OK so this post really is trying to understand how fruit trees respond to chill hours.
If a mature fruit tree meets it’s chill hour requirement, it blooms “on schedule” in a relatively short period of time measured in a week or two.
So here is the question:
Say you have a tree with a chill requirement that isn’t met. Will it flower at all? If it flowers, can it be pollinated?
Or will it flower some and drop any pollinated fruit when small?
The statement that a tree has too high a chill requirement for your area means you shouldn’t expect good production of fruit. Does it mean you won’t get ANY fruit? Does it mean it may set fruit and drop them all? Does it mean it won’t even flower? Or perhaps if it flowers the flowers are not formed properly? If it flowers at all, does it mean it cannot pollinate another compatible tree that is also flowering (if this tree is a pollinator for another that has met it’s chill requirement).
It’s kind of a read between the lines thing, like the statement that two trees will not pollinate each other. Is it because they can’t physically do so (compatibility) or does it simply mean they don’t bloom together and if they did, they could cross-pollinate. Simply saying they don’t doesn’t tell you why.
It’s about perspective too… Since I live in the deep south (9a), if we see a single snowflake in the air we say it snowed. In colder climates they wouldn’t even mention it or notice to comment if they had a light flurry.
The commercial grower is the person living at a ski resort that gets 500 inches of snow a year. I’m the sorry chap out late at night in winter squinting at a floodlight hoping to see a snowflake.
Again this is a home grower with only a handful of trees on a 1/4 acre lot asking, not a commercial grower. If I get a handful of fruit on a small tree I may still be happy while real growers would never spent any time or energy even entertaining that variety. The literature is generally funded and written for commercial food production.
This is a very generic answer. Each plant type has a response which may vary from this description.
When a plant does not get chill hours met, it usually won’t “wake up” for spring at the normal time instead staying dormant. However, chill hours are not the only trigger to break dormancy. There is also a requirement for heating units in the spring. Even if chill hours are not met, eventually heat units will trigger the tree to open buds and produce flowers. This may be several months after the normal bloom time putting the crop at risk in the fall. There may be strange sequences where blooms and buds expand at the wrong time leaving the tree with fruit but no leaves.
Pecan gives an unusual example when moved to a climate with inadequate winter chill. Pecan has female flower buds, male flower buds, and vegetative buds. Each bud type has a different chill requirement. Normally, a pecan either produces female flowers first (protogynous) or male flowers first (protandrous). But when chill hours are inadequate, this may flip so that a normal protandrous tree becomes protogynous. This is because the dormancy of male buds is separate from the dormancy of female buds.
Phil,
I think you are asking a good question to which very few people can respond with any certainty. I do a lot of referencing to member growers who are geographically near me to gain insight on what works well in our region. Climatic changes make such a dramatic difference that it pays to consult people with similar climates when you are trying to predict how plants interact. The blossom cycle on plums for example will be very similar year to year in the same climate. My area is really well suited for plums but not many other stone fruits. My efforts to grow peaches and nectarines here has met only disappointments year after year. Only a very few persimmons will actually ripen here in our cool maritime short growing season, but the trees do very well vegetatively even when the variety will not ripen its fruit. Any fig tree will grow very well here but only a few varieties will produce an edible fruit before winter sets in. I look forward to seeing if anyone can give you a scientific response that you can utilize. Meantime you can do your best research for what types and varieties do well in your particular area by consulting other members whose climate gives you similar growing degree days or growing zone. The members map is very helpful for that purpose.
Good luck, I hope you find varieties that work.
Dennis
Kent, wa
Chill hours are a proven necessity for some fruiting plants
Published chill hours have been proven unnecessary for domesticated Apples and Pears – however, individual cultivars might have other important ripening requirements
There are many chill hour formulas and they are all approximations, and often inapplicable to other regions
I’ve don’t use my desktop often and didn’t know there was a map. Let me looks round the app (though apparently the android app is no longer supported as I can’t find it to check for a later version).
If you can just click on the correct location below, it will bring you to the members map. Once there, just expand it to show members near you or any other location with similar climate!
Indeed chill hour calcs are empirical at best. I have had a personal weather station for going on 20 years. I can tabulate my temp for every 5 minutes every winter.
If, for instance, I were to use the modified method that has a sliding scale and accumulates negative hours above 60°, I would end up with a total chill accumulation that is negative for many winters. That doesn’t work as I have gotten fruit from my young trees…not a lot yet but they’ve just begun bearing.
So the many methods of calculating don’t work well for where I live. Zone 9a here isn’t the same as, say, Zone 9a in SoCal.
Also it’s hard to know when to start and stop the chill hour clock here because our late fall, winter, early spring is anything but regular.
What it comes down to is planting low chill varieties and just seeing what sets fruit and what doesn’t. It is what it is regardless of how I try to calculate it.
I used it to locate several members in my region, got to know them and turns out we keep in touch on many variety and common interest questions, they have helped me gain a lot of specific facts that were not otherwise readily available
Dennis
That’s definitely drastically overstating the current state of understanding for at least apples (I cannot speak to pears).
What’s been shown is:
Many (especially older) varieties have wildly over-estimated chill requirements - Cox’s Orange Pippin’ is a perfect example of this, and will adequately bloom as low as 450, but does not bloom maximally until closer to 650 (which is of course far lower than the 800+ often repeated).
With enough man power you can lengthen the dormant period to increase the total chill hours received (by hand removing leaves at the end of autumn), and forcibly wake trees “early” in spring with nitrogen (and other more technical) sprays.
It’s true that apples create their flowers before they go dormant, and so do not require chill to have blooms, but typically they will bloom so poorly that you will be lucky to get almost any fruit at all, as the apple chooses to save many of it’s buds for what it hopes is a better winter the following year.
