2026 Prune Fruit Set, GDH30, and Cost of Thinning
In 2026 the Sacramento Valley prune industry is experiencing high fruit sets, high GDH 30, and a high cost of fruit thinning because of very diesel prices. Jaime Ott (UCCE Tehama), Domena Agyeman (UCCE Butte), and Franz Niederholzer (UCCE Colusa) break down prune grower crop management considerations given this tricky set of circumstances.
TRANSCRIPT:
Luke: [00:00:00] There are numerous upcoming orchard education meetings to take note of. The Almond Board of California also has upcoming IPM meeting , tackling rodents, redleaf, blotch, Carpophilus , and other pests. Bakersfield Thursday, April 30, and finally Wednesday, June 16. If you are an almond variety nerd like me, you can't miss the Almond Boards crack Out event at Merced College
in the Sacramento Valley. Nickels Field Day is Tuesday, may 19 in Arbuckle. I have two . almond variety meetings at the Chico State Farm in June, Thursday. june 11 and Thursday, June 25.
Today on Growing the Valley Podcast, we're back to talk about prune production here in 2026. In 2025, we had a great two part podcast series talking all the way back from the flower buds through to bloom and, fruit [00:01:00] set and thinning and fruit sizing and harvest. So we had the two episodes with Franz and Jamie and Becky, and now we're back in 2026.
With Domena Agyeman, he's a Ag Economist,based in Butte County with me, also covering Glenn and Tehama counties. It's great to have an Ag economist. welcome to the podcast Domena, and then also great to once again have Jamie Ott and Franz Niederholzer back on the podcast.Jamie, can you take me through the wild weather of the 2026 bloom and the immediate results that you saw with fruit set as we have this conversation in late April 2026?
Jaime: So this year you say wild weather and I certainly agree with that. . This year our improved French up north bloomed, March 9th or 10th, depending on the orchard you were [00:02:00] in. And three days later, we had temperatures up around 84 degrees and the temperatures didn't come down below 80 until at least a week and a half later.
So ideal bloom weather on full bloom day, but then just the heat ramped up really fast thereafter. , And if we take a look at Denny Bloom, which was a little bit earlier this year, the Denny's bloomed around February 14th for us this year. So, a little more than the standard two weeks before French. That's because the weather really pooled off right after the Denny's bloom. So we actually have some freeze damage in the Denny's up north, from some freezing temperatures about a week and a half after their full bloom. all of that said though, the improved French, even though we had lots of fear of heat at bloom with that hot weather three to four days after full bloom.
I'm seeing enormous sets in most of the orchards that I monitor. So, I keep track of fruit percent set in four orchards, ranging from Red [00:03:00] Bluff down. there's one in Corning and then a couple in Vina. And we've got fruit sets ranging from 23 to 52% in those orchards with an average of 35.
So substantial set this year in spite of weather that we were concerned about coming into bloom.
Luke: Excellent. And Franz, what's the nuance difference there? If you look to the Southern Sacramento Valley.
Franz: much like, in the North Valley, we had early bloom, full bloom in South Sutter County around March 7th or so, and get up towards Gridley. It was more like March 10th, , and there was dry North wind on the front end of that bloom.
But, the set looks very nice. The numbers aren't quite as good as Jamie's, but, on average about 20% set, really wild swings. some shoots didn't have any fruit. Some shoots had 30%, 40%. But if you look in the orchards, there's a nice crop load there. this'll be a good year to check for sure. There's definitely a lot of fruit. I [00:04:00] noticed, growers pruning, couple of weeks after harvest, so they knew that it was, a good set. looks like up and down the valley. Hopefully, fingers crossed to be a good year crop set wise.
Luke: Franz, you've been working with a large team for numerous years now, funded by the Prune Board of California, trying to model out and understand all sorts of different weather conditions at Bloom and prunes, and how that either results in too much fruit is set, or way too little. Where does this fit in with your current understanding of prune bloom conditions?
