Dr. Michela Centinari, Assistant Professor of Viticulture, Department of Plant Science
Another growing season has started for many Pennsylvania grape growers. Unfortunately, but not surprisingly, we are seeing and hearing of situations of vine winter injury across the State. This past winter, the lowest temperatures occurred at the end of January and during the first two days in February, with values around -5 °F (-20.6 °C) here in State College (central PA) and even lower temperatures were recorded at other locations.The injury seemed to have mainly affected Vitisviniferavarieties with reports of bud kill up to almost 100% for the most cold-sensitive varieties and, in some cases, trunk splitting.Growers also noticed uneven /nonuniform budburst which is typical of winter-injured vines. We ask that more growers share their experiences with us; in particular, we would like to know if growers made any pruning adjustments and what the results are/have been.
Since winter injury is a reoccurring issue for the eastern US, during certain years, we have covered topics related to vine cold hardiness, injury assessment, and pruning techniques for winter-injured vines at Extension meetings. Also, we have posted an announcement that focused on Pruning strategies for cold climate viticultureon the Penn State Viticulture and Enology Facebook page in January 2019, just before the “Arctic Vortex” event hit our region. Please do not hesitate to contact us if you have questions on how to manage cold-injured vines.
We heard from several PA growers in southern and central PA that budburst occurred earlier this year, a week to 10 days is what has been typically reported, than in 2018. This was also true for the hybrid varieties grown at the Penn State research farm at Rock Springs (central PA). I checked the growing degree days (GDD), a widely used index of heat accumulation, data calculated by the Network for Environment and Weather Applications (NEWA Cornell) for weather stations located in North East, Erie (northwestern PA), Biglerville (south-central PA), and Reading (southeast PA). Although historic data are not available, I compared the average GDD accumulated from January 1 to May 15 for 2013-2017 to those accumulated for the same period in 2018 and 2019 (Figures 1, 2 and 3).
Trends across locations/regions
Not surprisingly, it was cooler in Erie compared to south-central and southeastern PA between January to Mid-May, not just in 2019 but for each year analyzed. In 2019, approximately 158 GDD accumulated between January 1 to May 15 in Erie, while GDD were at least double in south-central and southeast PA. Differences in temperatures across regions and locations explain why budburst typically occurs much earlier in southeast PA compared to the northwestern part of the state.
Difference between years
In Erie, the GDD accumulated between January to mid-May 2019 (red line) were slightly lower than those for the same period in 2018 (blue line) and for the 2013-2017 average (black line). Also, note that there was no accumulation of GDD for a few days in May 2019 due to cool temperatures (Figure 1). The trend, however, was opposite in south-central and southeast PA, at least at the locations reported in this post. April was warmer (higher GDD) in 2019 compared to 2018 and the 2013-2017 average. While warmer spring temperatures favor earlier budburst they also increase the chance of freeze injury to green, tender plant tissues (Figure 4).
At several locations across PA, temperatures were below freezing in the early morning of April 29 and some varieties were close to or already passed budburst. Below freezing temperature does not necessarily mean freeze injury as many factors affect the temperature at which the plant tissue is damaged or killed. However, the freeze event on April 29 did cause freeze damage to vines at several locations, while others avoided the damage by using frost protection methods, such as frost dragons. Some of the varieties grown at the Penn State research vineyard at Rock Springs, chiefly Marquette and young LaCrescent vines, sustained freeze injury. It is too early to estimate crop losses, but at least we are seeing some secondary shoot development (Figure 5).
How to recognize a secondary from a primary shoot
A relatively easy way, especially for caned pruned vines, is to check the angle of projection from the cane. Primary shoots typically grow with an angle of 45°, while secondary grow at an angle of 90° (figure 5).
You can learn more about the basics of spring freeze injury and methods of protection at https://extension.psu.edu/understanding-and-preventing-spring-frost-and-freeze-damage
It is almost time for some early season canopy management practice. Please check the following articles if you need information on shoot thinning or early leaf removal:
How does delaying spur-pruning to the onset or after bud burst impact vine performance? Insights from recent studies
By Michela Centinari
Now that harvest is finally over and wines are tucked away in the cellar, it is time to prepare for the next year. One of the first concerns that many growers feel in a new growing season is that worry of spring frost and the associated potential risk of vine injury. In the spring of 2016, for example, an unusually warm March was followed by a very cold start to the month of April, which resulted in damaging frost incidences in some vineyards of the Mid-Atlantic region.
