135: Cold Hardiness of Grapevines

There are three levels of cold hardiness in grapes and understanding these can help growers select and manage the best varieties for their region. Imed Dami, Professor of Viticulture in the Department of Horticulture and Crop Science at The Ohio State University, explains cold tolerance and new information about the role of abscisic acid in ripening.

First an overview of cold hardiness. The first level is very cold tender. A lot of these varieties are grown in California and they are not native to North America. Second, are cold hardy, or tolerant grapes. These are the native species to North America. And third are hybrid crosses. The majority of grapes being farmed fall into this category.

New research is being done on abscisic acid, a plant hormone that induces dormancy. When sprayed around version, it can help send the plants into dormancy earlier and maintain a deeper dormancy which makes the grapevines more cold tolerant.  


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Craig Macmillan  0:00 

Our guest today is Imed Dami. He's Professor of Viticulture in the Department of Horticulture and Crop Science at The Ohio State University. Thanks for being on the show, Imed. So you're doing some pretty interesting work there in the Midwest or East, depending on who you ask. So you work in Ohio, and you work in cold hardiness of grapes, grape vines, which is the work you've been doing for quite a fair amount of time. Now, you did that as well, in your graduate work. What can you tell us about what the climatic conditions are like in Ohio, especially in regards to cold weather, that may not be seen another grape growing region to the US. So other listeners may not be familiar with kind of the challenges there are.


Imed Dami  0:35 

So in terms of the growing season in Ohio, it could be similar to many other grape growing regions in the country. However, during the dormant season, in the winter, it is different, it is much colder in Ohio. So the way we determine you know, how cold it is, in Ohio, is there is this maps, it's called the plant called hardness map developed by USDA. And these maps are based on lowest temperature for 30 years during the coldest month of the year. So they put that information together, you know, and they come up with averages, and then they divide, you know, each state into different zones based on those minimum temperature. Okay. So for example, in Ohio, we have three zone. The minimum temperature range between zero and minus 15 fahrenheit. So that is cold. And then they designate you know, these zones, like a number and the letter. So, for example, in Ohio, we have the three zones, you know, designated as six B, six A and five B. However, the majority of the state falls in the what we call six A's zone, where temps range between minus five to minus 15. So that is very cold. So, how do we relate this to the grapes? Grape vines, typically, they start to see cold damage at temperature of zero fahrenheit, or lower. So you could see, you know, like grown grapes in the majority of the location in Ohio is challenging, you know, just because of how cold it gets during the winter. Just to give you like a point of reference, these numbers, you know, of the zones, the higher the number, the milder the winter is. So, for example, in California, the majority of the grape growing region, they fall in the region of 9, 10, and 11 zones. And in those zones, the minimum temperature doesn't fall below freezing. So that's why in California, you don't hear any, you know, problems about cold damage during the winter.


Craig Macmillan  2:44 

That's in that brings up an interesting point. So, you know, you say damage happening at zero degrees fahrenheit, why is it the vine damaged and cell walls breaking and whatnot, when we get below freezing? If that vine is full of, you know, water, that it's taken up in the fall and things like that? Why is it why does it need to get so cold before we see damage?


Imed Dami  3:03 

That's a good question. So the, the plants in general, they have a different mechanism of survival, you know, to cold. So when it comes to grape vines, the most sensitive part of the vine are the buds. And these buds, the way they survive winter is with the mechanism that we call a super cooling. So what super cooling is, is basically, the water or the solution, and the plant remains liquid, even below freezing temperatures. So that's how the grapevines you know, buds survive those minimum temperatures. So that is the mechanism by which grape vines survive. Of course, when you have a super cooling up to a point, in general, plants could super cool down to like minus 40 Fahrenheit, which is very cold. But it ranges you know, from let's say, like I said, you know, just below freezing all the way down to that. And the difference is the type of grape variety or species which I will be talking about later on.


Craig Macmillan  4:08 

What is severe cold damage on grapes look like? Again, a lot of folks outside of the Midwest or the northern United States may not have ever seen anything like it before. And what kind of economic impacts are there?


