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October 2015 WINES&VINES 75 freeze episodes has been isolated to single, non-consecutive years with vines having a chance to re- cover. 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 condi- tions 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 far- ther south, occasionally reaching southern Canada and the north- ern central plains, upper Midwest and northeastern regions of the United States. The winters of 2013-14 and 2014-15 in the Mid- west and eastern regions were significantly impacted by this phe- nomenon. Temperatures plunged into sub-zero territory (°F) and stayed there with the frequency, severity and extended duration of the cold episodes severely affect- ing 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 deter- mining how to manage the vines to salvage the growing season and minimize economic losses and, with an eye to the immediate fu- ture, how to maintain (if not in- crease) vine health and, hence, cold hardiness using the appropri- ate 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 culti- vars at 60%, and American culti- vars at 30%; even the super-hardy cultivars were extensively dam- aged (Dami, 2014). The Cornell extension team reported that av- erage 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-Peter- son, 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 ques- tion: 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 sea- son freeze episodes is complex at both the level of vine physiology and also very dependent on the timing of the stress and vine hardi- ness status when the episode oc- curs. In the winters of 2013-14 and 2014-15, the cultural strategy for coping with the extensive in- juries began with a choice to delay pruning as long as possible during the dormant period. The delay al- lowed an assessment of the extent of winter damage and then per- mitted adjustment of pruning strategies in relation to bud and vine injury and mortality levels. Most grapegrowers carefully KEY POINTS Two consecutive harsh winters damaged many vines in the upper Midwest. Cultivar choice and site selection are the most important factors for grapevines surviving low winter temperatures. With severely winter-damaged vines, the best option for growers is to leave five or six suckers and guide their growth to the trellis. "Spare parts" viticulture, graft union protection, seasonal quality assessment and crop adjustment can lead to vine balance, where the crop level will allow fruit to ripen. WINE EAST GRAPEGROWING