In this episode, Dr. Brown finishes up his discussion of frost mitigation best practices with Bill and Nick Anderson, Cherry growers in the Dalles, Oregon. They explore bud hardiness and how models under development might lead to improved frost mitigation.

This transcript has been edited for flow and clarity.

Brad Newbold (Host):

Hello and welcome to POMETA Coffee Break, a podcast produced especially for tree fruit and nut producers.

In today's conversation, Dr. Dave Brown will be discussing frost mitigation best practices with Bill and Nick Anderson from Anderson Fruit Cherry Growers in the Dalles, Oregon area.

Dr. Brown served for 20 years as a faculty member at Montana State University and Washington State University, where he pursued research on soil sensors, spatial data science and digital agriculture. At both universities, he served in many leadership roles for major research projects, academic programs, and most recently as director of the WSU AgWeatherNet program. In this capacity, he hired and supervised a team of meteorologists who pursued research and extension activities focused on evaluating and improving the quality of weather data used for agricultural decisions. I'll hand it over to Dave, Bill and Nick.

Bill Anderson:

We've talked about this before, but during an inversion event, our buds may be a few degrees colder than our chart criticals.

Dave Brown:

The chart criticals should be correct. You may not be exactly in that bud stage or whatever. It's more to do with the fact that you're not measuring the bud temperature. POMETA has a project with WSU where we have research sensors installed with probes stuck into the bud itself to measure the bud temperature. Then we have a sensor to estimate the bud temperature. We're building models, and eventually, growers want a sensor that tells them the bud temperature. And you make your decision about when to turn on the wind machine based on the bud temperature, not the air temperature.

Bill Anderson:

That would be amazing. In one of the districts we farm, we've noticed that in the last two years, we've gone with our normal Ross program. There are more factors involved, such as a bad bloom season, but we've lost some buds that didn't get pollinated, and it just seems like we've had a little more of a kill in those areas. This year, we're being a little more proactive in setting ourselves up for a couple of degrees to see if we can better protect them.

Dave Brown:

Yeah. Systematically, buds are colder than air. If you don't account for that, you're missing by a few degrees usually. And that's the thing about frost – a few degrees matters.

Bill Anderson:

That's pretty critical.

Dave Brown:

Yeah.

Bill Anderson:

A lot of things we'd like to talk about are lethal temps and at what temperatures we should take action. We go off bud charts showing critical temps, done by WSU. We go off the old bud charts, which we found worked best for us. Can you explain a little about lethal temps and how they're determined?

Dave Brown:

The basic idea is that someone takes a bud and puts it in an enclosed chamber, then drops the temperature at a certain rate. There are different ways of measuring how a bud dies, but let's not get into that. The important thing to realize is that, at wet temperature, a certain fraction of buds will die. By doing enough experiments, you can start to put together a picture of a distribution. The main thing to keep in mind is that it's always a bell curve. At any given point, some buds will die at a higher temperature, and some will die at a lower temperature. There's a tail end to that as well. The key takeaway is that whether it's a chart, a model, or someone who samples your orchard and gives you a report, the reported temperature is the temperature at which the kill occurs.

In that chamber, the air temperature and the bud temperature are the same because everything is enclosed and insulated. Unless they drop the temperature too fast, the bud temperature matches the air temperature inside the chamber. However, a bud can lag behind the air temperature if they drop the temperature too quickly, especially with a big fat bud later in the season. Even when growers directly sample and feel like they dialed in where their buds are at, they can still experience some death. That chart, model, or report is based on the bud temperature, which is always going to be a bit colder than the air in the orchard.

Most people measure air temperature to make their decisions, whether it's the autostart sensor for Orchard, their own weather station, or some other sensor. The buds will be colder than the air, and that difference can be pretty dramatic at times, especially early in the season when the bud temperature can be four or five degrees colder than the air. That difference is the difference between minor damage and losing your crop, which is why we need to get the message out that people need to understand that overnight, as the temperature declines, the buds will lose heat much faster than the air.

This differential can be greater on clear calm nights when we get inversions. As the bud gets bigger and fatter, and as there are more leaves to protect it in the canopy, the difference will get smaller. However, in general, the leaves, stems, buds, and ground will always be colder than the air at night, which is why we need to take that into account when making decisions. The charts are reasonably accurate, but they are ballparkish, and quite often, they don't know exactly where you're at. Early in the season, when you're at bud stage 0, 1, 2, you know what's happened recently, and you know if there has been a warm spell or a cold spell. That hardiness can bounce up and down a little bit.

Bill Anderson:

I was going to ask you about de-acclimation and reacclimation. They're not talked about a lot, but we know we can gain some reacclimation with colder nights, cooler days, and less sunlight with some shade. Is that correct?

Dave Brown:

Yes. Cooler temperatures will help you reacclimate as long as you haven't passed the point of no return in terms of dormancy loss.

Bill Anderson:

Is that more likely to happen in the first couple of stages than the latter stages?