Franz: Well, it, um, it just fits, it was very warm. The weirdest, warmest bloom I can remember. With still a crop set, but, in our experience over the years, and we've been doing this for about 20 years now, when the heat hits, when the over 80 degree temperatures hit from full bloom to a [00:05:00] couple of days later, right after bloom, that is where we get the most damage. And this year, we flirted with 80 through much of the bloom, but it wasn't quite, too hot.
this particular year I see some shoots that I've been tracking that bloomed a little bit later March 12th, or so, those don't look so good. I need to go back over the data, but I think we're pretty consistent with, what the expectations of the model is, that if you're under 80 degrees at full bloom and the day or two after, Those are conditions for good bloom, especially when we had, warm temperatures going into bloom and warm temperatures coming outta bloom When it's really cooland not very good bloom conditions early and then it spikes up hot and then backs down to cool. Those are years where we see, very poor set.
Jaime: When I've been talking to people, , I've told them that we kind of squeaked in this year, with those high temperatures after bloom, because,, like I said, the high temperatures came three to four days after Bloom, and I think. If they had come a day or two earlier, we would be having a very different conversation right [00:06:00] now.
Franz: We delayed bloom and a couple of plots at, Matt Bozos and , the bloom some of those shoots, the set is not so great. But it's funny, I also talked to a grower, who said that they did not run water in an older block that they did not want to set very well, and they set a big crop. And he was annoyed him. He said, look, Franz, it was hotter than heck. And you said , that cuts down on bloom set and we still set a nice crop. So, it was, as to Jamie's point, I think we just squeaked in.
Luke: Franz, let's turn now to what happened just after full bloom and take us through this concept of, GDH 30 the growing degree hours, the 30 days after full bloom, and then, what those numbers were like this year in your region , and what the implications for that are.
Franz: Plant growth is driven by temperature, but in a certain window, cooler temperatures in the forties activate growth, but not very rapidly. temperatures above [00:07:00] 75 tend to start to decrease, net growth in stone fruits This growing degree, hour 30 or, GDH 30 is something that the researchers have developed as a measurement of heat units, which accounts for, less activity, below 60 and less activity over 75.
a really strong activity between basically 50 and 70. the relationship looks kinda like a child's slide you'd see at a park. It's flat at the bottom and steep in the middle, and then flattens off at the top. this year, with all the warm temperatures we had, the GDH 30, at least in the Yuba City area, was 8,300 degrees,
which is the highest we've seen in five or six years. and one of the highest that I can remember, so , high numbers for Gdh 30 coincide with rapid fruit growth and earlier, harvest timing in research that, has been done in prunes, but [00:08:00] really the bulk of it's been done in fresh market peaches.
Luke: And then Jamie, can you take us through your shorthand for, explaining GDH 30 and then, what you are seeing in the Northern Sacramento Valley and the implications there.
Jaime: GDH 30 is kind of a weird concept. so it definitely takes some,
time to wrap my head around it.
I think about Gdh 30 as the. Difference between the fruit growing in size and hitting certain developmental milestones. if you can kind of imagine that, pit hardening is a developmental milestone and when that happens, other things are triggered in the fruit development.
the amount of growth you get before some of these developmental milestones happens. defines how much growth you can get later in the season because you're setting the number of cells in any particular part of the [00:09:00] fruit. so GDH 30 is an attempt to , pull together this idea of, temperature affecting the growth of the fruit, the actual size increase. Differently than it's affecting the developmental processes that the fruit is going through. , So when we have a high GDH 30, the development happens much more quickly. at warm temperatures, development happens faster. so the pit hardening is happening, sooner earlier in the year, fewer days after bloom.
The size the fruit has reached at these particular developmental milestones, charts its trajectory for the rest of the season. All other factors being equal, which they never are.
in terms of GDH 30 in our area, it's bang on with what Franz was saying. the orchards that I'm monitoring are right around 8,300 hours. just to put that in perspective, last year we were talking about GDH 30 being high. last year all the orchards I was monitoring were [00:10:00] between 7,500 and 8,000 hours.
