GRAPE WINTER HARDINESS: Winter hardiness and the short growing season here are my greatest concerns here at the Rist Canyon Vineyards. I have come to think of winter hardiness as not only varietal selection to deal with this problem, but a year round concern of matching cultural practices with the growing season to ensure survival of the vines and maximize winter hardiness. I will deal with how the grapevine survives the winter by acclimation to cold weather and the cultural practices that can be taken advantage of by the grower to maximize this potential in the following article.
Varieties are the first step to dealing with winter hardiness. But hardy varieties don't ensure the grower that the vines will survive the winter. There are many factors that contribute to the actual survival rate of any variety. Hardy varieties only give the vine the genetic potential to endure cold winters.
I have grown Frontenac, (with it's potential to live through -35F), that has not survived a mild winter where the lowest temperature was only -1F. Factors encountered during the growing season contributed to its poor performance during a winter where it should have easily survived.
A winter hardy variety may not live up to its potential because it is not adapted properly to the local environment. A good example of this is growing a late-midseason, winter hardy variety under short growing season conditions. This will not allow that variety to gain the necessary hardening before frost and thus it will never live up to the winter hardiness for which it has the genetic potential and that the grower expects.
What is considered a winter hardy variety in one area of the country may not live up to that potential in other areas. A variety may go through -25F in an area where the winter remains cold with little fluctuation in temperature and the weather warming up quickly in the spring. This same variety may do poorly in areas where the winter temperatures fluctuate widely throughout the winter, going from very cold to warm and back to cold. Varieties vary greatly in the amount of temperature fluctuation they can endure. Vitis amurensis hybrids are known to be able to withstand very cold temperatures but they succumb to the late winter-early spring temperature fluctuations because they de-harden quickly with any warm weather.
A given variety comes with its own genetic potential to survive cold winters but this is only a potential. Whether this potential is realized depends on many factors associated with the growing season. To better understand this, we have to fully understand how the grapevine acclimates to the winter temperatures.
Acclimation to the cold of winter occurs in two phases with the grapevine. This is a continuous process that occurs over the months from August to January. The two phases are:
Late Summer-Fall Hardening: During August the vine begins to slow growth at the apical tips of the shoots, eventually stopping growth altogether. This is in response to the shortened day lengths of the late summer and the cooler night temperatures. The slowing of growth is accompanied by the deposition of suberin and the formation of a periderm to create an outer bark. This is seen by the grower as a color change starting at the base of the shoot and migrating upward to the tip. As the color changes on the shoot, the shoot is effectively cutting off the loss of moisture by sealing itself off from the environment. The shoot hardens into a woody brown to become the canes of next years growth. The texture of the shoot also becomes woody.
As this hardening process is occurring, the vine is also depositing callose in the vascular tissue within the trunk and shoots. This callose deposition at first slows and then stops the upward movement of water from the roots, thus effectively cutting off the vine from water which could freeze and injure the vine.
It is important to understand that varieties differ in the timing, speed, and completeness of this hardening process. Some Vitis riparia clones from the northern and western areas of its range complete this process very quickly and completely. They are masters of the deposition of callose and do this so efficiently that they cut off water to the leaves and impart an early leaf senescence compared to other species and cultivars from warmer regions. Some cultivars of Vitis vinifera are very poor at the deposition of callose. As the temperature becomes warmer during the winter they allow water to move upward into the trunk and if the weather becomes cold again, they are susceptible to freeze damage.
The ability of a variety to maximize its winter hardiness potential is critically dependent upon its ability to properly harden off before a killing frost in the fall. Cultural practices during the growing season must be performed to insure that maximum hardening off occurs. Without an effective hardening process, the vine will never survive the coldest of winters and freeze damage will occur.
Early-Mid Winter Cold Acclimation: The hardening off period ends with the first killing frost of the fall. At this point the vine enters into dormancy. Dormancy is not a period of inactivity though. The vine is still metabolically active and undergoing some crucial events to insure its survival through the coldest portions of the winter. Two processes occur to insure this survival. The first is a process that allows the vine to "supercool" and the second is the formation of various cryoprotectants that allow the vine to dehydrate without lethal damage being inflicted.
