiences that relate to growing fruits. Some amateur pomiculturists might find it interesting or useful. Others won’t. I don’t think anything is simple, and this info will relate to each cultivar, each microclimate, and each gardener, in a different way. Since my oncologist told me that unless my health plan kills me first, I could have 15 good years to go, I feel less urgent about having to get a taste of something this year or next year, and more willing to look at the long term. But I really want a taste of those pawpaws and persimmons, not to mention the more typical fruits.


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I don’t know about their chill requirements, but this seemed like a decent place to throw in my observation that pawpaws don’t seem to enter dormancy based off temperature. I never got around to moving my seedlings from the windowsill to the outdoors, and they dropped their leaves when fall came. I assume they’re triggered more by day length.

A few have lagged and held on to a leaf. These were invariably the ones that germinated at the very end of summer, after I had written the seeds off as dead.

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@tjasko, somewhere I read that for pawpaws, going into dormancy was based on daylength. That you can fool seedlings into growing taller the first year by giving them artificial light, like we do with chickens to keep them laying eggs.

The chill requirement would be for flower buds coming out of dormancy. Looks like they need 400 chill hours. Mine have nice furry flower buds now. If this El Niño fools them into opening then zaps them with a freeze, I’ll be unhappy. On the other hand, if they bloom early, giving them a longer season to mature, and I get a crop of pawpaws, I’ll be a pretty happy guy.

My pears and apple buds are not swelling just yet. Plums and peaches, maybe a little bit but still tight. Lilac buds are looking green and plump now.

Wow Muddy! You guys are having a VERY warm winter so far! Heck we see better numbers than that even on warm years. Yikes! Unless it turns cold for you soon you are going to see some serious lack of chill repercussions next year.

El Nino on the other hand brings us here in Phoenix a excellent chill profile for a change. Its been cloudy and cold most of December. We saw more chill this year by Jan 1 than ive seen in entire seasons for the last 4 years. Looking forward to seeing normal growth resume on some of the higher chill types we are growing. Our pistachios in particular dont grow right on low chill years, lots of blind wood/slow growth etc.

Here is where we are at today.
Below 45 Model: 624 chill hours
Between 45 and 32 Model: 504 chill hours
Utah Model: 363 chill units
Positive Utah Model: 717 chill units
Dynamic Model: 24 chill portions

We are a reporting weather station for the weather underground, so I know the numbers are spot on. Weather station is in the middle of the orchard.

The weather here quickly changed with the new year. It may have been the first time in my life that I’ve wanted cold temperatures. Not for me; I don’t enjoy cold. Cold for the fruits. Those very warm and wet months from Oct to Dec had some chill time mixed in with lots of growing degrees. I suspect that one more week delay before freezing temps arrived would have guaranteed close to total loss. It’s “Southern cold” now with days in 40’s -60’s and nights in the upper 20’s to 50’s. I had hoped that the colder weather would halt bud advancement. It has only slowed it for my trees. The Japanese plums and cherry/plum hybrids seem the least at risk, with the most dormant appearing buds. I think the highest risk is for the apples that get the earliest morning sun. Here’s a pic of the Double Red Delicious taken this morning.

Eleven days ago I could see the pink, but the calyxes were still tight. Peaches and other apples are about one warm week behind, with the pears and cherries right behind those in readiness. This is nearly opposite the normal bloom sequence here. I even had a pomegranate start to leaf out, for pity’s sake.

It is 56 here now and a low of 35 last night. The chill accumulations as of today are:
Below 45 Model: 227 chill hours
Between 45 and 32 Model: 199 chill hours
Utah Model: -120 chill units
Positive Utah Model: 404 chill units
Dynamic Model: 10 chill portions

compared to what they were on the first:
Below 45 Model: 163 chill hours
Between 45 and 32 Model: 144 chill hours
Utah Model: ** -208** chill units
Positive Utah Model: 315 chill units
Dynamic Model: 7 chill portions

Can chill hours still help once trees (I’m not even going to go into strawberries and bramble reactions) get this far along?

These are my chill hours so far this year. Not much to speak about. I’m holding my breath!

Below 45 Model: 734 chill hours
Between 45 and 32 Model: 625 chill hours
Utah Model: 883 chill units
Positive Utah Model: 925 chill units
Dynamic Model: 44 chill portions

Here in NE Florida, we’re not even close…The good thing is we get some of our coldest weather in February. The bad part is that I’m already seeing bud swell on some of my pears and blueberry bushes…

Below 45 Model: 43 chill hours
Between 45 and 32 Model: 43 chill hours
Utah Model: -738 chill units
Positive Utah Model: 149 chill units
Dynamic Model: 6 chill portions

Wow, those are some crazy numbers! Here are my numbers in San Jose, CA

Below 45 Model: 491 chill hours
Between 45 and 32 Model: 462 chill hours
Utah Model: 593 chill units
Positive Utah Model: 826 chill units
Dynamic Model: 33 chill portions

The next 10 days are forecasted with above average temperatures with no chill. I’m hoping it’ll turn cooler after that. My spot is supposed to get about 800 chill hours.

I also hope the Utah model is more relevant. I get about 1000 chill units with that model on an average!

I’m starting to believe the model is relevant!

Based on the Utah model, I’m assuming most of my chilling requirements have been met. Although I’m about 300ft above the station, but I think it’s pretty accurate. I’m seeing lots of flowers and leaf out on my peaches and nectarines and even my Apple has some flowers starting to show.

Below 45 Model: 0 chill hours
Between 45 and 32 Model: 0 chill hours
Utah Model: -287 chill units
Positive Utah Model: 0 chill units
Dynamic Model: 0 chill portions