Imed Dami  4:19 

Sure, the vines you know, they have this parts that are above ground, and any part above the ground that is exposed to those minimum temperature could sustain cold damage. And then because there are different parts, the one that is the most sensitive are the buds, like I mentioned earlier, the way you identify a cold damage is by cutting through the bud. And then the typically, the buds are alive, they look green, and then when they are damaged, they look brown, so it's very easy to tell, you know, by visual observation. And then within grapevines, we have what we call the combpound buds, they call them compound because there is a primary, secondary, and tertiary bud. Primary bud actually is where the fruit comes from. Unfortunately, that is the most sensitive to cold. So what that means is every time you have cold damage, the primary bud is damaged, that means that's going to become like a loss, you know, in crops. And then the other tissue that is important that could also sustain damage is what we call the vascular tissues. These are basically the pipeline's you know, in the grapevine, primarily what we call the phloem, and the cambium. And those two actually, same thing, when you will cut through the cane, you know, or the corridor or the trunk, you'll see the color change to brown, which indicates cold damage. Though the worst case scenario is not only you have the canes damaged the cordon, the trunk, but also the whole vine, and basically, cold actually could cause complete vine death. So in terms of economic impact, it depends on the severity of the plant part that is damaged. So if you have only buds damaged, that's typically becomes you know, like a more like a crop loss. Same thing with more severe damage with the vascular tissue, let's say in cane, cordon, or trunk, you lose, you know, those parts of the vine like a cane, you may lose it or the cordon, or the trunk. The most severe damage is vine death, basically, the whole vine dies. In terms of economic impact for the vine damage. Actually, there is a study done in New York, where they estimated the loss, you know, from a dead vine due to cold. And their estimate is a loss of $155 per vine, and this is for vinifera. So if you extrapolate that to per acre is equivalent to $125,000 per acre in terms of losses. And the main reason is not only you have to buy new vines, you have to replace you know, remove the vines, do the replanting, and then also, the main thing is you have a loss of production for four years. And then basically, when we talk about wine grapes, you're not only losing the grapes, you are also losing the product, which is why for four years, and that's why the number is so high and significant.


Craig Macmillan  7:20 

That's a good question. So if you have a bad winter, and you go in and you evaluate buds, and they're dead, and you then go into the cordon and look and say oh, that's not good. And then you work your way down into the trunk, and look at that, is it ever the case that you will hit a spot where there's still some vinifera that's alive towards where the rootstock graft union is that you could graft on to or that you might get some latent buds to come out.


Imed Dami  7:45 

So, actually, I will talk about this later on in terms of the practices, you know, in the vineyard when we talk about grafted vines, which is, you know, common for vinifera. The reason why I mentioned this, because typically here we also grow hybrids, they're not grafte. With the grafted vines, you typically hear in the east, we mound soil around the graft union. So at the base of the trunk, you know, you see this what we call mounting or healing up around the vine. And the main reason is to protect you know, that graph tune because it's very sensitive. And then also the base of that trunk to avoid any damage in case of the worst case scenario, like you said. When do you have damaged, you know, all the way of the cordon and then the trunks. If you have that situation, then you save the base of that trunk. And then you have regrowth, you know, of the vine. So that's typically how it is done, it is not common here to graft over, you know, the vines when they are damaged. But that is typically the practice, you know,


Craig Macmillan  8:48 

Which reminds you something else. So is this a temperature over time kind of a curve? Or is it a threshold? Once we hit this temperature it's done or is it need to be at a temperature for a period of time before the damage really shows up?


Imed Dami  9:03 

Typically, when you reach like the temperature outside, you basically have the temperature outside and then the temperature of the tissue. When we talk about the small tissue like gray buds, they reach an equilibrium between the two. So as soon as it gets cold, you know, let's say two minus 10 in the outside it will be minus 10 in the buds as well. So in that case, you know, the the freezing of the water is instantaneous. It's like immediately. When you have tissues that are bigger, like the cordon or the trunks because they're thicker you know, just like you take a piece of wood you know and then the temperature usually outside is colder colder than the temperature in the trunk. So it takes more time for trunk to get damaged. You know it may be like hours before it really, yeah. So typically here in our situation, you know, when we have some these big events like the polar vortex back in 2014 Not only it got cold, but it's  stayed cold for a long time. And those are the worst scenarios, you know, because not only a damaging or the bad, but also, it damaged the trunks as well, because it's so cold, you know, for a long time.