Dave Brown:

It doesn't happen much in the later stages. For cherries, there might not even be that much at bud stage three. It's really more at 0, 1, and 2. Once you're past that point, there's no return.

Bill Anderson:

So, are you thinking our most critical stages for frost mitigation are bud stages one through three?

Dave Brown:

I wouldn't say they're necessarily more critical, but they're more vulnerable later. The trickiest part is the early season because of how dynamic it is. Once you get to stage four and beyond, the amount of change is not that great. Bud stage observation gets you pretty close to where you're at. However, at 0, 1, and 2, it's sometimes hard to know your hardiness and your LT 10.

One thing I wanted to mention is when I was at WSU, we started thinking about how to communicate this. Because you have a bell curve in terms of hardiness, the LT 10 is out on that tail, and you don't get a lot of buds out there. It's hard to determine accurately what the LT 10 is, where 10% of your buds die. That's why WSU, OSU, and UW are starting to communicate more about the LT 25. You don't want to lose a quarter of your buds.

Sometimes it's better to look at a stable measurement and adjust up a few degrees because the LT 10 can bounce around and not give an accurate assessment of where you're really at.

Bill Anderson:

We've noticed that on really cool days, followed by cold nights, our bud stages seem to slow down significantly, either from one to two or two to three. It can feel like the buds are hanging onto one stage or just not moving at all.

Dave Brown:

Yeah, I see what you mean. So, you're saying that the cool days kind of give the buds a chance to recover some hardiness?

Bill Anderson:

Exactly, it's usually X amount of days in your bud stages. Sometimes, it feels like even on those cool days, they don't necessarily stunt the buds but rather make them hardier and slow down the actual bud growth a little.

Dave Brown:

It definitely slows them down. When you have the cool days – we don't really take into account with these models – but the bud temperature in the daytime will depend on how sunny it is. On a day that’s 50 degrees and sunny, that bud's a lot warmer than 50 degrees and cloudy.

Bill Anderson:

It would be really helpful in the industry if we could test bud hardiness once or twice weekly. We're trying to do some testing through the experiment station, but they get backed up easily. It's challenging to get up-to-date information from different areas since the testing process is slow.

Dave Brown:

If we can get all the data figured out, we could make better judgment calls on acclimation and reclamation. The charts we have don't work well in the early stages because the visible information doesn't tell us enough about what's going on. But, having a model based on weather data that predicts hardiness should get us pretty close. And then, there's a general model for the next stage that has a single progression.

Bill Anderson:

You're right, if we had all this data figured out already, we could look and say, ‘okay, we just have to make a little bit of a judgment call on acclimation and reacclimation.

Dave Brown:

Exactly. My hope is that the researchers can build a model that can predict within a degree or two.

Nick Anderson:

Can you explain to people who might not know about the different bud stages and how super cooling works in the earlier stages?

Dave Brown:

Sure, super cooling is essentially about having certain materials in the water that prevent it from freezing. There are a couple of different things at play here. Part of it involves moving the water out to parts of the tissue where there won't be any damage when it does freeze. Another part involves adding certain constituents to the water, such as salt, which lowers the freezing temperature. This allows the bud to withstand colder temperatures without freezing. The tree can also add some constituents to the water to allow it to get even colder without freezing.

Bill Anderson:

We lose most of that water in the bud by about stage two, right?

Dave Brown:

Yeah. Now I'll go look it up and find out I'm wrong, but my understanding is that's why it swells – it's gaining water. That's why it's also more vulnerable, because you’ve got water coming into the bud and when it freezes, it breaks the cells.

Bill Anderson:

Oh, I see. I had heard something about losing that water in the first few bud stages and making it more vulnerable to damage. I'm not sure if that's correct though.

Dave Brown:

From my understanding, it's actually the opposite. I think the water is moved to safer areas first and then rehydrates as the bud develops. But I could be wrong. One thing I've learned from interacting with you guys is that we both learn a lot from each other.

Bill Anderson:

Definitely. Your weather program and stations are going to be a fantastic tool for farmers like us who are in temperature-critical areas. It's really been helpful.

Dave Brown:

Firstly, I want to say I appreciate your time coming on, especially during go time when everything's moving forward on the cherries. I've been working with Bill and Nick for six months in this capacity, but I knew about you guys before that. I've been looking at your data, your cold hardiness data, and your weather stations for about three years, I guess. So I always saw Anderson as a name and a column of data.

Bill Anderson:

The cold one is the one that stood out.

Dave Brown:

Yeah. I've seen that Anderson name for a long time. So Bill and Nick, we've been working together for about four or five months now, I guess. Both learning from each other. I'm learning about your operations, and you’ve really helped us in developing our app that goes with our weather station. This is a chance for us to talk and share our interaction with the wider group.

Brad Newbold (Host):

Alright. Our time's up for today. Thanks, Bill and Nick, for being with us today. We really appreciate you spending time with us, and thanks to Dave for taking time to share information and ideas with us.

If you have any questions about this topic or want to hear more, feel free to contact Dave Brown.