March 2016 Issue of Wines & Vines

Cold Hardiness and Dormancy

More challenges for grapevines in eastern growing regions

by Linda Jones McKee
Dormant Vines
Snow drifts cover the graft union of these dormant vines during the 2014-15 winter in the Finger Lakes, N.Y.

After two bitterly cold winters, almost everyone on the East Coast was pleased to have warm, almost spring-like temperatures for the 2015 holiday season and even the first few days of 2016. However, after several days of temperatures rising into the 60ºs and even 70ºs (F) in late December, grapegrowers began to get concerned. When temperatures dropped into the single digits for several nights in early January, that concern turned into anxiety.

Some growers with long memories were reminded of the “Christmas Massacre” of 1980. That year was characterized by a warm November in New York state, with more normal highs and lows in December. On Christmas Eve, the temperature rose into the mid-30ºs and then plummeted to approximately -20° F in the Finger Lakes and -24° F in the Hudson River Valley. The drop of more than 50º, through the freezing zone to such extremely cold levels in less than 24 hours, was seen as a major calamity for the grape and wine industry. As it turned out, the Christmas Massacre did cause damage, worse at some sites and for some varieties than others, but it did not completely decimate the entire grape industry in New York.

The temperature change from December to early January this year was even greater than the drop in 1980, but the major difference was that the warm temperatures were higher during the daytime, and the low temperatures were not below zero. For example, the high in Richmond, Va., on Christmas Day 2015 was 75° F; on Jan. 5 the low in Winchester, Va., was 7.5° F, a change of almost 70° F. In Ithaca, N.Y., the weather was a balmy 64° F on Christmas Eve, with a low of 35° F; by Jan. 4, the high temperature was 10° F with a low of 7° F.

Now the major question is: Had grapevines from Virginia to New York acclimated to cold temperatures?


  • This January, temperatures dropped from the 60ºs and 70ºs (F) to single digits across eastern grapegrowing regions.
  • Grapevines need sustained exposure to temperatures between 32º F and 45º F before they can begin to deacclimate.
  • When grapevine tissues have been exposed to the necessary chilling requirement, the endodormancy stage transitions to ecodormancy. In this stage, bud development is inhibited not by the vine itself but by environmental factors such as late-winter warm spells.

Cold hardiness and dormancy
In the article, “Questions from the Vineyard: Dormancy” in the December 2015 issue of Viticulture Notes, professor Tony Wolf and Tremain Hatch, viticulture extension research associate at Virginia Tech, said, “Vines have become reasonably cold hardy for this time of year.” They noted, “Although we in Virginia are not currently measuring grapevine cold hardiness, we’ve used a bud cold hardiness model developed at Washington State University that we have previously found to be fairly accurate in terms of predicting dormant bud cold hardiness at a given time of the fall, winter and early spring.”

After plugging in data about minimum daily temperatures in Winchester, Va., from Sept. 7 to Dec. 13, 2015, the duo predicted that Cabernet Sauvignon would be able to tolerate temperatures as low as 0° F. At the same time in 2014, Cabernet Sauvignon buds were estimated to be cold hardy to about -8° F.

“Other varieties would not necessarily respond to ambient temperatures in the manner that Cabernet Sauvignon is predicted to do,” they said. “Single-digit temperatures (or even temperatures much below freezing) have not been forecast for the remainder of 2015.”

Of course, temperatures did drop into the single digits in January, and some plants that weren’t watching the calendar in late December pushed a few flowers out. “Grapevines, fortunately, are a little smarter,” Wolf and Hatch commented. “Multiple mechanisms help grapevine tissues survive through the winter. For simplicity, let’s lump these mechanisms into two categories: cold hardiness and dormancy. Both of these are dynamic processes that are dependent on the grape tissue and the temperature stimulus to which the tissue is exposed. Experienced grapegrowers should be familiar with the U-shape of grapevine bud hardiness during the dormant period (see “Grapevine Bud Hardiness in the Northern Hemisphere”). Sharp vertical changes in the seasonal cold hardiness curve are due to the vine’s physiology related to cold hardiness, in which the vine’s tissues acclimate and deacclimate to cooling and warming temperatures.”

Wolf and Hatch continued, “Dormancy can be thought of as a protective feature that keeps grapevines from taking a miscue from warm periods in the fall. It can be separated into multiple stages including endodormancy and ecodormancy. Endodormancy is triggered by cooler temperatures and shorter days at the end of the growing season and is due to intrinsic features of the grapevine itself, which suppress metabolic activity in the face of unseasonably warm weather such as we experienced in November.”