So we're substantially above what we were looking at last year and last year we were really, Pushing the idea that the GDH 30 was high, so people needed to think about thinning early and thinning down to a really reasonable crop load for their trees. and so , we just need to reiterate that this year, that the GDH 30 is high when GDH 30 is high that means the development happened quickly and not as much growth could happen before those stages got hit. So the trajectory of the fruit is that it is likely to be smaller. last year we had a great size year and a great sugar year, which I definitely attribute to the mild summer temperatures we had last year., So maybe if we could order mild summer temperatures like that this year, we'd all be able to size these crops really well. But , I'm not willing to bet on summer temperatures in Red Bluff. Certainly. , And I'll say that some of the growers that I've been talking to who've been doing their thinning have been, calculating that they have around 10 to [00:11:00] 15,000 pieces of fruit per tree. And so, I mean, those are just massive crops that we need to thin those down to get good size anyhow. And the Gdh 30 is just another pebble on the scales, tipping the scales against us.
Luke: GDH 30 is, just one more indication, on top of the already most important indication, which is just way too much fruit on this tree, that thinning, is important, especially in a year, like this one. with such high GDH 30's , you don't know what you're gonna be handed with summer temperatures, but, you need to set yourself up the best you can. by thinning down to a sustainable crop size, and maybe doing on the more conservative end of that range you would think about of how many fruit you like to hold. on this tree, you might target something, on the lower end of that typical range that you go with.
Alright, so now Domena has been very patient and we get to, to turn to the Ag Economist because here we can. [00:12:00] Talk about things just in a horticulture science, lens and say, oh, the temperatures did this. This is the resulting fruit set. It's really high. We have this high GDH 30. It's so important for you growers to go out , and thin aggressively if you have a lot of fruit on your trees.
But it's just not that simple. We're operating in a difficult, context in terms of really high prices to farm. Could you talk a little bit about that Domena and some of the takeaways from an article you did about, that decision to thin or not thin, and how that worked out, at least in one example, for deciding whether the economics, were worth, the cost .
Domena : You know, Luke, I think this season is shaping out to be one of the seasons that a lot of growers will step back and ask whether some operations are worth doing, and I think thinning is definitely one of those decisions that will be under scrutiny. And a big part of that comes down to diesel. We have seen , diesel prices jumped, [00:13:00] pretty sharply this spring and in California. Currently, I think diesel prices is around $7.50 per gallon, and that is roughly 50% increase from the beginning of the year, which is, huge. And this increase is mainly stemming from the uncertainties in the Middle East when you have distractions around key routes or passageways, like the straight of, Hormuz in the Middle East which is very critical to global or your supply, you can expect, fuel prices to push up and that is what we are experiencing.
And when that happens, it goes straight into the cost of performing field operations like thinning, and in areas like California, that impacts things to be even stronger. Tighter fuel supply conditions here due to the closure of some of the refineries in California. So when global prices move, the [00:14:00] growers here feel it more. And that is why decisions like thinning depends heavily on the machinery being reevaluated. So I would say it makes sense for growers to reconsider whether to thin or not thin.
But the main point, thinning is not just about the cost. It is tied directly to the fruit size, the overall quality of the fruits that you get, and ultimately , the price that a grower will receive.
So while diesel prices makes thinning, , expensive. The real question is not just about, what it'll cost to thin, but instead what happens if you don't thin, and that is why it is important to weigh the benefits against the cost. It is very important that you look at the full picture. How thinning affects your yield, your fruit size, your price, and even next year's, crop. So in the article I did talk about partial budgeting. partial budget is a very [00:15:00] simple,but effective approach that you can use to assess, small decisions that you make on your farm.