Craig Macmillan  10:11 

When we see this kind of an event, are we looking at a 100% loss across the vineyard?


Imed Dami  10:16 

No, not typically. So, again, you know, I'd be mentioning this later on. The vines, you know, they tolerate cold based on their genetic makeup. So there are some varieties like the vinifera, they are very cold tender, they may sustained some damage, or more damaged than more cold tolerant or resistant varieties, like, you know, Concord, or some of the native grapes, you know, here grown in the east. So there are differences, you know, that you see, in terms of cold damage.


Craig Macmillan  10:49 

Tell me more, you mentioned hybrids before and we're talking about different varieties, what can you tell us about the cold tolerance to cold hardiness of different varieties and what the genetic background might be on those and how plant breeding has addressed this problem maybe.


Imed Dami  11:03 

So as I mentioned earlier, the cold hardens is actually a genetic trait. So what that means is the genetic makeup of each variety determines the cold hardens level of that variety. So it is in the genes, you know, of the viru. Based on that we have, like mainly three groups of grape types, if you want to call them The first group, what we call very cold, tender or cold sensitive, these are the group of the species vitis vinifera, these are basically the varieties grown in California. And the main reason why they're so tender, because they're not native to North America, they are used to more what we call a Mediterranean climate, you know, which is characterized by mild winters. So when we imported them here, and we grow them like in climates, like here in Ohio, it is very challenging because they're so tender. The other group, kind of the other extreme is what we call the cold hardy group, you know, or cold tolerant. These are native grapes. These are native species to North America, and we find them a lot here in eastern US, like vitis labrusca, an example of variety, like very popular varieties Concord. We also have other species like vitis riparia. So these are all native to this region. And those species are very cold, hard, you know, because they're used to the type of climate, you know, they are grown in. And then the third group is what we call the hybrid. So the hybrids are crosses between the vinifera and the native grapes. And the main reason why they develop these crosses is the vinifera basically provide the quality of the fruit, and eventually the wine. And then the native grapes provide the cold hardiness. In our industry, and generally, in the east, most of the grapes we grow actually are hybrids, because they tolerate a more cold, you know, than the vinifera. And there are several examples of varieties, old varieties like Vidal. Seyval, example of Chambourcin. And, and then we have now like a lot of new hybrids, like Traminette, Chardonel, are varieties are developed from New York program. And then we also have other very cold hardy do they call it super hardy varieties from the University of Minnesota like Frontenac, Marquette. These are very hardy varieties. And they are hybrids.


Craig Macmillan  13:28 

Yeah, they were developed in Minnesota that have to be pretty darn hardy. You can see the challenge there. Tell us about your work around abscisic acid, I know that that's related to cold hardiness, that's related to sugar and other things. First of all, tell us what what is abscisic acid, ABA, what is abscisic acid.


Imed Dami  13:48 

So abscisic acid acid actually is a plant hormone, it is naturally produced by the plan. And typically it is, it is associated with a lot of like physiological response by the plant. And one of them actually induces dormancy. Our idea when we use the abscisic acid is we try to kind of enhance dormancy by applying abscisic acid, you know. So we are adding more abscisic acid to the plant that produces its ow. And by doing that we could, our hypothesis at the time is can we change the dormancy like in terms of occurrence, you know, can we make it happen earlier because the earlier the vines enter into dormancy, the more prepared they are for the winter. And then number two, yeah, and then number two is the level of dormancy, the more dormant the vines, the better they do in the winter. And so when we applied abscisic acid actually did both. So while we are pleased with the responses. And then eventually vines, you know, not only they enter into dormancy earlier in the season, they also have a deeper dormancy. But then that actually was reflected later on in more cold hardy response by the vines that are treated with abscisic acid. So it was really a very positive response, you know, by applying that product. It was the first time this product was used in grapevines. And we're very excited to know by the response.


Craig Macmillan  15:21 

How is it applied?


