You can ponder charts all you want but you never know what temps will kill flower buds when because so much depends on how much water in is the cells at the time of the freeze event. This year we got down to maybe -7F on Feb. 4th, which usually would be no big deal, but preceding mild weather apparently made cot flower buds unusually vulnerable for this deep in winter. There are only a scattering of viable flowers, even though the branches are trained right next to the walls of my house, which offers them several degrees worth of protection.
Sugar Pearls seems to be about my toughest cot and has the most viable flowers (not sure about the ovaries, but male parts seem fine in the few flowers that survived. Alfred is a variety Jim Cummins said was most reliable at the Cornell station, but it has fewer living flowers than Sugar. Tomcot is in the most protected spot and has almost nothing… I don’t really like it anyway, at least compared to other, juicier cots, meaning every other type I’ve grown.
I used to look at the bud freeze charts fairly often in the spring. I don’t bother anymore because they only work as a very rough estimate, in my opinion. There is no way I can factor in elevation, wind direction, hours at a given temp, weather in the weeks prior, etc… Also, I have too many trees and too little time to do anything about it. When someone asks me if the freeze is going to kill the buds, now I just say, “Maybe, we’ll see.” And can I do anything about it, “Nope.” I try to be nonchalant about it, but it still pains me to lose a crop.
I’ve been cutting buds open and checking the last few weeks. I think my sweet cherries are 100% toast, green flower buds but dead brown ovules on every one I checked. Most of my tart cherries seem at least 90% gone as well but I saw the occasional green ovule on some Carmine Jewels. My peaches look about 50/50, maybe a little better. Plums all look green inside as far as I can tell. The euros are so small it’s a little hard for me to judge, but everything seems vibrant in them. I checked a few Jerseycot apricot buds, they seemed ok. I’ll check some more when I get home from work. If not for that one sudden plunge around here, this would have been an ideal winter. We ended up getting down somewhere around -5 degrees that one day then I don’t think it got below +15 the rest of the winter.
One thing I’ve learned in the last couple of freeze outs. The male flower parts can take freeze better than the ovules, so you can have full bloom, amply tended by pollinators but still get no crop. Funny how that’s not mentioned in the literature I’ve encountered.
Checking the green flower buds for browning in the center is the only way to know for sure besides just waiting.
This is what so sucks about growing fruit in non-Mediterranean climates, especially as we seem to be entering more erratic weather. One night usually is what makes all the negative difference. Last year it was a single night on the last week of March where it only got down to about 19F and nothing was in bloom. J. plums were just starting to show some green. All nectarines, plums and pears were destroyed on my prop- but I did get some cots. .The bloom was full and well tended.
If I had a penny for all the hours each spring that I have spent looking at those freeze charts and the photos of supposed freeze damaged buds and so on, I’d be rich. But like @alan and @ztom have just said, I find those freeze charts/expected damage stats to be extremely undependable. I’ll take others more experienced opinion for the cause (relative water content at time of freeze, whether freeze event was preceded by a warm spell, wind/humidity/other weather conditions coinciding with freeze), but I can say without question that the same temperature one year will result in a 10% loss and the exact same temperature another year may be an 80% loss! It really is that dramatically different for me. So for the newer growers, I say there is nothing wrong with consulting those freeze charts and letting others tell you how much damage you can expect from that 26 degree (for example) spring freeze, just know that your results may vary from the chart and even from your own observations from one year to the next.
This year, I had a rare event (just like Alan’s) where I lost almost all my stone fruit when it was still mid/late winter. It was early february and temps here in northern middle TN got down to about -6. Buds were, I thought, still quite tight and asleep. But it did come right after almost 2 weeks of unusually warm weather. I didn’t even know how severe the damage had been until bloom time when I noticed almost none of my stone fruits were blooming! (Yes, I should have noticed the buds hadn’t been swelling but I just hadn’t been in the orchard enough to notice).
Normally, I’d be crushed. But I have more non-fruit things going on this year than I have the last 15 years, so I’m actually kind of looking forward to taking a year off in the orchard. Stone fruits represent the majority of what I grow, and a large percentage of the other things are low maintenance. Of course I’ll still prune and do some other routine maintenance and even a spray or two to keep things in check, but compared to spending about 7 hours every 10-15 days all spring and summer, I’m going to have my easiest orchard year in the last 15, and I’m actually pretty happy about it. I’ll still have grapes, persimmons, apples, pears, blueberries, cherries, etc so I’ll be fine! This year is also a good example of the importance of diversification. If I grew only stone fruits, I’d have a fruitless year and that WOULD upset me. Having the simply get by on my non-stone fruits isn’t a huge problem.
Anyway, my point is that I agree with Alan and Ztom - freeze charts and past experiences may provide partial insight into what you can expect from a certain temp in a given year, but don’t think for a minute that those things are perfect or even very strong predictors of what you can expect in another year. Conditions vary, and so do results of reaching a given temperature in any one year.
