Insulation of vinifera vines with soil (or mulch) by hilling up above the graft union has provided the best protection from cold damage (especially to the graft union area and those viable scion buds above the union).

The sight of a normally beautiful and bountiful vineyard in any spring following a winter’s devastation is depressing, with vine after vine and row upon row showing outright death or loss of aboveground vine structures right down to the soil or snowline. Beyond the eye’s immediate aesthetic judgment, industry insiders and those with the knowledge may quickly leap to imagine numbers of dead buds, percent of cane mortality or losses in the season’s projected production volume.

That was certainly the case in spring 2014 in the upper Midwest, where frigid arctic air masses ravaged the region’s sensitive vinifera vines. As field reports trickled in, it became clear that the year’s yield would be much reduced—perhaps lost entirely. The resulting high vine mortality highlighted the necessity of either: 1) replanting and enduring the requisite three-year wait to return to full production, or 2) retraining vines with sucker shoots from the base of the trunk (from scion, above rootstock), with potentially only a single year’s crop loss.

This has happened before, and the region’s growers and winemakers have learned how to adjust and move on, absorbing the losses and the lessons while remaining generally optimistic. However, what was most unexpected and tragic this time was the repetition of that devastating experience the very next year, in spring 2015, an unprecedented event that has rightfully resulted in thoughtful evaluation of the future of fine wine production from regionally grown grapevines—a question of serious consequence.

Grapevines in nature
In nature, the survival of a plant species depends on maintaining its genetic diversity through cross-pollination, so that changes in its native environment can be readily accommodated. A similar rule of adaptation holds true for the culture of any economic crop. For these, there are two key components that influence growth and productivity: a) the genetic constitution of the plant, and b) the ability of those genes to be expressed not only in the environment where cultivated, but also under the managed conditions of culture.

The situation is true for all plants regardless of life cycle, but it is especially important when culturing perennial plants with goals of plant survival over the dormant period, the production of both an economically acceptable yield and economically acceptable crop product quality. Further, they must possess the capacity to do this consistently over multiple years, since the perennial crops commonly require a significant investment in years of initial growth when no economic yield is produced. Grapevines epitomize this financial and agricultural challenge.

What are the environmental limits for the profitable production of grapes? There are several, but in the Great Lakes and the northeast cool/cold-climate viticultural regions of the United States and Canada, the most limiting are the terrible and seemingly unpredictable winter freeze episodes, which can push vines beyond their threshold of survival. Unfortunately, this devastating prospect is an annual threat even in the face of a warming climate trend. The recent rise in acreage planted to grapes, fueled by expanding interest by states in the economic value of wine tourism, makes the losses no laughing matter.

Further, the introduction of “super-hardy” resistant cultivars from breeding efforts in Minnesota and Wisconsin has increased the potential for commercial viticulture in regions where it was previously thought unacceptable, thereby expanding the boundaries for both gain and loss. However, for most of the relevant growing regions, damage from winter freeze episodes has been isolated to single, non-consecutive years with vines having a chance to recover. The prospect of “what could happen if…” such an event were to occur with greater frequency has largely been the stuff of casual conversation—until now.

The polar vortex and the winters of 2013-14 and 2014-15
The unique set of weather conditions that created these damaging events has been termed the polar vortex. This is best understood as a large pocket of very cold air, typically the coldest air in the Northern Hemisphere, which sits over the polar region during the winter season. The frigid air can find its way into the United States when the air mass is pushed farther south, occasionally reaching southern Canada and the northern central plains, upper Midwest and northeastern regions of the United States. The winters of 2013-14 and 2014-15 in the Midwest and eastern regions were significantly impacted by this phenomenon. Temperatures plunged into sub-zero territory (°F) and stayed there with the frequency, severity and extended duration of the cold episodes severely affecting grapevines.

Many cultivars that normally survive our winters with minimal injury were heavily damaged by the extreme cold. The extent of the damage depended mainly on cultivar and location. The two consecutive winters affected vines with damage expressed as varying degrees of bud, trunk and vine death. This range of damage, higher and more destructive than typically seen, left growers with the immediate challenge of determining how to manage the vines to salvage the growing season and minimize economic losses and, with an eye to the immediate future, how to maintain (if not increase) vine health and, hence, cold hardiness using the appropriate and best viticultural practices.

Reports of the extent of winter injury and damage in vineyards across the Midwest and the East showed that Ohio had almost 98% bud mortality to vinifera grapes, with interspecific resistant cultivars at 60%, and American cultivars at 30%; even the super-hardy cultivars were extensively damaged (Dami, 2014). The Cornell extension team reported that average vinifera bud damage in New York was 67%. However, the level of damage varied depending on vineyard location; for example, in Pinot Noir, bud mortality ranged from 18% to 100% (Walter-Peterson, 2014). At the time of this writing, precise data about vine mortality does not yet exist, but it is believed to be very high.

