MSU Extension

A year ago when reports came out on severe winter kill to wine grape buds in northwest Michigan, everyone assumed that such unprecedented cold would not likely happen again. Unfortunately, the assumption was wrong. The 2014-15 winter in the Midwest and Eastern United States was once again impacted by the polar vortex – extreme cold temperatures that normally float around the poles year-round, but “thanks” to climate change these are unfortunately propagated farther, transporting warmer air to the North Pole and polar air into lower latitudes, directly into our vineyards.

Many sites in Michigan’s Grand Traverse region were hit with numerous evenings of extremely cold temperatures in February and March. The Michigan State University Enviro-weather system recorded temperatures at -15 degrees Fahrenheit or lower on six dates from Feb. 15 through March 5 on Old Mission Peninsula in Grand Traverse County, with -19.9 F as the coldest reading on Feb. 20. East Leland, Michigan in Leelanau County also recorded six nights below -15 F, with four of those nights below -20 F. The Northwest Michigan Horticultural Research Station (NWMHRS) in Leelanau County only reported -10 F or lower four times, with the lowest temperature at -16 F on Feb. 20.

The minimum low temperatures at East Leland, Michigan and the NWMHRS were even lower than temperatures experienced in early 2014. To make matters worse, there was far less snow cover during this year’s cold events, so more of the wood and buds of the vines were fully exposed to the cold.

This year, the strategy for coping with the extensive winter injury should start with delaying pruning as long as possible during the dormant period. The delay should be used to assess the extent of winter injury and then adjusting the pruning strategies (and spring-summer vineyard practices) in relation to bud and vine damage and mortality levels. Therefore, before pruning, grape growers should carefully evaluate each cultivar for bud damage.

Thirty-one varieties of wine grapes in the NWMHRS experimental vineyard were sampled between March 16 and 31 by collecting a minimum of 10 1-year-old canes per variety. Buds and the vascular tissues of each cane were carefully sectioned with a razor blade and examined under magnification to look for indications of cold injury. The results were as expected, with Vitis vinifera varieties showing high mortality to buds and many instances of injury to the canes as well. The cane injury was localized, occurring just above where the winter snow cover had protected the wood; the tissues several inches above this zone looked much healthier.

Until further into spring and the beginning of bud swell, it will be difficult to tell if the injured region of the canes will be able to recover enough to support the growth of shoots higher in the vines. Shoots collapsing during the growing season up to veraison are symptomatic of trunks and canes damage.

Looking just at the condition of the buds, Riesling showed a 61 percent primary bud survival rate and Chardonnay at 66 percent. Each had almost 80 percent of their secondary buds looking alive. Pinot Noir and Cabernet Franc both had about 30 percent live primary buds. These numbers may not mean much if the injured vascular tissues in the canes does turn out to be a problem. If so, all of the more distal buds or shoots may eventually die, and the effective bud survival rates would be very low. Reports of more severe levels of bud injury have been received from commercial vineyard sites that recorded much lower temperatures than the research center.

The hybrid wine grape varieties fared much better, but there were still significant levels of injury to buds. The best primary bud survival rates were found in St. Croix (93 percent) and Frontenac (92 percent). The worst injury occurred to Aromella (59 percent live primary buds) and Chambourcin (43 percent live primary buds). At this point in time, our estimates of bud injury may not be good indications of the potential yield of the varieties during the growing season ahead; a level of 70 percent primary bud damage does not necessarily correspond to a 70 percent reduction in yield due to the potential for fruit production by secondary buds and the possibility there could be a compensatory increase in the weight of clusters.

The 2014 and 2015 winters are a forceful reminder that cultivar choice and site selection are still the most important tools we have against low winter temperatures. While we need to prune vines to mitigate damage as best as possible for the 2015 growing season, we should also keep in mind the 2016 season and crop, and the effect our choices during pruning and training will have on it. Several viticultural strategies can mitigate the impact of winter cold, such as:

• The use of multiple trunks, sometime defined as “spare-parts viticulture.”
• Always having replacement canes (suckers) growing under the vine, which is fundamentally important for grafted Vinifera cultivars.
• Covering the graft union with soil during the winter. Covering, and uncovering, vines every year is very labor intensive, but the most efficient technique to guarantee vines and fruiting canes survival for the following season.

In 2012, the USDA released a new plant hardiness zone map because of the need to be in line with the temperature increase around the United States. We are using the extra heat and the longer growing season to ripen cultivars that were impossible to grow only few decades ago in cool and cold climate viticulture. Unfortunately, this warming trend is also producing extreme winter cold temperatures (polar vortex), and going back to the basic methods of grapevine protection during the winter is our best option to keep a sustainable grape industry in the Midwestern and Eastern United States.

A complete listing of the varieties that were examined and the bud survival percentages will be released in an upcoming Northern Michigan FruitNet report, or by request if you send an email to Michigan State University Extension small fruit educator Erwin Elsner at elsner@msu.edu.

The MSU Enviro-weather system can be used to check the low temperatures at dozens of locations in Michigan. See www.enviroweather.msu.edu for more information.