It is the preceding weather that largely determines the moisture content in the buds… at least the hardening off process influenced by temps. Length of days also affects the coming in and out of winter hardening, but is likely influenced by both species and cultivar.
What is clearly determined by the science is that plants harden off by shuttling water out of cells so the walls aren’t fractured by expanding ice. As they come out of dormancy water is shuttled back.
At least this is how I understand it… but I’m a lot more farmer than botanist.
See…that’s why I said I’d leave the reasons to experts like you! That makes perfect sense to me. But whatever the reason, I just know that I’ve absolutely had years where 26 degrees in mid-March caused major damage to my peaches and years when 26 degrees caused fairly minor damage to the same peaches in the same time frame (calendar wise I mean).
The funny part is that in spite of knowing all this and having seen it for many years, I found myself looking up those same freeze charts again this year about a week ago when it got down to 25 (A few of my stone fruit trees still had a hand full of blooms after the winter kill). I guess its just natural to want more information in times of trouble, even if that information is known to be dubious!
I’ve found the charts reasonably correct for spring freezes. We don’t get much winter kill. It all happens here after flowers start opening. My current crop got taken out by 25F after bloom. The apples and pears that haven’t bloomed will be fine. Stone fruit that had bloomed is gone. Just what the charts would say.
What it comes down to is before bloom they’ll take a lot. After bloom they won’t. That’s about all you need to know here.
I think people expect too much. Your anecdotes aren’t better than the charts. Most people don’t really have that good of a handle on the temperature.
Are you sure of that Temp. 25 doesn’t spell doom here at that stage. Unless I’m the one not getting temps exactly right. That is certainly possible. The temps can vary a couple of degrees just from the ground up to the top of a tree.
You make a fair point…I probably sounded too harsh on the charts. Obviously I still think they are a decent tool for predicting damage or I wouldn’t have consulted them at all recently. But they are just that- one indicator of what might be the result at a given temp. I think I am trying to reach out to the new growers who - and I can say this because it 100% used to be me doing this - who have a strong belief that if the temperature hits a given low, they can just look up that temperature on the chart and know exactly what percentage of their crop will be lost. But as we are all saying on this thread, it just isn’t that simple.
You make another good point, I think, when you say people don’t have that good a handle on the temp. I think you are saying their measurements may not be that accurate. If so, I couldn’t agree more. I myself just never trust my thermometers to be that accurate. Plus, the temps can vary by 1-2 degrees from the highest point in my orchard to the lowest, even though it isn’t a big elevation difference at all. So yea, all these things make it almost impossible for me, and probably most people here, to do any really serious research or reporting on things like freeze damage at various temperatures when I/we can’t measure those temps with a high degree of accuracy.
Edit: We were typing our posts at the same time, that’s why it sounds like I just repeated what @alan and you said exactly! ha. So we independently were saying the same things about how temps can vary within the orchard and not everyone can precisely measure temps. Great minds think alike. haha
The temperatures in those charts are usually the critical temperature for damage. Depending on the dew point, the temperature experienced by the bud may or may not be the same as the air. If the air is 28 degrees and the dewpoint is also 28, you will probably see much less damage than if the air was 34 degrees with a dewpoint of 24 degrees. Dry, cold air causes plant tissues to lose water, which absorbs energy as it evaporates, cooling the tissue to below the temperature of the air. At or below the dew point temperature, water condenses on plant tissue and releases energy, keeping it from getting colder than the air.
You can see this effect when you see frost form on your car despite the temperature being slightly higher than 32 degrees. Although the mechanism here isn’t evaporative cooling, the effect is the same. The windows and metal lose heat rapidly and cool to below the temperature of the air, allowing water to freeze.
Part of this is frustration at losing a crop. I understand that well. 50 yrs growing fruit in west Texas is a lot of freezes. It caused me to move to CA to get away from it and when that fell thru to build a greenhouse when I moved back to TX.
Even with all of that I had a freeze this spring that cost me at least $50K in losses. The greenhouse heater went out on the coldest night of the year.
The researchers who developed the charts likely have NWS grade temperature measuring equipment. I’ve got two of those spring loaded round thermometers that I compare all the time to the airport 2 miles away.
Consider how much time and effort that went into making the charts. I’m not sure how that was done but consider that in commercial fruit growing districts they don’t get nearly as many freezes as west TX or TN. All those stages and all those temperatures takes a long time to see it all outdoors in an area with few freezes. I’m amazed they did it at all.
Any adjustments we might make would just be undone by the next guy that sees the opposite.
i would consider purchasing or making a wet-bulb thermometer to better measure the actual temperature experienced by your trees.
Between brown rot from our excessively wet spring and numerous frost events, I think I have lost nearly all of my pluot and peach flowers despite the air temperature never dropping below 32 degrees since budbreak.