They added that endodormancy transitions to ecodormancy after the chilling requirement of the grapevine tissue has been met. Grapevines require exposure to “chilling” temperatures (32° to 45° F) for a period of time before they can deacclimate and begin the bud growth of the next growing season. The period of time required varies somewhat by grape species and possibly among varieties. During ecodormancy, bud development is no longer inhibited by the vine itself but by environmental (or “eco”logical) factors. Bud development is blocked by cool temperatures (less than 50° F) but is encouraged by warm temperatures. The requirement for chilling hours of endodormancy greatly retards grapevines from starting bud growth during seasonal temperature variations in late fall and early winter, such as the warm November temperatures we often experience in the mid-Atlantic region.

Wolf and Hatch said, “For example, let’s say you were to bring a cane into room-temperature conditions in the fall following a frost that defoliated the vines. The cane would not break bud rapidly (it would probably take a month or more), and bud break would be erratic along the cane even though it was exposed to warm temperatures. The grapevine tissue inhibits bud development because it has not yet satisfied its chilling requirement; the cane is in endodormancy.

“However, if you brought a grapevine cane inside in January, it would break bud more rapidly, and bud burst would be consistent along the cane (unless there had been some cold injury to the buds). The cane collected in January is in ecodormancy, having already satisfied the chilling requirements of endodormancy. While in ecodormancy the buds deacclimate and grow more readily when exposed to warm temperatures. The specific requirements beyond chilling hours that grapevines need to transition from endodormancy to eco­dormancy are not clear.

Wolf and Hatch said grapevines have modest chilling-hour requirements compared to other perennial crops like apples, and warm temperatures in late winter and early spring could accelerate bud deacclimation and result in an advanced bud burst.

Christmas Massacre redux?
As noted above, while the temperature drop in 2015 was even greater than the one in 1980, there were several different factors involved. The decline this year took place not in 24 hours, but over more than a week, when the daytime highs gradually came down, and the nighttime lows dropped first into the 20°s, then the teens, and finally to single digits. And the ultimate low point of temperatures in both Virginia and New York—while cold—was still above 0° F.

Hatch went into the vineyard Jan. 8 to cut buds and check canes on both Merlot and Cabernet Sauvignon. He reported that “all the buds and canes were fine, with no apparent winter injury.” Wolf added that the Merlot vineyard had had a low temperature of 7.5° F on Jan. 5. So far, “No problem,” Wolf commented, but there is “lots of ‘winter’ left.”

Tim Martinson, senior viticulture extension associate in the department of horticulture at Cornell University, has also been tracking the cold-hardiness of grapevines. He told Wines & Vines, “We have been freezing buds from Geneva since early November. Basically, bud hardiness to date is tracking what we saw last year: Riesling LT50 (median bud freezing temperature) is pretty much equivalent to last year (-10° F); Concord is more responsive, and LT50 seems to be down to -18° F or so. This suggests the vines are not any less hardy than last year at this time. That’s good news.”

On the issue of chilling requirements, Martinson commented, “We’re ahead on ‘chilling hours,’ with enough to satisfy endodormancy, meaning that if temps rose well above freezing, time until bud burst would be the shortest possible, and responsive to ambient temperature alone. So this may leave vines somewhat more at risk for early bud burst. The caveat is that we never worried about chilling hours in the past, so I’m not sure this is a ‘real’ issue. Bottom line: I’m not worried about any lasting impact from a warm December.”

Explaining how grapevines acquire cold hardiness
In the February 2011 issue of Appellation Cornell, Martinson described how grapevines transition from a cold-tender to a cold-hardy state in a process known as cold acclimation and then deacclimate, slowly, which leads to bud burst. This process allows vines to survive low winter temperatures. Martinson wrote:

“It is a gradual process, which starts around veraíson in response to low temperatures and decreasing day length and continues after leaf fall when temperatures are below freezing. As temperatures rise after mid-winter, grapevine tissues deacclimate in a gradual process, culminating in bud burst and active growth at the start of the growing season. How fast this process happens, and to what extent vine tissues survive extreme winter low temperatures, depends upon the cultivar (its genetic makeup), seasonal temperatures and how they vary, and the vine’s condition as it enters the dormant season.

“During the growing season, green, actively growing vine tissue is composed mostly of water, which will form ice at freezing temperatures, expanding the cells and disrupting their integrity. In preparation for the dormant season, cells become resistant to lower temperature through two mechanisms: dehydration through movement of water to intercellular spaces and accumulation of sugars and protein complexes that bind water and serve as cryoprotectants. These cryoprotectants lower the freezing point of water and allow cell contents to supercool without forming damaging ice crystals. The acclimation process starts well before freezing temperatures occur, but buds continue to gain hardiness from the onset of freezing temperatures through the coldest part of midwinter.