As temperatures drop in the early winter, the grapevine begins to pump water into the intercellular spaces of the various tissues. Water in the intercellular spaces can safely freeze without injury to the cells themselves. As water is lost in the cell by this process, soluble sugars and other cryoprotectants concentrate within the cell causing a freezing point depression. This means that the cell fluids can withstand greater drops in temperature without freezing than can pure water. -40F seems to be the lowest temperature that cell fluids can supercool to before freeze damage will occur. The northern most range of Vitis riparia is correlated with -40F as the minimum winter temperature. The ability of the grapevine to supercool is favored by slow temperature reductions versus quick drops in temperature. With rapid drops, water can't be pumped out of the cells fast enough and the cells can be ruptured by water freezing from within.
The second process occuring at this time is a shift in the products of metabolism. The vine begins to manufacture various cryoprotectants. A shift to the metabolism of different sugars is chief among these. This shift entails the production of more mono-saccharide (glucose and fructose) sugars than the di-saccharides (sucrose) that the vine has been producing through the growing season. The vine also metabolizes a sugar know as raffinose which has been shown to have cryoprotectant properties. Raffinose is thought to be the sugar responsible for the freezing point depression exhibited in supercooling. There is a direct correlation with the amount of raffinose production and the time of the coldest temperatures of the winter. The hardiest of species and varieties are extremely efficient in the metabolism of raffinose.
Sugars aren't the only cryoprotectants formed during the acclimation period. Late embryogenesis accumulating proteins and other de-hydrins are produced within the vine's cells. Theses proteins act as protectants of the cell's membranes. They have been shown to increase the membrane's permeability and stability. This is important as the cell pumps water out of the cell and into the intercellular spaces and the cell dries out. Often, the ultimate death of the cell is not through freezing rupture but by dessication. These proteins aid in making cells more dessication tolerant. Cell membranes themselves undergo reconfiguration to withstand the drying out and to rehydrate properly without rupturing.
It should also be pointed out that the morphology of the buds is such that it allows water to be safely frozen in certain tissues while allowing the primary tissues to supercool properly. All of these processes are genetically controlled and are thus species and variety dependent.
Having survived the coldest part of the winter, the vine is not out of the woods yet. Freezing injury can still occur before growth begins in the spring. Just as the vine an acclimate to the cold temperatures, it can also deacclimate in response to warmer temperatures. The notorious January and February "thaws" just speed up the deacclimation process and make the vine susceptible to freezing injury as the temperatures drop again. Deacclimation takes place quickly and undoes the benefits that cold acclimation imparted to the vine.
During the cold acclimation period, the vine is in what is referred to a endo-dormancy. During this phase, the vine cannot begin growth no matter what environmental conditions it is put into. But as winter progresses, dormancy shifts and the vine begins to prepare for spring. Deacclimation and the shift in the dormancy leave the vine in a precarious position. Temperature fluctuations during the late winter can often cause more freezing injury that the coldest periods of the winter. I have taken cane samples that had survived -27F episodes that were totally alive, yet canes in the spring from the same vines were totally dead after passing through wild swings in temperatures in February and March when they were becoming less dormant.
Survival at this time is also variety dependent. Some species and varieties retain their endodormancy longer. Vitis amurensis hybrids are notorious for losing their endodormancy with the first onslaught of warm weather. Some varieties have been shown to reacclimate to the cold better than other varieties. This allows them to be able to survive future cold episodes with some protection and the acclimation is never to the cold temperatures that it attained in mid-winter. But the drop to the colder temperatures must occur slowly for this reacclimation to proceed. Sudden temperature drops may cause freeze injury regardless of the degree of acclimation.
Fluctuating temperatures continue to be a problem up to and past bud break. But once growth begins in the spring, then the problem becomes one of frost and frost tolerance.
