In this episode, Dr. Brown continues exploring frost mitigation best practices with Bill and Nick Anderson, Cherry growers in the Dalles, Oregon region. This session focuses on the use of water for frost mitigation in both ground and bud applications.

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:

Another practice that I have been doing for many years, which has proved to be effective, is watering the ground during the day to add moisture. Could you talk a bit about this in comparison to the common practice of starting sprinklers as soon as the frost event occurs at night? Many people tend to do that.

Dave Brown:

We'll discuss both of these topics, starting with the fact that watering the ground and putting water on trees are two completely different practices. Watering the ground is beneficial for mitigating frost, unless there are environmental concerns, a limited water supply, or negative impacts from overwatering the soil. There are three reasons why it works.

Firstly, it adds thermal mass, which means it can absorb heat energy during the daytime and release it more slowly at night, resulting in slower temperature drops.

Secondly, it darkens the soil, which enables it to absorb more solar energy during the day, leading to more warming.

Finally, it makes the soil more thermally conductive, which is important because sub-soil is a source of warmth during wintertime. Increasing conductivity in the sub-soil means that the ground won't be as cold at night, providing more thermal energy to the buds and less cooling of the air. This, in turn, results in less strong inversion. If water is available, I definitely recommend this practice.

Bill Anderson:

Well, I understand covering your buds for protection, but there are some people that do under tree watering, but they start at night when they're getting ready for a frost event.

Dave Brown:

Yeah. You're talking about micro sprinklers, is that correct?

Bill Anderson:

Yes, exactly.

Dave Brown:

So, whether water is applied over the trees or through micro sprinklers underneath the trees, the effect is the same. However, by using spray under the tree, you are targeting the bottom of the tree where frost damage is more likely to occur. This is because it is colder there. It makes sense to treat it that way.

That process is completely different from watering the ground. The idea behind it is that melting ice takes time. A block of ice has to sit in the sun for a while before it melts. However, when water freezes, it releases a lot of energy. If a wet surface is created on the buds through a frost event, it will continue to release heat into that bud and keep it warm throughout the night. This method has proven to be very effective.

However, to ensure success, it is important to start before the event and keep the surface wet throughout. The outer side must never dry out. If it does, a lot of damage can occur.

Bill Anderson:

Yeah.

Dave Brown:

Are you guys familiar with wet bulb temperatures? Is that something a lot of growers talk about?

Bill Anderson:

Not, not really.

Dave Brown:

The concept behind the dry bulb and wet bulb temperatures is that dry bulb represents the normal air temperature. Although it is not commonly referred to as dry bulb, that is its official name.

To get a wet bulb temperature, in the past, a small piece of wet cloth was wrapped around the thermometer and spun. This caused a little bit of evaporation from the wet cloth to cool off the bulb of the thermometer. As a result, the wet bulb temperature is a little cooler than the dry bulb temperature.

It is important to note that if water is applied to the buds, it will ultimately warm them and prevent the freezing process. However, initially, there will be a cooling effect on the surface because of the evaporation of the water. When deciding whether to use water directly on the tree, the wet bulb temperature should be used instead of the dry bulb temperature.

To determine when to turn on the water, consider your critical temperature, which might be 26 degrees Fahrenheit. You'll need to start watering before the wet bulb temperature dips below 26, which could mean your dry bulb temperature is around 28 or 29, depending on humidity. Remember, the wet bulb temperature is always lower.

We hope to add the wet bulb temperature to our app in the next couple of weeks. We can estimate it using humidity and temperature.

When deciding whether to water the buds, use the wet bulb temperature to guide your decision-making. Start watering when the wet bulb temperature is above your critical temperature, and continue until it's above your critical temperature again on the other side of the frost event.

Nick Anderson:

Later on in the frost season, does that help us keep a little bit more moisture and help bring temperatures up? Or should we have a shorter ground cover? That's

Dave Brown:

That's a good question. I was just researching this recently, and the advice is to go with a short ground cover. Technically speaking – I'm not recommending this at all – is not having any ground cover. Bare soil saturated with water is ideal, but you don't want to do that for other reasons like the risk of erosion and other problems. However, you do want to access that thermal mass down in the soil.

Bill Anderson:

So the taller grass is kind of insulating against the absorption of that heat?

Dave Brown:

Yeah, apparently that's what the research shows – you get more solar energy being absorbed, warming everything up, and that good thermal mass carries you through the night.

I'm curious, how many people apply water directly to the trees during frost events? And how many run into crop damage?

Nick Anderson:

Not too many people actually go directly to the tree. Most people around here turn it on as soon as they turn the wind machines on. Someone drives around and turns all the water on.

Then there's a small group of us that are turning the water on early in the day to start getting all the ground wet.