Imed Dami  15:22 

We looked at the timing, you know, when is the best time to apply it. And what we found is actually, right after verasion during fruit ripening, when the leaves are still on the vines, you know, actually, the fruit is still maturing, we found that is the best time to apply abacisic acid. So this is basically sprayed, you know, it's a liquid, that you spray it on the canopy. And then that's it, then basically, we look at the response, you know, later in the fall, and then during the winter, so.


Craig Macmillan  15:56 

You also mentioned a deeper dormancy. What does that mean? When is deeper dormancy?


Imed Dami  16:03 

A deeper dormancy, that means the vines, you know, basically, during the fall, they began to enter into what we call dormant, you know, basically, they go into a state of like, asleep. When they do that, sometimes they don't go like into what I call, like a deep sleep, you know, and then that has to do with the climate, the conditions, you know, that they were exposed to every year depends also on the vine health, etc. When the vines enter into deeper dormancy, that means it takes more time and more effort to wake them up. And then when they are more dormant, they actually gain a more cold hardiness as compared to when they're not as as dominant. So, so that's what we mean by deeper dormancy.


Craig Macmillan  16:55 

And that can be achieved by applying abscisic acid into the canopy, right around verasion?


Imed Dami  17:02 

Yeah, we weren't able to do that. Exactly. So again, you know, like anything else. When you apply a product, you know, it works in some varieties, it doesn't work in others. It works in some climates, you know, not under others. So, we see sometimes this kind of inconsistency. But when we have a controlled environment, let's say in a greenhouse, we consistently see the response to abscisic acid by the plants. So basically, absciscic acid you could think of it as it mimics the environmental cues that typically the vines, you know, get from the environment because the vine, for the vines that go dormant and begin acclimating it has to have two clues from the. It's short days, as soon as the days become shorter, the vines you know, start to get dormant. And then the second clue or cue is the temperature. When he started to get colder, the virus begin to become more dominant and become more cold tolerant. So those are the two. And then we could replace those two cues, actually, by applying ABA.


Craig Macmillan  18:10 

You know, actually, this brings up something that often or continually be been kind of confused about. How does the vine sense photoperiod? If the leaves are falling off or becoming cut off from the rest of the vine, is there another organ or way that the vine can sense what's going on with the light?


Imed Dami  18:30 

Well, actually, they do that sensing when the leaves are still on.


Craig Macmillan  18:33 

They do. Okay.


Imed Dami  18:34 

Yeah, so the receptors really actually are by the leafs you know, and I think that's why probably even when we applied the ABA was more effective, you know, when the vines still have their leaves on. So that I mean when you think about like short days, you know, during the growing season that starts back in June. So it's such like a way early see and then actually, by that time the vines begin already sensing you know, this short days, they begin the process actually of cold acclimation. So cold acclamation, or we call the hardening off of the vines. Actually, it begins right when the fruit begins to ripen during verasion. So it happened like way early, you know, like in the summer, basically, you talking about July, you know, and you start to see the tissue, as they, as the fruit is ripening the vines actually at the same time is preparing for the winter at the same time. And then it will continue after the leaves are dropped. And then the vines become more sensitive to the temperature rather than the full period. So it becomes the second step is based mainly on the temperature.


Craig Macmillan  19:38 

Interesting, interesting.


Imed Dami  19:39 

And that's why during the winter because that is the coldest month, the vines you know, they sense these cold temperatures, and they reach actually their maximum cold hardens during the winter because they need it.


Craig Macmillan  19:51 

And then it's the response to the warming temperatures that brings them out of dormancy.


Imed Dami  19:54 

Exactly. And that basically happened like late winter like right now or early spring and that's why basically, the winds, you know, begin to wake up. And that phenomenon is typically driven exclusively by temperature. As it starts warming up, you know, in the spring, the vines, you know, begin to do, what we call deacclimate. So that means they lose their cold hardness, and then they start growing again.


Craig Macmillan  20:18 

What can growers do, are specific practical things that growers can do to prepare for, or manage, or prevent damage to vines in these really cold areas.


