Grapes need to be cold hardy to turn out fruitful

That sound on the roof? It’s not reindeer, it’s the other shoe dropping.

The lingering late-fall weather might be great for travelers, golfers and shoppers, but it is just another reason to worry for grape farmers.

They know it is only a matter of time before the really cold weather arrives.

This winter has been quite different from 2009, when early December temperatures plummeted, in some cases to minus 20 and lower, killing up to 75 percent of the grapevines in the western half of the valley.

The cold came suddenly, catching many vineyards not yet cold hardy, and growers and winemakers saw their hopes die for 2010.

This year, except for a short-lived drop to near zero at Thanksgiving, temperatures this late fall and early winter across the Grand Valley have been unseasonably warm.

That keeps grape growers awake at night, wondering if the vines are going to be cold hardy enough once the really cold temperatures arrive.

Speaking recently with Horst Caspari, state viticulturist at Colorado State University’s Orchard Mesa Research Center, I discovered cold weather can be complicated.

The following is what I took from the conversation, so if it’s unclear, blame me and not Caspari.

Most wine lovers already know some grape varietals are genetically more cold hardy than others, which is why Germany grows great Riesling while the temperate Napa Valley excels in cabernet sauvignon.

All grapes, however, need gradually colder weather to prepare for winter (becoming cold hardy) as well as a certain amount of cold weather (chill units) to become fruitful the next spring.

Cold weather triggers acclimation, Caspari said, while warm temperatures, such as a midwinter thaw, cause plants to lose cold hardiness.

Caspari regularly checks cold hardiness in grapes at the Orchard Mesa station and grape growers around the region use his research as insight to their own vines.

Last week, the station’s Syrah and chardonnay vines were “5 degrees more susceptible than they are normally at this time of year,” Caspari said.

This means the mild weather hasn’t allowed the grapes to adjust and they are susceptible to cold damage at temperatures 5 degrees warmer than is normal.

Five degrees doesn’t sound like much, but a table Caspari prepares on cold and damage shows damage to chardonnay goes from none at zero degrees to 35 percent at minus 5 degrees and 100 percent loss at minus 10 degrees.

With temperatures in the valley forecast to reach close to zero this weekend, survival depends on how cold hardy the grapes are.

“This time of year, they wouldn’t be able to handle” temperatures to 5 below, Caspari warned last week.

Cold hardiness is influenced by many different factors, including variety, localized weather conditions and the duration of a cold event.

It’s also dependent on recent temperatures, which is why gradual declines, even to sub-zero, don’t do as much damage as quick drops.

Research indicates it is the high temperatures on the three days prior to the cold blast that determines cold hardiness.

There’s not much wiggle room, either, Caspari said.

“At Thanksgiving some parts of the valley got close to zero degrees while we had 7 degrees here,” he said. “I heard some vineyards had zero and that was the number they could handle.”

It’s a similar story in the spring, when warming temperatures bring plants out of dormancy.

This is a crude summation, but all plants need a certain number of chill units, roughly speaking as hours between 40 and 45 degrees, to be most fruitful the next spring.

Once those chill unit demands are met, the plant is ready to break dormancy.

That’s why a February warming, after the plants attain the needed chill units, can be disastrous.

“During a thaw the plants can lose cold hardiness, which can make it very interesting in spring because then it doesn’t have to get that cold” to cause damage, Caspari said.

After an early thaw last spring, a late frost dropped temperatures to around 20 near Cedaredge and killed grapes there.

The damage to grapevines didn’t disappear with the heat of summer.

September brought uneven ripening and grape buds five months or more behind.

“I had plants here breaking bud in September due to the injury from past December’s cold,” Caspari said. “It took a long time for the injuries to heal.”

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