As we sit today, the fallout of the tragedy poses the critical question: Were the experiences of the past two years simply bad luck, or are they portents of a new climate reality requiring new tactics and decisions for profitable viticulture in the affected regions?

Assessing and adjusting to vine damage
Damage to vine organs as a result of one or a series of dormant season freeze episodes is complex at both the level of vine physiology and also very dependent on the timing of the stress and vine hardiness status when the episode occurs. In the winters of 2013-14 and 2014-15, the cultural strategy for coping with the extensive injuries began with a choice to delay pruning as long as possible during the dormant period. The delay allowed an assessment of the extent of winter damage and then permitted adjustment of pruning strategies in relation to bud and vine injury and mortality levels.

Most grapegrowers carefully evaluated each cultivar for bud damage. At each node there is a compound bud; a complex of three shoot primordia. The primary bud is the largest meristematic tissue of the three and is in the middle of the compound bud. The secondary bud is located toward the base of the cane, between the primary bud and the petiole scar, while the tertiary bud (generally not fruitful) is apical to the primary bud. For most of the cultivars grown in the eastern United States and Midwest, the primary bud possesses 70% to 75% of the cropping potential of the compound bud (primary + secondary + tertiary). Knowing this simple bud morphology and making cross-sectional cuts through the bud with a sharp razor blade allows growers to identify the survival status of each bud at the node. If the buds are alive and healthy, they should be green; a brown or black color, unfortunately, is an indication of mortality.

Grower response to this circumstance is dependent on knowing the number of buds healthy enough to offer potential for the season’s growth and production. The number of nodes retained per vine at dormant pruning depends on an assessment of the amount of bud damage sustained after the dormant season nears completion.

This assessment is simple. Collect a sample of 40-50 node positions on canes of desired quality from positions appropriate to be retained upon final pruning. If all buds are dead (dark in color), the chances of finding living buds is very unlikely, and there is no need to continue the assessment. Alternatively, highly variable results (alive and damaged buds in samples coming from the same cultivar and the same vineyard location) suggest that it’s advisable to enlarge your sample to 100 nodes to get a more accurate idea of the level of damage. If the vineyards are not uniform (different vine size, slope, soil), then it is better to keep samples from vines/areas separated to evaluate the potential impact of those variables.

Secondary bud mortality is usually similar to primary bud mortality, yet some indication of the amount of secondary bud damage is important, especially when more than 60% of primary buds are dead. When the assessment of bud damage is complete, the next step is to adjust the dormant season pruning strategy accordingly. General suggestions are provided in the table above. When primary bud mortality is greater than 70%, the pruning effort should be directed to reestablish the fruit-bearing zone of the vines and to balance the growth of the vines during the spring and summer to avoid excessive shoot vigor, which can lead to vine weakness and higher potential for winter injury.

Damage to vines and recovery
The past two winters resulted in vines that were severely damaged. Growers used every remaining living bud to cope with the damage, including retaining base buds close to pruning cuts that have the potential to produce shoots with needed leaf area. This physiological phenomenon dilutes the individual shoot vigor available from the large root systems of mature, bearing grapevines. Mature, established vines have large root systems containing large reserves of carbohydrates to fuel growth. Retaining multiple shoots creates a dilution effect that helps avoid excessive shoot vigor (e.g., bull canes), which are much less cold hardy. Excessively vigorous shoots tend to have long internodes with larger diameter, lighter periderm color, poor lignification and larger cell size that make them more susceptible to future winter damage. Field assessment of the two consecutive harsh winters indicated that poor choices of canes at pruning time after the first year’s damage (winter and spring 2013-14) led to greater levels of damage in winter and spring 2014-15.

The extreme cold during the past two winters killed cordons and trunks of several cultivars. Damaged cambium tissues were brown to black color. Thus, when damage to permanent cordons and/or trunks was present, the pruning effort was directed at reestablishing vine structure. Renewal canes from the base of the trunks were the best options for replacing damaged bearing structure—especially for Vitis vinifera cultivars. Severely injured but surviving trunks were generally replaced. In cold-climate locations, damaged trunks have proven to be highly sensitive to crown gall disease. The diseased portions need to be removed and replaced with new shoots of moderate vigor.

Severely winter-damaged vines will often have shoots (suckers) arising from basal trunk positions protected by snow cover. Sometimes, due to extensive damage, the suckers are the only resource for leaf area for the vines in the spring. Several extension bulletins, supported by the anecdotal experience of growers in cool- or cold-climate growing regions, suggest that the suckers should not be removed or even thinned to one or two per vine (the latter a common standard).

The better option is to leave at least five or six suckers and to try to guide their growth to the trellis system. Other suckers, if present, are allowed to grow without training on the ground to bolster photosynthetic leaf area, resulting in additional carbon fixation during the summer. These additions help limit the production of excessively vigorous growth on the potential trunk replacement canes, enhancing their survivability. In summary, overly vigorous growth during vine recovery from freeze damage is undesirable and leads to increased temperature sensitivity and potential for future freeze damage to perennial vine structures.