I’m in a 9a region, and make a point of pushing my boundaries with apple varieties, and while almost all of my nearly 100 varieties perform just fine, there are absolutely some that even in a good winter (we can be lucky some years ang get nearly 800), with manual leaf removal, and spring nitrogen sprays, struggle to wake up correctly (worst so far has got to be Devonshire Quarrenden, hands down).
Many studies show that while “chill” factor is less significant that previously thought, it’s a great measure for the converse issue which is for those whose peak above 18C regularly during winter Apples fail to register any chill for that period, which can make it appear higher chill.
@schme16
Here’s what’s been observed in southern CA coldhardiness zone 9b regions with chill hours (whole hours below 45°F) between 100-250: all domesticated apples in U.S. retail circulation produce viable fruit.
The situation for southern CA coldhardiness zone 10 is similar: all will produce fruit but many require an efficient progression of degree days for the fruit interior to ripen before the exterior starts to rot (over soften).
Georgia lost more than 90 percent of its peach crop this year due to warm winter. Most news articles mentioned the crop didn’t get enough chill hours and those that did got zapped by late frosts. One grower estimated he received only 700 hours of chill on the varieties that require 850. The articles as expected ventured into climate change as the culprit.
Just last week Georgia got a federal declaration for a natural disaster making growers eligible for low cost loans. With the trend to warmer winters, this year may not be just a freak event. So what do you do? Plant low chill varieties like they do in Florida and deal with a normal year’s early warm spell and later frost?
Can’t help but to think it was the late March freeze.
Just north of Lake Pontchartrain in southern Louisiana I got down to 27 degrees (March 20th I think). It killed what little fruit set I had (Pluot). I have to assume it was worse in the GA peach orchards.
There’s is no such thing as normal in weather. The NWS uses a 30 year floating average for temps. Chill hours have always and will always vary by year. The day you hit exactly your average high and low is a rare event. The year you accumulate your exact zones rated chill hours is rarer still.
Climate IS change. Regardless of our part and whether we can affect it or not, it will always be changing so we shouldn’t be surprised. We’ve lost so much biodiversity by developing single variety crops for market that are grown across large areas. Nature’s own insurance has always been cross pollination that evolves enough diversity that there are varieties that survive as climate does it usual thing.
Not trying to start an argument or discussion on anthropogenic climate change…im sure there is a thread somewhere.
The only commercial fruit tree crop grown in Louisiana anywhere close to me is satsumas which are grown south of New Orleans along the Mississippi River. Every so often they get his with a freeze cold enough to affect the crops…less often than more continental climate locations like GA peach territory.
Depends on the variety. Some will outright never wake up and die. Heck once filberts go dormant they can be kept pretty much at room temperature because they will not break dormancy until they get enough chill hours. At least Romeo from the romance cherry series is like that.
Not to speak for the tree but it seems to be a protective mechanism to ensure that the tree doesn’t wake up during a warm spell in the middle of the winter. Where I am in Alaska we get to -20f but we also get a meltdown in January with temperatures hitting the 50’s. Anything foolish enough to shed hardiness, let alone break dormancy, gets hit with sub zero temperatures the next week and dies.
The above is also why many trees that would otherwise be hardy low temp wise are in fact not, because of a propensity to wake up at the wrong time.
I’m sure there are several things going on. Evolution knows what it’s doing even if we don’t.
For you of course any fruit tree that is native won’t be fooled by a January thaw.
As far as never waking up just because of not getting a chill hour requirement I’m not so sure of. The warmth of spring should be enough for any healthy tree to break dormancy. It just doesn’t feel the stress from winter to reproduce.
Fruit tree flower buds are set in the late summer to early fall from what I read, so they obviously don’t care for temperature as it’s summer still.
My amateur understanding is the tree wakes all together after chill is obtained and it’s warming in spring. The tree sets all it’s blossoms in a short period of time. Trees that didn’t get enough chill have sporadic drawn out bloom time with much fewer blooms. I saw this myself at my house as my Flavor Grenade Pluot bloomed beautifully but my other Pluot had only a handful of blooms oder about a months time.
If chill requirement isn’t met the trees must do something with the flower buds… Either not pushing them that spring or if they do perhaps the flowers are not formed normally? Or you get fruit drop? That was the question I was looking to get info on.
Were all in the business (or play) end of growing fruit trees. We deal with the result of whatever nature does.
I always want to know the why of things and have found it actually quite hard to find answers. Local COOP horticulturalist aren’t of help here because none of them deal with fruit trees. Citrus (which is quite a different animal than stone fruit) is the only commercial crop grown anywhere in the area.
I love testing things just for kicks. One winter I moved a potted Romeo cherry bush indoors, it basically froze in time. It stopped growing altogether even though I had it under artificial light with my tropical plants. The next spring it was basically in a coma, holding onto the same leaves that it had the previous year. It didn’t quite die until the fall.
I was amazed when I found that out a couple years ago.
I mentioned in another thread I had a tree fruit culture 'professor’in college…
He was knowledgeable in other areas, however one area that he really stuck was chill hours, and I’ve since learned better
So domesticated apples have been proven to not ‘need’ chill hours as was once thought, though in areas that have reasonably warm weather September through November fall apples are notorious for not coloring sufficiently well to fetch top dollar with consumers hence in VA and areas warmer Red Cameo and Simmons Gala have gained a solid follow because the commonly grown Cameo strain and Gala strains (there’s in excess of a dozen Gala varieties… sheesh) don’t redden particularly well in the warm falls of those areas.
I’ve also noticed Magnolias flowering out of season in my area (a Philly suburb).
So more research needed in chill hours and how they actually hold up.