See More on Frost Tolerance
There are things that can be done to insure that your vines enter the winter in the best of form. Cultural practices that favor vine health and fruit quality aid also in imparting the highest degree of winter hardiness. Most of these are just commonsense. Here are my suggestions to the grower for the cultural practices that will get your vines into the winter in the best of shape:
Varieties: I've already talked about varieties but there are some commonsense thoughts that need to be addressed when deciding on which grape varieties to plant for short growing season, cool winter areas. Choose the earliest maturing, winterhardy varieties. I stress the early maturing because the vines will have time after harvest to properly mature the wood for next year and to get through the winter. Harvesting just prior to the killing frost will leave the vine in poor shape to survive a cold winter.
I have included in my varietal trial some marginal grape varieties. These are my indicator plants for judging the quality of the growing season and the harshness of the winter. Compare GR7, which was totally hardened off by the first of September this year, with a variety like Landot 4511 that obtained only 50% hardening by the first of October. The GR7 vines have a much better chance of winter survival as they are properly prepared to endure the cold where the Landot 4511 vines will most likely die back quite far (up to 50%) as they will still be trying to harden off when the frost strips them of their leaves.
Many growers in this area are trying to grow Vitis vinifera varieties to obtain wines similar to Europe and California. It's my opinion that a grower is better off growing short season, winter hardy hybrids and obtaining sound quality wines than ending up with a mediocre vinifera wine due to poor ripening and problem growing conditions. I am into regionality when it comes to wines. This means growing those varieties that are totally adapted to the region.
Early varieties will easily reach 20-25Brix and still have time to properly ripen the canes for the winter season coming up. Contrast this to later varieties that struggle to reach 20Brix and have poor cane hardening.
Vine Health: Leaves and shoots should be in the best of health. This means good nutrition, disease prevention, and insect control. Fertilization should be performed based upon soil tests or petiole analysis. Any fertilizer should be applied early in the spring. Late applications run the risk of rampant growth into the period where the vine should be hardening off. The cane wood will be weak. Winter survival will be jeopardized.
A regular spray schedule should be followed based upon the diseases known to do damage in your area. Routine inspection for insects should be followed by control measures specific to the insects found doing damage. Some insects make yearly appearances at known times so that the grower can take appropriate measures before the insect's appearance and damage.
Irrigation and Water Management: If you live in a dry climate as I do, irrigation is a necessity but also one of the best tools to promote winter hardiness. I water the vineyard through a drip irrigation system until late August-September. As I begin seeing signs of cane hardening, I water less and less, thus promoting the rapid maturation of the shoots. I completely stop watering by Labor Day. This insures my vines to be in the best condition for winter survival.
Control of Fruiting: The grower must take measures to keep the vines in maximum health. Many varieties will over-produce and weaken themselves for winter survival and for future vine health. I advocate cluster thinning to balance vigor with productivity. Cluster thinning to one or two clusters per shoot has the added advantage of advancing the ripening of the fruit. Thus with an earlier harvest there is more time for the vine to harden off for winter. In addition to cluster thinning, I perform basal leaf removal around the clusters of fruit to allow maximum sunlight penetration to the berries. This aids in fruit maturation and quality of the fruit.
Frost Control: Frost control is important to winter hardiness. The longer a vine retains its leaves, the greater the cane maturation and hardening. The grower must take frost into consideration when selecting the vineyard site. Proper air flow through the vineyard helps prevent frost. Trellis systems also affect frost prevention. Most frosts kill from the ground up. Therefore, higher trellises will hold the shoots and leaves above the cold temperatures to some extent. I am in the process of researching trellis systems and their effects on frost and winter hardiness. At this time, I don't have enough data to draw real conclusions. But I will be addressing this issue in the near future.
The Rist Canyon Vineyard is proof that grapes can be grown under very short growing seasons and cold winters with success. Commercial viability of such a vineyard is still questionable over the long term. But as a research vineyard, cultural practices and variety selection can be assessed that will aid in the proper fruit ripening and vine maturation to insure maximum winter survival. I still have much to learn. Over the years I hope to continue finding methods that will get vines through the winter in top shape. Any thoughts on research directions I should pursuit would be appreciated. I hope this page has been helpful to those sharing my goals of growing grapes and aid them in establishing their own vineyards.
Ft. Collins Temp
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