You don't need a full budget, farm budget. You don't need a full enterprise budget. All that you are interested in is what changes. you can do this by answering four main questions. The first I would say is what will be the additional revenue or the new revenue that you get if you decide to thin, of course, when you thin you expect to get bigger fruit size. And overall increasing quality of their fruit, which transcends into, higher prices. So that is something that you take into consideration. And another question, which I believe is the reason why I'm invited here, is the added cost. So whatever cost that you incur because you are thinning, you account for that as well. And another important question is how to deal with cost savings. Most likely you're going to lose some yield and that yield loss will also mean that your hauling and drying activities maybe [00:16:00] reduced, which means you might not incur as much cost as you would if you don't thin so that may lead to some cost savings there. Another important question that you have to ask yourself is what is the revenue that you are going to lose when you thin most likely you are going to lose some yield. So the value of the yield that you lose is what you also account for.
So once you are able to answer these four questions, you just weigh the positives against the negatives. So what are the positives? The positives are the additional revenue that you get from the increase in fruit size and the cost savings. Those are the positives, and the negatives are the , cost that you care for thinning and the value of the yield that you lose because , you thin, then you just subtract the positive from the negatives, then you are done.
It doesn't need to be complicated, but having that structured comparison gives you a clearer answer than just focusing on the cost of thinning. And of course in the article I have, a very simple [00:17:00] example that I would like to share here. So suppose that you have a hundred acre prune orchard and, you have done your crop load evaluation.
Jamie and Franz have mentioned this, it is very important that you do your crop load evaluation before you decide to thin or not. So suppose you do your crop load evaluation and , the numbers come out that you have to thin, now you are looking at the cost because maybe you've asked for quotes from your customer operator, and is quoting a value that you think is higher. So let's say your custom operator quotes something like $150 per acre. This value is higher than you anticipated. So you are thinking maybe it is not worth it to . thin this year, which is a valid question, but the key is you have to run the numbers before you decide, because even if thinning is expensive, it could be that the benefit that you get from the increased fruit size and quality which commands higher price [00:18:00] may outweigh the cost that you incur in thinning. So from this example, let's say you anticipate that when you don't thin you are going to get about four tons per acre in terms of yield, but , that comes with smaller fruit size and that will sell from let's say, $1,800 per ton.
But then if you thin you are going to lose some yield, so let's say your yield reduces from four tons to 3.5 tons, but then you anticipate to get some cost savings from, hauling and, drying. And also the 3.5 tons in yield, which is reduction from the four tons will command the higher price of, let's say 2000.
with this, if you arrange this in a simple partial budget form, you'll see that even though you are incurring a cost for thinning, The increase in price and the cost savings outweighs the cost that you incur from this example, I think the grower, if he decides to thin, is going to have [00:19:00] about $60 per acre in return, which is an increase in profit.
So if the grower had just looked at the cost of thinning was like, oh, this year I think it's too expensive, to think I was anticipating a hundred dollars per acre, but my customer operator , is quoting $150 per acre. So I think I can't afford , that would've been, , an error on his or her behalf.
So the main idea is that run your own numbers before you decide. Don't just base the decision to thin on the cost only.
Luke: Domena walking through that example from your article, which we'll link to in the show notes Resulting in, at least in your example, and it's good for everyone to run this for their own operation and the numbers that they're seeing with this very simple formula that you have there of the pros and the cons, with this partial budget approach
The return on investment of thinning, might be reduced in a year, like this year with higher, [00:20:00] diesel prices. But, it was still a positive. And that's even without factoring in that benefit that you mentioned for 2027 in this case, that you're, potentially reducing this risk of alternate bearing or putting way too much fruit on your trees that causes tree stress and other problems.
a great way to capture it there with, just four inputs, this partial budget approach that everyone should look at your article for, because they can adopt that to all sorts of other decisions too. so it's a great framework.
