Bud Break Could Come Early For Northeast And Midwest Grapevines

After two consecutive freezing winters, the mild winter of 2015-2016 was a welcome change. But, the unseasonably warm temperatures could potentially pose some risks to grapevines in the Northeast and Midwest.

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Hans Walter-Peterson, Cornell University Viticulture Extension Specialist, says chilling hours (the number of hours vines spend in dormancy in a certain temperature range) have been on his radar more in the last few years.

“November and December around here were much warmer than normal,” he says. “We spent a lot more time in that temperature range where the vines are counting these chilling requirement hours.”

Most varieties reached their chilling requirements before Christmas, which is very early. The problem is that once those chilling hours are fulfilled, the vines are primed to break bud once they feel the temperatures warming up. If there’s a string of warm days before the threat of frost has passed and bud break occurs early, a spring frost could be potentially devastating.

“So that’s the concern we have,” Walter-Peterson says. “How warm does it need to get, how long does it need to be that warm? We don’t know, but it seems that we’re at an increased risk for an early bud break, given those warm temperatures we had earlier this winter.”

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It might seem that vines would reach their chilling requirements sooner in a particularly cold year, but that’s not the case. The temperature where the vines actually count chilling hours is above 0°C. Last year’s extremely cold temperatures meant grapevines didn’t fulfill their chilling requirements until much later in the season.

“By the time the vines reached that chilling need, we were further into the later winter/early spring, and therefore we had less chance of an early bud break,” Walter-Peterson explains. “In fact, we ended up with a bud break that was just about close to average, if not a tiny bit later than usual. Most everything came out of dormancy last year pretty close together – we didn’t have a stretched out bud break.”

He adds that there’s no magic number or sweet spot when it comes to timing of chilling hour fulfillment. For Riesling, which needs about 700 to 800 chilling hours, the requirement is usually met in February or March in a normal year. This year, the requirement was met by mid-December.

“Even though in most years we will still hit that chilling requirement before we get that first warm up, achieving it earlier means that, theoretically, it takes less of a prolonged warm-up to get the vines to wake up, and the vines that have fewer chilling hour requirements will react faster to a warm-up than other ones.”

In other words, something like a ‘Cabernet Sauvignon,’ which is from a much warmer region, needs more chilling hours because it comes from an area where it spends more time in the chilling hour range during the winter. That means it’s one of the later bud-breaking varieties in the area. On the other hand, varieties like ‘Frontenac,’ ‘Marquette,’ and ‘Concord’ have relatively small chilling requirements and will react to warmer temperatures sooner and more rapidly.

Mitigating Risk
For varieties that are quick to break bud, such as some of the hybrids, Walter-Peterson suggests considering leaving some extra buds on the vine. This will offer protection if some of the buds are lost to a cold weather event.

But, if you do leave more buds and there isn’t a late frost, it’s important to go back out to the vineyard and thin some of the extra shoots. “If Mother Nature doesn’t do it for you, you’re going to have to do it yourself,” Walter-Peterson says.

Varieties Of The Future
As part of the VitisGen project, researchers are looking to hone in on some of the genetic markers that are responsible for influencing chilling requirement timing.

“They have made some progress in finding those markers, so I think in the not-too-distant future that breeders may actually be able to make crosses and look for individuals that will have a longer chilling requirement in order to take into account more extreme changes in temperatures during the wintertime,” Walter-Peterson says. “That could potentially delay bud break a little bit more and therefore give you a chance to avoid a major frost issue.”

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