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The 'low chill' controversy

Posted by axel_sc z9b/Suns16 (My Page) on
Mon, Jan 12, 09 at 0:12

Has anyone clearly observed what symptoms appear as a result of low chill? I've read multiple reports on this, but I've never had the opportunity to observe for myself.

What I've heard so far is:

1) Delayed blooming
- I think applenut has reported that some of his so-called high chill apples simply bloom a lot later and therefore fruit a lot later, which doesn't seem to be a big problem.

2) Reduced blooming, or blooming over a longer period of time.
- I've heard about this showing up on cherry trees, leaves come out first, and blooms follow, but less and over a longer period of time.

3) Significant reduction in vigor
- One of our local CRFG members lives right on the coast and he has observed that a number of higher chill apple trees will grow much less vigorously on the Winter sunny side of the tree (south side).

i don't think this chill thing is really well understood. For one, I've noticed that a rather large number of so-called low chill apples and even low chill cherries happen to be self-fertile and early bearing. Pettingill, beverly Hills and Gordon are apple trees that come to mind, and lapins and Stella are a couple more. Are these really "low chill" or are they just self-fertile.

Now there are real low chill fruit trees: Anna and Dorsett Golden apples are not self pollinating, but they will produce up to three crops a year in mild climates and seem oblivious to the seasons. The new Royal Lee and Mini lee cherries from Zeigers will bloom and fruit when spurred by manual defoliation after a zero-chiil Socal Winter.

Now I happen to be in a medium chill area, (800h on average) but fairly freeze free. My first attempt at growing Dorsett Golden doesn't suggest it's capable of doing three crops here. it's bare right now, although it seems to be getting ready to bloom.

My basic understanding of chill is that the higher chill requirements simply keep a tree dormant through occasional Winter warm spells, or by requiring a longer early Springtime chill exposure. If it's below freezing, apples don't accumulate any chill. So most of the chill accumulated in Northern latitiudes takes place either during the late Fall or early Spring.

So the questions I have are:

- How often does it turn out that chill accumulation is erroneously attributed to poor fruiting when in reality the issue is either pollination or fruit tree maturity?

- How many "chill ratings" do nurseries pull out of thin air simply by looking at where a fruit originates from? The typical erroneous logic goes something like this: "German, Canadian and Upper-Midwest originating apples must be high chill because they come from places that have high chill."

Follow-Up Postings:

RE: The 'low chill' controversy

Axel, what a great thread starter.
I have noticed over the years that chill is not the only thing that causes the symptoms you mentioned.
Poor nutrition and post harvest stress will make a tree look like it is lacking winter chill.
I think that deciduous trees need to build up nutrients in the wood behind the buds, before the buds will burst properly.
I am not saying chill is not important, I am saying you can do a lot of things to enhance cropping in areas where chill is lacking.
I have read a paper on growing stonefruit in marginal chill areas, while they recommend using dormancy breaking chemicals, they also recommend preconditioning your trees with KNO3 sprays.
I think part of the benefit of the KNO3 spray is the build up of nutrients in the wood.
I had used urea sprays to defoliate plum trees in order to reduce disease carry over on the leaves.
While I found it nearly impossible to defoliate the trees with urea, I did find that the next years blossom on those trees was advanced and very compact (just like they had had 100 hours more chill than the untreated trees.)
A friend of mine is a mad collector of books specially anything horticultural.
He had found a book published in 1904 and it had the results of a trial using NaNO3 sprays in autumn on an ancient apple variety and the results were the same, tighter blossom and much bigger crop.

My advice if you are borderline for winter chill is to post harvest irrigate , enough to stop water stress but not enough to send your trees into a growth spurt.
Make sure the tree has enough nutrients to look deep green, but not enough to start shoots growing (KNO3 is good for that)
Make sure the tree has all the nutrients and trace elements it requires (no deficiencies)
Do some late nutrient sprays, so the leaves can absorb some nutrients and store them in the wood behind the buds.
Defoliate the tree with ZnSO4 (choosing the right time is the hard part )

I must have a hunt round and see if I can find the name of the study and the author and let you find it

RE: The 'low chill' controversy

I think you have to separate it out into fruits--you are mixing apples and cherries. There IS a controversy about low-chill apples.
I haven't heard anyone argue it about cherries, or other stone fruits.
SO are you asking about apples, or cherries, or what in particular?

Carla in Sac

RE: The 'low chill' controversy

Mike, the KNO3 is a good idea. I don't have an issue with trees going dormant, they loose their leaves on their own, except when the trees get planted the first year. But still, I've heard it's very important to feed post harvest, and in low chill areas, unless you use the right feed, they might go into growth spurts. Basically, the next year blooms develop before the tree goes dormant. One big mistake California gardeners make is to let the drought stress cause the tree to go dormant. That's really bad and will screw up the tree's internal clock. As a case in point, I have a raisin tree that lost irrigation in August. By mid September, it defoliated. It got hooked up again, and then went into full growth mode in late Fall. It's now totally green, foliated and going gangbusters as if it is Summer.