Imed Dami  20:30 

Sure, yes. In terms of like things that the growers could do, there are three main category, if you want to call them. The first one is what we call a preventative. So how do you prepare for the cold before even it happens. One of the main ways to do that actually is site selection. You have to have a very good site to grow the grapes, and avoid, you know, this minimum temperatures. So that is very critical. You know, especially for us here in the east. The other thing is selecting the proper variety. So like I mentioned, we have variety that are very sensitive dive right at a more cold hardy. So it's very important to match the cold hardness of that variety with the site where you're going to grow them. You cannot grow for example, vinifera in a site where it gets to minus 10, you know, every year, that is not possible. The other thing in terms of practices. And again, this is more unique, you know, to eastern US, is we train vines with multiple trunks. If you look at the vines, you know, in California, they all have a single trunk. For us here we have multiple trunks. So you could see a vine with two trunks, you know, they look like they are two vines, but they're not like one vine with two or more trunks. And the main reason is when we have a cold event, that cold event doesn't kill both of those trunks kills one and not the other, so they don't die simultaneously. So that is kind of like a kind of an insurance, you know, practice. And typically we see this in almost all grapevines. When we have injuries, you see one trying to get damaged and not the other. The other thing that I mentioned earlier, is we heal up the vines to protect the grafting union. So this practice actually is done every year, it is done in the fall, and then the vines have to be dehilled, or removing those mounds in the spring. So this is a common practice that we use for vinifera here in the east. And then the third one is what we call cultural practices. In terms of fertilization, crop management, anything basically improves fruit quality also is favorable for improving cold hardiness. In terms of during the cold event, the main thing that our growers, you know, some of them they use is what we call the wind machines. So when machines I know for example, in California, they're very common not to use for spring crops, but here we could use them for spring frost events as well as cold events in the winter, they are an effective tool. So finally, what I call practices by the grower after you do, you have like a cold event. So even though you do everything by the book, you don't have a good site, you have good varieties, you know, a suitable variety then you do the multiple trunks and cultural management, you still it gets cold enough, you know that you have damage. In that case, vines are trained or adjusted, you know, depends on severity of the damage. So for example, when we have only bud injury, we adjust pruning to compensate for those losses. And by doing that you could have a normal crop even though you lost some of the buds. So for example, if you lose 30% of the bugs, you could compensate pruning by adding you know 30% more buds you know, then how you typically prune those vines. And by doing so, you could have normal crop up to a point of course. Another more severe damage when we have trunk damage and basically die back you know of the vines. In that situation, we have to retrain the shoots. And typically the way we retain thse you know is kind of unique in the east. Like I said, we do multiple shoots or multiple trunks, it's very important to do that. And then the size of those shoots are important. We have to select shoots or canes that are pencil size. The main reason is big shoots or large canes we call them bullcanes are not favorable because they are more damaged by the cold. So selection of these shoots and canes, and how many shoots you trained are typical practices, you know, for retraining, winter damage vines.


Craig Macmillan  24:35 

Well, is it better to cane prune or spur prune? Does that make a difference?


Imed Dami  24:40 

Yeah, it doesn't. If your vines you know are typically prune spur or cane you know, it doesn't make difference. However, when you have injury after the fact and your vines let's say they are cane prune. What we found is it is best if you convert those vines into spur prune. And this is only when you have a severe damage of the buds. We found that when you do spur pruning, you had a better recovery, better crop than cane pruned vines. Again, this situation is only true when the vines sustained damage and like more severe damage of the buds. Then you could convert the virus into spur pruning. And then of course, you know, you could always go back to your original in our pruning later on to cane pruning, again, after like year one of the winter injury.


Craig Macmillan  25:31 

All right, what, what's the best timing for pruning in a cold situation?


Imed Dami  25:36 

In our situation here, timing, you know, is not critical. However, when you have large large vineyards, you know, you have to prune like over many months, we typically recommend that you prune the cold hardy varieties first, and the main reason is, you know, if you get like a cold damage, you haven't pruned you know, the code sensitive yet. So you could still leave, you know, more buds or more canes, like I mentioned, with the pruning adjustment. You leave the sense of variety last in terms of pruning, so we prune those last. That is kind of typical recommendation for our growers.


Craig Macmillan  26:09 

Tell me a little bit about the role of ABA and sugar.