Domena : Thank you, Luke. Yeah, and I think it's not , just like you said, it's not just about thinning for any small change that you're making on the farm, it's very important that you consider something like partial Budget before you make the decision
Franz: Just to reinforce Domena's points and, things that Jamie and I have been saying for a long time, now is the time to make an effort to manage the crop that you get at harvest. this is the only time you really get to make a significant impact on it. [00:21:00] And that will be a major impact , especially with the crop numbers that, currently set that Jamie's talking about.
you don't want to be peaking at seventies, for fruit size, and now's the time to make that move. , check your crop load, check carefully and where you have to drop it to a number that you're comfortable with. But as Jamie pointed out earlier, keeping in mind that chances are you're better off this year on the lower end , of the acceptable window for fruit number per tree that you've developed over the years , and or , your processor has , and try to hit that.
Luke: Jamie, what would you wanna leave, growers or consultants with today?
Jaime: The most important part our conversation Is what Franz just said, that it's really important that you understand what your crop load looks like and if you are sitting at, 12,000 pieces of fruit per tree, I think it's really important to balance that against the cost of thinning because I think it's very easy to look at the cost of thinning.
That's a very concrete number that [00:22:00] somebody hands you. And the benefits of thinning are a little more, vague. And we know that it leads to larger crop size. We know it leads to, you know, more consistent sugar. but I do really encourage people to, sit down and really think about what the benefits would be, to balance out that cost like Domena was talking about. And, , with such big crops on these trees. We really need to think about how to get those down to a manageable number.
Luke: Excellent. And finally, Domena, round us out with,any other final takeaways that you wanna leave people with.
Domena : Yeah, I think I'll echo what, Jamie and Franz have just said, it is very important that you run your numbers before you, make the decision. when you do your crop load assessment, then you find out that you need to thin. Don't let the cost of thinning or mechanical operation [00:23:00] this year due to increasing diesel prices and fuel prices tell you, run the numbers and see whether you are in a good position to thin or not.
Because as Franz said, the impact of thinning is not just only this year. there might be ripple effects the subsequent years. So if you decide not to thin because the cost you might even incur more costs, trying to, recover the issues that you have brought upon yourself. So run the numbers and decide.
And again, another thing that I will add is that if possible, ask your colleague growers who have gone through these phases before. If someone has done this, maybe there was a time that someone decided not to thin. If you know such a person, ask them what's happened the following year. How did the numbers turn out? If they are willing to share, I think that would be very helpful as well. The main idea of partial budget is that you make those most changes [00:24:00] and then you use realistic numbers. Don't just throw numbers out there Run the numbers, use realistic numbers and draw the conclusions.
Franz: I just wanted to point out, since we're talking about prunes here and thinning time that, Themis Michailides has done work over the years, showing that latent brown rot infections, have a, greater, potential during the timing of pit hardening and into, reference date when we have rain and we have had rain.
I urged growers and PCAs to make a mental note to, consider fruit brown rot, as we're going into harvest, into second half of July as we're getting close to harvest And have a conversation about how the weather looks and what they've been seeing in the orchard and the possibility that this year may have set up latent infections in the orchard.
And, um. Just something, something to be, to be considered.
Luke: . Yeah. , There are so many, as Jamie put it, the benefits of thinning are [00:25:00] numerous and vague. And, the disease angle is a really tricky angle too. Franz not only having wet weather at thinning and the risk there for brown rot, down the line, but also, you have too much crop on the trees, then you're gonna have more fruit to fruit contact, which is gonna increase your chance of brown rot. If you have too much fruit on the trees, the trees are gonna be more stressed. You might get into, potassium deficiency and Cytospora issues. So it's just so complicated. but, a healthy orchard, thinning , is a great part of that foundation in a crop that in the vast majority of years it, it over crops.
Thank you all so much. I'll link to your articles in the show notes. , Thank you for your excellent research and extension in the Sacramento Valley , and thanks to the Prune board, , for their support as well. Thank you folks.
Franz: Thanks Luke. [00:26:00] Thanks for having us on and for maintaining this terrific resource.
