Trunk renewal essentials pruning approaches preserve cold-damaged vineyards

Dealing with extreme low temperatures can keep winegrape growers on edge, but some preliminary research in Michigan pinpoints potential methods of preserving a crop in the current freeze event year and sustaining it into future seasons.

Mike de Schaaf, farm manager at Michigan State University’s (MSU) Southwest Michigan Research & Extension Center in Benton Harbor, Michigan, shared his research into pruning methods to adjust for winter damage during his presentation at the 35th annual Michigan Viticulture Field Day.

“This is our pruning strategy for post-freeze or frost damage in the vineyard,” he told attendees. “We wanted to take a look at how we could get more regenerative growth down low at the graft union, especially for vines that haven’t been trunked or renewed in quite a while.”

The effort is in response to a changing climate. “It wasn’t necessarily the cold temperature that got us, it was the quick drop in temperature because we weren’t cold enough for the cold,” de Schaaf said.

Mike Reinke, MSU Extension viticulture specialist, said the cold snap occurred in January.

Chilling hours challenge

“Until Jan. 14, we were having the warmest winter on record,” Reinke said. “We had no point here at the station that got below 22º F. That was the case for the six weeks before that point. Then, within two days, we went from 22º F to -8º F.”

Typically, at -8º F, if the vineyards had enough chilling hours prior, de Schaaf explained that primary damage will be seen, “but we’ll have plenty of secondaries out there. We’ll have a crop.”

He shared that this year, when they were cutting buds, observations weren’t good.

“It’s all site-specific, but some of the Chardonnay did not fare well through the freeze. Cabernet Franc did not do well. Riesling, on the other hand, came through with flying colors,” de Schaaf said.

In several panels where pruning was done differently, shoot counts varied. Interestingly, panels with the least amount of pruning were the ones that pushed the most new shoots below, de Schaaf said. He anticipated most new shoots would have come from the vines that were most severely pruned, but that wasn’t the case.

“In Chardonnay, there were some differences, but in Riesling, it didn’t really matter in the shoots that were most severely pruned,” de Schaaf. “It pushed the same number of new shoots at the graft union no matter how we pruned it.”

He found that vines that weren’t typically producing new shoots from the base. Now, new shoots emerged at the base after a frost.

While it is possible to crop after a frost, de Schaaf cautioned growers that those cold temperatures damaged the trunks’ vascular systems, leading to degradation and the need for replacement.

De Schaaf said there should be spurs at the bottom, at the graft union where “you can bring them up the trunks and keep healthy trunks growing. It helps minimize trunk disease.”

Growers must renew the material, particularly if temperatures continue to drop, and be ready to install new material and be able to crop it, De Schaaf said. If a grower leaves long spurs, the grower can crop close to a normal tonnage in some varieties, depending on the year, without having to bury canes, which can be expensive, de Schaaf said.

“This is a new way of dealing with your winter stuff. If it’s a different kind of freeze, where it gets to -20º F, all bets are off,” he said. “In a year like this year, we have a normal crop (in this panel). You could run a machine harvester over it, you can handpick it. It’s right in the areas you would normally be finding your fruit. In other areas here, the crop is all over the place. It’s at the top of the trellis. It becomes too time consuming to try to leaf pull.”

De Schaaf wanted to review the numbers of shoots that grew at the base and the number of fruit, adding those in combination to see if a grower can get a crop and if the grower has the ability to renew those trunks. Growers need to produce more live tissue below where it’s needed because growers can’t know what the climate will be like in any given year.

As growers face more climate uncertainty, less expensive ways of maintaining a crop and renewing trunks are needed, he said.

Written by Gary Pullano

Gary Pullano is a Michigan-based journalist with more than 45 years of industry experience. As a semi-retired former managing editor for Great American Media Services, Pullano has covered the specialty agriculture sector for the past decade. He can be reached at garypullano@gmail.com.