Imed Dami  26:15 

One of our research focus, I mentioned, you know, ABA, but before that, actually, we looked at sugars. And what we found is like sugar production by the vines go hand in hand with the cold hardness of the vines as well. So what I mean by that is during the fall, when cold hardness keeps increasing during the fall, the sugar concentration also increases in the vine, in the bud and the vascular tissues. And then when it reaches the cold hardens its maximum during the winter, the level of sugar is also reached maximum at the time. And then in the spring, when the vine lose hardness, the level of sugars goes down again. So there's a very close relationship between cold hardiness and sugar accumulation. And one of the explanation is the sugars that are produced more by the vines, you know, is because they they provide what we call protection to the tissues, you know, they call them cryoprotectant. What we found in our research also is there is a specific group of sugars that we call raffinose family oligosaccharides, RFO. And these are like larger sugars, they have even closer relationship with cold hardiness and cold acclimation as well as dormancy. So in our recent research, what we found is when we apply abscisic acid to the plant, actually, that acts as a signal to produce sugars in the vine. So basically, ABA and induces sugar production. And we have demonstrated that in our recent research, and this is why we have this close relationship, you know, between the ABA role, and sugar production in the vines.


Craig Macmillan  27:50 

Does that affect the sugar accumulation in the cluster? And the berries?


Imed Dami  27:53 



Craig Macmillan  27:54 



Imed Dami  27:56 

Because, you know, the time when the sugars are accumulating, let's say in the winter, the clusters are already gone, you know. So the vines actually, they don't only not only they accumulate sugars in the clusters, at the same time, they are accumulating sugars in the dormant tissues. They do it faster. Of course, once the vines are harvested, they do it at the bigger, faster rate, you know, so they call that actually kind of becomes like a major pool of sugar accumulations, you know. And that's how the vines you know, they have to have this reserves to overwinter, you know, properly.


Craig Macmillan  28:27 

We've talked about a lot of different things. But is there one, one thing that you'd recommend to our listeners that are facing cold hardiness issues? What's the top? What's the top thing?


Imed Dami  28:38 

The top thing? Wow, you know, like I mentioned earlier, it is very simple. I mean, really, in our industry, even though we've been around for a long time, one of the major issue and challenge, you know, in the east is selecting, you know, the proper variety in a given site. And that is really, it's work in progress. It's you know, like, in Europe, you know, they found this matching that we call terroir, over hundreds of years, you know. Even in California, you know, it's still a young industry. So imagine here in the east, we are still really learning about the best varieties, you know, in the best sites, and especially a lot of these hybrids are new to our industry. So we're trying to find you know, that match because really, and for me, that's I always find that the most challenging, you know, to our grower to find out, so.


Craig Macmillan  29:28 

Where can people find out more about you and your work?


Imed Dami  29:30 

One of the ways is, obviously my email address I could give it to you could contact me directly. It's And then there's more information in our website. You could Google grape wherever you know, Ohio and it will show up you know, it is called Buckeye Appalachian. There's a lot of information there about what I talked about. And then also it talks about our extension work you know, working with our industry as well. So I would say those are two good ways or resources, you know, to find out. In terms of information called harness. I mean, I know this is a long title. But we published you know, years ago, a book on cold hardiness of grapevines, it is available through Michigan State Extension. It is called Winter Injury to Grapevines and Methods of Protection. Everything I talked about, with a lot of details, and with more technical stuff, you know. It's like a over 100 pages book. And it is really an excellent resource, you know, for any growers, especially dealing with cold damage.


Craig Macmillan  30:51 

That's fantastic. We're out of time for today, I want to think a guest, Imed Dami, Professor of Viticulture in the Department of Horticulture and Crop Science at The Ohio State University. This has been a fascinating conversation, for me. It's not an area that I really knew very much about, and I feel much more educated than I did. That's a book I might need to get from my bookshelf.


Imed Dami  31:12 



Craig Macmillan  31:13 

So I want to thank you. I want to thank you again Imed. Check out our website for more podcasts. We've got many different topics and many different speakers at the Vineyard Team website. And thank you all again for listening to Sustainable Winegrowing with the Vineyard Team.


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