Carla, the controversy exists with all fruits, not just apples. Often people mis-interpret poor cherry bearing on low chill when in fact it's due to lack of maturity, poor pollination, or Summer drought. But you are right in that each fruit is different in it's behavior. For example, apricots need their chill early in the season, and do best when we get the coldest weather in December. Cherries need their chill mid-Winter, and apples do best with late Winter chill.

RE: The 'low chill' controversy

I think sometimes cherries suffer more from post harvest neglect, since they crop so early, there is a lot of season to neglect them in

RE: The 'low chill' controversy

My experience with fruit trees receiving less chill than required is the bloom is erratic over a long period, rather than the whole tree bursting into bloom at once. Very little fruit is set. Al

RE: The 'low chill' controversy

Compactness of the bloom period is how I determine if an apple is "low chill" or not. Northern Spy has a very compact period here and sets good crops (too bad the quality is absolutely hideous in the heat). Nittany delays blooming until almost August but then has a compact period, so I would call that low-chill also.

On the other hand, Hawaii blooms over a 4-month period, but has a very good fruit set on those blossoms. This would drive a commercial grower nuts, but is quite convenient for the home grower. I would call this inadequate chill, but is no reason not to grow it. I've never had any trees die from lack of chill (borers usually get them first). Trees do seem to lack the vigor of colder climates, but that may be because I stuff them so close together.

My Rainier cherry literally explodes in blossoms, but only sets one or two cherries. Haven't quite figured that one out yet.


RE: The 'low chill' controversy

Well, I guess the topic I brought up is quite relevant as we're having a major mid-Winter heatwave. Looks like I will get to see first hand what happens when there isn't enough chill.

This year will be the least chill we've gotten that I can remember. Right now daytime temps are in the upper 70's with lows in the upper 50's as a result of a freak mid-Winter heatwave, and there is no end in sight for the warmth and dry spell. Normally we are in the low 50's during the day and the 30's to low 40's at night. We even hit the low 80's yesterday.

While there is finally a hint of rain in the extended forecast, it looks to be of the warm subtropical type. So the window for chill accumulation is quickly closing. Most years we get about 800 hours, this year, we're lucky if we hit the 400 mark.

RE: The 'low chill' controversy

A Rainier cherry with a good bloom and only one or two fruits says 'pollination problem'. Usually it means a suitable pollinizer is not near by. Sometimes it is too cool for bee activity, but you would get more than you are. When I had your problem I bought a Black Tartarian from a nursery in a five gallon container already in bloom and put it under my Royal Ann, also in bloom. That year I had a bumper crop of cherries. Sinse I planted the Black Tartarian close by I have regular crops of both cherries. Al

RE: The 'low chill' controversy

Well, I guess I should get my cherries straight. What I thought was Ranier is actually Royal Ann, and Lapins and Royal Ann explode in blossoms, but give meager fruit.

Last year we planted Royal Lee and Minnie Royal, which are proven producers in low chill climates, so we'll see if that helps any.

It was 87 here this week and we sleep with the windows open. I'm not worried a bit about it (the apples will fruit fine) just as I wasn't worried about the freeze in 2007 that wiped out so much citrus and avocados here. Apples are like the perfect fruit to grow in our climate, as you get something every time.


RE: The 'low chill' controversy

Applenut, cherry trees need to reach a certain level of maturity before they will readily set fruit. As long as they are blossoming all at the same time, then you should be all set, just wait for the trees to mature.

The trigger for fruiting depends on the rootstock. Colt and mazzard, although semi-dwarf still require the tree to be fairly mature to set fruit. The Gisela series rootstocks cause earlier fruiting than mazzard or colt. The best rootstock that causes the earliest fruiting on most varieties is supposed to be the zee interstem on citation rootstocks. They are very dwarfing and will trigger the tree into fruiting much earlier. Of course you can always girdle.

Nowadays, there are new releases that are bred for precociousness and will bear earlier, even on mazzard or colt. For example, Royal Rainier will bear even as a small tree on either colt or mazzard. Mine fruited literally the first year I planted them and they have gone non-stop since. Ironically, royal rainier has a longer more spread out bloom cycle than bing, lapins, tartarian etc... making it appear to require more chill.

Last year, I planted royal lee and mini-lee for the years when it's abnormally warm, such as this year, we're enjoying Summer temperatures here. I have them on mazzard and on the new zee interstem.