Dave Brown:

Yeah. So they turn them on, but they're using them to coat the buds, right? When you say they turn them on at the same time?

Nick Anderson:

Just to get the ground wet. They just do it right at nighttime.

Dave Brown:

Oh, okay. So that's really missing the benefit. You really want to get it on early in the morning before the frost event, so you can get a day of solar energy absorbed. I didn't realize that – they're applying irrigation to the ground during the event.

Nick Anderson:

Yes, and most of the time the sprinklers freeze up within an hour if they're running small micros. Then they're just doing nothing.

Dave Brown:

Yeah, it's really probably not going to have a whole lot of benefit at that point. Maybe some, because the water's probably a little warmer than the air, but you're better off applying it in the morning and giving it a whole day to absorb some solar energy and warm up.

Bill Anderson:

One of the districts we farm in has very sandy soil, and I'm wondering if that’s where we're picking up some damage, since it’s not holding heat as well at night. That's why we were thinking we need to start earlier, because if it's sandy soil, it'll absorb the heat, but might be releasing it sooner.

Dave Brown:

Yeah, it probably doesn't have the same thermal mass as soil that has some fines in it. It's going to be more responsive to the changing conditions. It'll cool down quicker.

Bill Anderson:

You think we should run the wind machines a little sooner? To start mixing air a couple degrees earlier?

Dave Brown:

That's an active research question we're exploring this year, even in your orchard. One of the things we want to ask is what are the benefits of running the wind machine early? It's not entirely clear. Firstly, in terms of mixing the air, that takes about half an hour to get pretty well mixed. But then, after mixing the air, there's a certain amount of time needed to warm the bud with that mixed air, right? Because the buds are colder than the air and you need some time for that. But it's probably around an hour.

So when you're considering starting an hour or more earlier, then you're probably thinking about whether you can warm up the ground. I'm curious to see the answer to that.

I've heard of some people turning on the wind machine at eight o'clock at night and running it forever. I'm not sure if science says there's a real benefit to that necessarily, as at the end of the day, whatever the wind is doing, the ground is still radiating out that energy. It would be interesting to see how much benefit you get, but there's probably some benefit.

Bill Anderson:

In the area where we are, people have been starting the wind machines approximately two degrees above critical for quite a few years. However, what I have always considered is, first, where is my dew point, and second, how fast is the cold air moving in? In some areas, the cold air moves in slowly, while in others, like our home area, it can drop three degrees in 45 minutes. So, we usually set the wind machines at about four degrees above critical in that area.

Dave Brown:

I'll say that when growers set the wind machine at a certain offset, they are essentially trying to make a forecast. The idea is to start the wind machine an hour before crossing the critical point and on the fly predict the rate of temperature drop to set the machine accordingly.

However, we believe that the better long-term solution is to have a nowcast model that uses machine learning AI to analyze trends in air temperature and dew point and predict when you will cross the critical point, say in two hours, one hour, 90 minutes, etc.

This approach will make decisions based on the forecast rather than a set point, which can lead to growers constantly adjusting the set point night after night.

Bill Anderson:

How is the progress coming with POMETA nowcast and forecast?

Dave Brown:

Currently, having a year of data works well. We're working on developing a reliable system with a shorter training period. Even with four or five months of data, it's still quite reliable. We're making adjustments to make it work with as little as two months or a month and a half of data.

That's the challenging part, but we hope to release a solution soon that you can experiment with for short-term solutions. When there isn't enough training data available, we learn from several stations and apply that to your station. However, that will never be as effective as having a year of data from your station. The true benefit of these methods comes from having a station for an extended period and gaining knowledge of what occurs in that area.

Nick Anderson:

I do want to tell you – your app, it is amazing. No joke. I'm not doing a sales speech right now. It does work great.

Seeing the actual alarms is nice. Getting a frost alert phone call and having to ‘press one’ to say, ‘Hey, I’m awake, I got the alert.’ It works really nice.

Dave Brown:

Glad to hear that! We've been sweating blood on that thing.

Bill Anderson:

Yeah, well, it is the way of the future. It’s allowing us to sleep. We still can't sleep all night, no matter what – we’re trained to get up every few hours and check temps even though we haven't had an alarm go off. But it's also nice to know when you're dead tired, you can go to sleep and we can count on that alarm.

Dave Brown:

That's really good to hear. That's our goal. That's actually how I motivate the software development team – tell them, “Hey, this is an opportunity to do something that actually makes someone's life better.”

Bill Anderson:

Yeah, my wife appreciates it.

Dave Brown:

Good to hear. Really good to hear.

Brad Newbold:

Alright, our time's up for today. Thanks, Bill and Nick, for being with us today and we really appreciate you spending time with us. We'll continue with more frost mitigation discussion in part three of the series.