“In autumn, green shoots turn brown from the base outward toward shoot tips as the cork cambium forms (a ring of cells outside the phloem), producing a layer of water-resistant cork cells called the periderm. As these cells are produced and die, they become impervious to water. Buds are only weakly connected to the vine’s vascular system, which isolates bud tissue and limits the potential for them to take up water. At leaf fall, buds are moderately cold-hardy and can survive temperatures ranging from 5° to 20° F.

“After the onset of below-freezing temperatures, buds continue to gain cold-hardiness through further desiccation and redistribution of water to the intercellular spaces. As ice forms outside of cells, differential vapor pressure draws water out of the cells and onto the surface of the ice crystals. This response is highly correlated with the vine’s exposure to low winter temperatures. For example, buds exposed to lower winter temperatures in New York have median lethal temperatures (LT50) 2° to 3° F lower than buds exposed to more moderate winter temperatures in Virginia.”

Losing cold hardiness
Martinson then describes what happens to the cold-hardy grapevines when the weather begins to warm up. “After attaining their maximum cold hardiness in midwinter, buds deacclimate in response to milder temperatures, and deacclimation is often more rapid than the acclimation process. Warmer temperatures increase ambient humidity, and vine tissues gradually gain water. As soils warm up, capillary action draws water up the trunk, and ‘sap flow’ occurs. By the time of bud swell, rehydrated bud tissue is vulnerable to freeze injury at only a few degrees below freezing.

“Each grapevine bud contains a primary, secondary and tertiary bud. The primary bud is most well developed and is typically less cold-hardy than secondary or tertiary buds. Freeze injury in response to low temperatures typically affects the primary bud first.

“We measure bud freezing temperatures by collecting canes from vineyards, cutting off buds and placing them on thermocouples in a controlled temperature freezer. As the temperature in the freezer gradually decreases, each bud will release a small amount of heat (called a low temperature exotherm) as it freezes, allowing a precise estimate of the lethal temperature for that bud. A collection of buds from a single vineyard will exhibit a range of bud freezing temperatures that vary from 2° to 6° F. The median freezing temperature from a collection of 30 buds, called the LT50, is a common measure of cold hardiness. Measurements of LT50 bud-freezing temperatures from leaf fall to bud burst reveal that bud hardiness undergoes constant change in response to weather conditions.

“The winter low temperatures that injure buds limit where a cultivar can be grown. Cold-sensitive V. vinifera cultivars may have significant bud injury below -3° F, but buds of cold-hardy varieties with V. riparia parentage can survive winter lows of -30° F.” In general, Martinson notes that cold-hardy Minnesota hybrids can withstand lower temperatures than Labrusca types, and Labrusca vines are more hardy than conventional French-American hybrids, which in turn are more resistant to low temperatures than V. vinifera cultivars.

He continues, “Cultivars also vary in the rate at which they acclimate and de-acclimate. Cold-hardy cultivars (e.g., Concord) may acclimate and de-acclimate faster than less cold-hardy cultivars (e.g., Cabernet Sauvignon), which occasionally can result in freeze injury in the springtime even in hardier cultivars.

“Vine stress associated with delayed harvest, drought stress, disease pressure or overcropping can reduce the vine’s ability to attain its maximum potential cold hardiness. Years in which frost is early or fruit maturity is delayed may also delay cold acclimation and reduce bud hardiness.

“Genetics determines a vine’s maximum cold-hardiness, but environmental conditions will influence how much of the genetic potential is realized in a given year. Growers can’t influence weather conditions, but they can understand the risks, evaluate potential bud injury and manage vines to limit or respond to bud injury in the following ways:

Cultivar selection: Match grape variety with your climate. With new cold-hardy varieties, cultivars are available that will survive even extreme winter low temperatures. If you choose more cold-sensitive varieties, be aware of the higher risk of winter injury in your climate—and be prepared to compensate for it.

Site selection: Plant your vineyard in a location that will have good air and soil drainage. Cold air moves downhill, so avoid low areas or “frost pockets” where cold air will collect. Mid-slope areas are less risky than low areas, both in midwinter and in the spring or fall. All else being equal, vineyards with heavier, more poorly drained soils will be more prone to winter injury than those on well-drained, lighter soils.

Bud injury evaluation: The extent of bud injury following a cold temperature event can be evaluated by collecting dormant canes and buds and examining them to determine whether primary buds are alive or dead. Guidelines for determining bud injury and a video for evaluating bud injury before pruning are available online.

Adjusting pruning severity: When the risk of winter bud injury has passed, it may be necessary to adjust the number of buds retained after pruning to compensate for buds lost to winter injury.

Other protection methods: Aerial “wind machines” (powerful fans mounted on posts) can be installed in vineyards and used during temperature inversions to mix warmer above-ground air with cold air, thus raising temperatures above bud-injuring levels at the trellis. Hilling-up soil over graft unions can protect scion buds for re-establishing trunks following a cold event that damages buds.

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