One note, we've clocked 470 hours of chill so far according to UC Davis, and we stopped accumulating about a week ago. So far, there are some interesting surprises as to what is coming out of dormancy as a result of mid 70's day, mid 50's nights, suggesting enough chill has accumulated for those varieties. What I recall I saw today was Calville Rouge and pink pearl, I'll have to take another look tomorrow. Of course, dorset Golden and Anna are already in bloom. Mini Lee and Royal lee have the largest buds and look like they might come out of dormancy the earliest.

RE: The 'low chill' controversy


Well, we're at 167 chill hours and none in sight. Big Bear Lake is only 40 miles from me and is at 1,444 chill hours, and so is a good reference for me to check the difference on how apples perform under different chilling conditions.


RE: The 'low chill' controversy

you mention that you have 800 hours of chill. That is medium? that seems like enough for pretty much anything. I would consider medium chill area about 300 to 400 hours. We are "lucky" (the tropical growers would say un-lucky), if we get 100 hours, that is low chill for sure.
another symptom in stone fruit is "tippy" growth. The new growth is only at the tip of the branches instead of a good full flush.
also low fruit set. just don't get much.
I have tried all sorts of things to get more production in the face of low chill. The only thing that seems to make any real difference is manually striping all the leaves as soon as the main leaf drop has happened. Helps a little little little bit.
also growing cover crop or mulching to keep soil temp lower. Can't say that it really helps but it does look pretty and builds the soil.

RE: The 'low chill' controversy

Not sure what is medium or high, I think medium chill is a range from 400 to 800, low chill is below 400, high chill is above 800. When all of my cherry cultivars go into full production, then I will be convinced that 800 is high chill. :) They are still young, so they have a ways to go.

We do vary from year to year. Here are the values for the last few years, total accumulation from Nov 1 through Feb 28:

2007/2008 816 hours
2006/2007 810 hours
2005/2006 976 hours
2004/2005 543 hours
2003/2004 669 hours
2002/2003 412 hours

So we are medium/high I would say, and borderline low chill on some odd years.

RE: The 'low chill' controversy

We've found stonefruit is much more picky about chilling hours, and if you don't plant the right varieties you get squat in a low-chill climate.

Fortunately there are at least a dozen low-chill peach, nectarine, and apricot varieties that do well at 300 hours and set insane crops each year. The quality of these are outstanding also, so we don't feel left out at all. Dave Wilson Nursery has a page by Tom Spellman that lists his favorite pics.


RE: The 'low chill' controversy

This is a fascinating topic, as I am particularly interested in growing fruiting cherries and ornamental cherries, and live in a warm climate without any chill.

I might be completely wrong here... but one thought that I had, one of the big reasons lack of chill hours could affect the fruit yields may have to do with pollination. As we all know, without sufficient chill hours the tree does not have a clear signal when to flower, so it flowers erratically throughout the year. But there is a reason why the fruit trees are designed to flower so vigorously within a short timespan. With so many blossoms on all the trees at the same time, there is a much greater chance of cross pollination. Also a tree with many blossoms is more likely to attract pollinating bees than a tree with only a few.

So if this theory has validity, it may be possible to counteract a lack of chilling hours by ensuring ideal pollination conditions. This theory might also explain why some of the anecdotal observations by members in this forum seem to suggest that the chilling requirement is not quite as important for obtaining fruit from self-pollinating varieties.

RE: The 'low chill' controversy

If I didn't get chill I would just grow tropical fruit. So many are awesome, not sure I would bother with the other stuff. But as it happens I get about 1600 chill hours here.
My problem is trying to grow low chill cultivars here. Not the best idea, but I have nothing better to do. I'm only trying it with blueberries because they are triggered to come out of dormancy by temperatures, so as long as I can keep the plants cool till the weather is favorable, I should be able to grow low chill blueberries. Also I have them in pots, so if danger of frost happens during bloom, I can move them out of the frost. If I can keep them alive during the winter is another challenge. I'm keeping them in an unheated garage, out of the drying wind, temps rarely go below freezing in there, and I can heat it if needed.

RE: The 'low chill' controversy

I have read some unpublished research that a 200ppm solution of gibberellic acid, sprayed onto the leaves or bark, of a 6 week old flowering cherry seedling can bring it out of dormancy. The gibberellic acid has to be continually reapplied, and should not exceed this concentration (otherwise the plant may stay in dormancy, strangely). The effect is noticeable after 48 hours, the buds start to open.
There has also been research on using giberellic acid to get cherry seeds to germinate without cold stratification, so this may not be surprising.

RE: The 'low chill' controversy

  • Posted by Drew51 5b/6a SE MI (My Page) on
    Mon, Dec 8, 14 at 16:15

Speaking of cherries and chill I heard the CA crop, or lack of a crop. That it was a terrible year for them. I can relate 2 years ago here we lost almost all of our state's crop due to late freezes. Thank goodness they are very rare here.
This was a good year for us.

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