Viticultural practices enhancing protection for the future
Covering and uncovering the graft union of the vines each year are certainly labor-intensive processes, yet they remain the most efficient and effective techniques to guarantee the survival of vines and fruiting canes for the following season. Soil is an excellent insulator. Cold winter temperatures only a few inches below the soil surface are rarely damaging, and they generally remain around the freezing point—even with much colder air temperatures just above the soil surface. Commercial growers employ a grape hoe to mound the soil in the fall and remove the soil in the spring, before the vines resume their active growth.

The timing of soil removal, of course, is critical to prevent root growth from the scion cultivar instead of the rootstock cultivar. Unions on grafted vines should be covered with a few inches of soil for the winter months, thereby protecting scion buds close to the graft union that could be very important in the case of severe winters damaging or killing the buds above the mounded soil.

In 2012, the USDA released a new plant hardiness zone map in response to the observed temperature increase around the United States. Growers in the regions discussed here are using the extra heat (growing degree-days) and the longer growing season to ripen cultivars that were impossible to grow only a few decades ago in cool- and cold-climate viticulture. Unfortunately, this warming trend is also generating winter temperature volatility, especially in the extreme lows, and it requires reconsideration of basic methods of grapevine protection during the winter to maintain economically sustainable viticulture. The low winter temperatures in our region are something we cannot avoid. The frequency probably will change, but not at the intensity demonstrated in the graph, which shows multiple killing temperatures in every decade since 1950. The impact of cold temperatures is greater in the 1990s, due to increased planting of cold-tender vinifera cultivars.

When reports in the winter-spring of 2013-14 quantified severe winter kill to wine grape buds, few thought that such unprecedented cold would happen again soon. Unfortunately, the assumption was wrong. Many sites in Michigan’s viticultural regions suffered multiple evenings of severe low temperatures in February and March 2015. The Michigan State University Enviro-weather system recorded temperatures at -15° F or lower on six dates from Feb. 15 through March 5 at several locations in Michigan, with -20° F on Feb. 20 recorded as the coldest reading.

Another climate condition made this even more problematic in 2015: There was much less snow cover, decreasing the natural insulation layer typically offered by a normal season’s snowfall. As a result, more of the perennial wood and buds were exposed to full effects of the severe cold episodes in 2015 compared to the year prior, and the results were truly devastating.

Conclusion
The 2014 and 2015 winters have been a forceful reminder that cultivar choice and site selection are still the most important tools we have against potential damage and loss from low winter temperature exposure. Once those decisions are made and vines are planted, we need to be smart about our viticultural practices and adjust when Mother Nature (or your bottom line) demands it. While we need to prune vines to mitigate damage as best as possible, we should also keep in mind the following season and its crop as well as the effect our choices during pruning and re-training will have on it.

Beyond our earlier discussion of the merits of spare-parts viticulture and graft-union protection, there are several additional viticultural strategies that can mitigate the impact of winter cold, particularly seasonal quality assessment, crop estimation and crop adjustment as required. All are focused on achieving vine balance, where the goal is to produce a crop at the level that will allow desired fruit ripening to occur while reducing neither the next season’s cropping potential nor the cold hardiness of the vine. When effectively applied, this approach can also contribute significantly to the diminution of crop losses due to spring freeze and the resulting conditions of poor fruit set.

Grapevine balance has been reviewed at length previously (Gladstones, 1992; Howell, 2001) and begins with the contribution of L. Ravaz (1911), who calculated a ratio of vine yield to the weight of annual cane prunings to give us the Ravaz Index. The efforts of Newton L. Partridge in Michigan (1931) and Nelson Shaulis in New York (Kimball and Shaulis, 1958) provided a basis to make the Ravaz Index proactive by using the weight of cane prunings in year one as an indicator of the ability of a vine to ripen a crop in year two. This was accomplished by allocating a specific number of nodes per vine at pruning based on winter pruning weight, and was called “balanced pruning.” Richard Smart (1985, 1991) has demonstrated that the ratio of the exposed leaf area to the weight of the vine’s crop was a more accurate way of expressing balance, and this has been shown frequently.

Several published reports support the idea that excess cropping in the climate of culture for the cultivar reduced the cold hardiness of primary buds both during the late autumn (after leaf loss) and early spring (three weeks prior to bud break), and in general the negative impact on fruit ripening is less than on either vine size or hardiness. Achieving vine balance allows vines to fully express the complex system of genetic controls of cold hardiness at the metabolic, cellular, tissue and morphological levels that combine to yield the characteristic resistance to freezing stresses for that cultivar. It is our goal as viticulturists to minimize limits to this full genetic expression. Though logical, this speculation must be subjected to rigorous critical evaluation because the potential for producing useful grapevine culture information is considerable. 

Paolo Sabbatini is an associate professor of viticulture and extension specialist in the Department of Horticulture at Michigan State University in East Lansing.
G. Stanley Howell is professor of viticulture emeritus in the Department of Horticulture at MSU.