Winter Dormancy and Cold Hardiness

I’m not so sure that when it comes to a vine’s winter hardiness, one can actually state in the affirmative that grape variety X is cold hardy to minus 35 deg F. Basically I thought I understood the winter dormancy stage of a grape vine. At the start the leaves start to change color, later they finally fall off, the sap from the upper part of the plant moves to the root system and underground portion of the plant and the vine goes to “sleep”. During the coldest of winter months the vines look like primitive and simply stick figures just hanging around a bunch of posts and wires.
Because of the recent cold snap of weather we have been experiencing lately, and while visiting the local wineries of the east end of Long Island, I had the opportunity to discuss just this topic.
The winter dormancy stage is a lot more complex then I originally thought. It consists of actually three phases (1) acclimation; (2) winter hardiness; and (3) deacclimation.
The acclimation phase, the start of the vines dormancy stage begins a lot early then I thought. It starts when the growth cycle of the vine actually stops; this means this could possible start occurring late August. During this phase the vine starts the “hardening” process. Fall weather conditions must be favorable for this hardening process to occur successfully and for the vine to obtain its optimum winter hardiness level. The next phase is winter hardiness, during this phase the vine might appear to be inactive and sleeping but in reality there is a lot of activity going on (especially on a cellular level). Winter hardiness is defined as, how well a dormant vine can tolerate cold temperatures. During the winter hardiness phase the vine goes into a growth cycle lockdown. In other words even if nature provides a stretch of abnormally warm days in January or February the vine will not begin a growth stage, this could be devastating. The last phase of the winter dormancy stage is deacclimation. Deacclimation could be thought of as a “softening” or “un-hardening” process so that the vine can continue its annual life functions.

Salute

..The last phase of the winter dormancy stage is deacclimation. Deacclimation could be thought of as a “softening” or “un-hardening” process so that the vine can continue its annual life functions. Salute

I would assume this stage ends when bud break occurs. And if it happens too early and a hard frost occurs afterwords, the vine, or at least that year's growth could be killed off. Different varieties have bud break at different times.

Victor,

That's the way it is supposed to work. But it doesn't always work that way:

1. an early and durable frost during but before the hardening stage is complete can weaken vines

2. a long enough stretch of warm weather toward the end of winter can awaken the vines, and then a cold snap comes in and zap.

3. a severe winter can freeze at the graft and kill, which is why "hilling up" is recommended in severe climates

4. severe winters or late spring frosts can kill off primary buds for the coming vintage

I'm sure I'm forgetting one or two more events.

Nothing is easy in a vineyard, not even after you understand the ideal cycles of nature as it relates to grapevines.

Just hopefully Howie, if an early spring frost does occur (and lets say the previous fall weather was extremely favorable for acclimation (the hardening phase) to occur successfully, the winter itself wasn’t too severe, and good balance pruning was conducted during the winter months, and during the deacclimation phase (softening or un-hardening phase) there wasn’t too much of a fluctuation in temperatures) just maybe secondary or tertiary buds could survive an early spring frost.

Salute

Thomas wrote:
Nothing is easy in a vineyard, not even after you understand the ideal cycles of nature as it relates to grapevines.

Thanks Thomas, but at least if one understands the natural life cycle of the vine it makes him/her a better vine grower or winemaker.

Salute

Thomas wrote:
Nothing is easy in a vineyard, not even after you understand the ideal cycles of nature as it relates to grapevines.

Thanks Thomas, but at least if one understands the natural life cycle of the vine it makes him/her a better vine grower or winemaker.

Salute

It certainly should, Victor. I wasn't commenting on that, only commiserating over the situation as it stands.

One thing I forgot to mention: if grapevines go through a period of stress, like an extended drought during the growing season or maybe a disease stress of some sort, they can be weakened enough not to respond as they normally would in winter. i.e., when a severe winter follows a drought, the results can be devastating.

One thing that interested me the most about the conversation with this knowledgeable young winemaker was that the dormancy period of the vine begins when the growing cycle (of the vine) ends. Basically what I got out of this conversation was that growing days (all wine growing regions love giving statistic about how many growing days the region has, basically it represents the time from flowering to harvest) does not really coincide with the actual growing period or cycle of the vine itself. Man determines the actual time of harvest, when he/she thinks the fruit is at its “optimum” ripeness for the style of wine he/she wants to produce. But to the vine when the seeds become viable and “ripe” (this is basically around the time of veraison) the fruit is “ready for the taking” and its growth cycle is terminated and it prepares itself for dormancy and commences its acclimation phase. So basically, vine grower most not only concern themselves with getting the fruit “ready” for harvest they most be aware that the vine itself is terminating its growth cycle and biologically preparing itself for dormancy, so basically during this time they must concern themselves with aiding the vine through its acclimation phase. Piling on to what Thomas pointed out- basically what ever is done to the vine prior to this (I guess we can even go back to the previous year) or during its growth cycle (spoiling the vine, letting it over crop, stressing it to much, not enough water, too much water, letting the disease state of the vineyard get out of control, etc) will not only effect how the fruit ripens but also how the vine acclimation phase proceeds and at what cold hardiness level will be obtainable by the time winter cold weather settles in.

But as far as predicting (or making generalizations about) the annual life cycle of a “domestic” plant it shouldn’t be that difficult. Because the annual life functions, even though they are greatly influenced by ambient temperatures and weather, they more or less proceed in a very step-wise oriented way.

Salute

"...what ever is done to the vine prior to this (I guess we can even go back to the previous year) or during its growth cycle (spoiling the vine, letting it over crop, stressing it to much, not enough water, too much water, letting the disease state of the vineyard get out of control, etc) will not only effect how the fruit ripens but also how the vine acclimation phase proceeds and at what cold hardiness level will be obtainable by the time winter cold weather settles in."

Salute

Yep. An important factor.

In the Niagara region, temperatures can drop low enough to cause bud damage and even vine loss. Alot has been learned to mitigate losses, but as recently as 2005, widespread damage is still possible. Even colder than Niagara is Prince Edward County. Here is an interested discussion on how grape-growers there survive tempertures arppoaching -30C :
http://www.bychadseyscairns.com/
(Note: There is no direct link, so you will have to click on "Vineyards", then select "Cool-Climate Grape Growing")

Cheers,
Norm

Thanks for the Link, Norm.

Salute