Susceptibility to frost injury increases with advanced phenological growth stage , therefore, growers and scientists have explored different techniques for delaying bud burst of grapevines to increase the chance of avoiding spring frost damaging events. Vegetable-based oils (e.g., Amigo oil) can be sprayed on the canes/buds during the winter to slow down bud de-acclimation and delay the resumption of vegetative growth in the spring [2; 3; study at Penn State]. Delaying pruning until late winter can also be used to delaying bud burst of vines growing in frost prone areas.
Canes of cordon-trained vines can be pruned to 2-3 node spurs late in the winter or even when apical buds begin to open to delay bud burst of basal buds. Due to the strong apical dominance of Vitis vinifera cultivars, apical buds of an unpruned cane tend to burst first, which inhibits development and growth of median and basal buds  (Figure 1).
What may happen if we wait until the onset of bud burst or even later to prune the vines?
Spur-pruning the vines when the apical buds of un-pruned canes are already open may not only delay bud burst of the basal nodes, but may also postpone other phenological growth stages such as bloom, fruit-set, or even veraison with potential consequences for vine yield and fruit chemical composition at harvest .
I recently read two articles on this topic published in the American Journal of Enology and Viticulture (Post-bud burst spur-pruning reduces yield and delays fruit sugar accumulation in Sangiovese in central Italy  ) and in Frontiers in Plant Sciences (Phenology, canopy aging and seasonal carbon balance as related to delayed winter pruning of Vitis vinifera L. cv. Sangiovese grapevines ).
The studies described in these articles aimed to assess if and how delaying winter spur-pruning of Sangiovese vines to the bud swelling stage or later, after bud burst, impacted the annual growth cycle of the vines and its productivity.
The studies were conducted in Italy and the researchers were specifically interested in assessing if vines pruned around or after bud burst exhibited a delay in grape ripening as compared to those pruned during the winter, resulting in lower sugar accumulation and higher acidity in the fruit at harvest. A steady trend of increased warming is, indeed, pushing some Mediterranean grape growing regions toward accelerated ripening , which could lead to excessive or overly fast sugar accumulation in the fruit, high alcohol in the wine, unacceptably low acidity, high pH, and also atypical grape flavors and aromas .
Although excessive or overly fast sugar accumulation may not be a problem in our region, it’s still important to understand if delaying winter pruning to extremes could be used to delay bud burst and reduce risk of frost damage, as well as the impact this practice may have on vine yield, and fruit and wine chemistry. This is a topic of further interest in light of changing climatic conditions and the potential increase of unpredictable weather patterns like early spring warming and late spring frosts .
Below, I will summarize the two previously mentioned studies emphasizing results which can be of interest to wine grape growers in our regions.
Both studies were conducted on mature Sangiovese (Vitis vinifera L.) vines. The first study was established in a commercial vineyard in central Italy, whereas the second study was conducted on vines growing outdoors in 10-gal pots at a research station in northern Italy. Groups of vines were assigned to different pruning treatments. Vines assigned to the standard grower practice treatment were spur-pruned to 2 basal nodes during the winter when buds were dormant. Vines assigned to the other treatments were spur-pruned at more unusual times from the bud swelling to full bloom (Figures 2 and 3A).Did delaying vine spur-pruning to after bud-burst consistently delay the whole annual growing cycle?
Basal buds of Sangiovese vines spur-pruned when apical shoots were about 1.6″ long (called late pruning treatment; Figure 3A central panel) burst 17 days later than those of vines pruned in the winter, when buds were dormant (called standard pruning treatment; Figure 3A left panel). Pruning the vines even later, when apical shoots were about 4.7-5.5″ long (called very-late pruning treatment; Figure 3A right panel) extended the delay in bud burst to 31 days as compared to vines pruned in the winter Unfortunately no phenology data were recorded for vines pruned at bud swelling stage (study 1).
The delay in phenological growth stage decreased over the season. For example, late-pruned and very-late pruned vines reached veraison 3 and 13 days, respectfully, after those pruned in the winter (Figure 3C). Shoots of vines pruned after bud burst developed later in the season under higher air temperature than those of vines pruned during the winter. Greater air temperature may have helped shoots of late- and very-late pruned vines to reach bloom and veraison in fewer days as compared to those pruned earlier .
By harvest the delay was fully off-set for the late-pruned vines: they reached the sugar level set for ripening (~ 19 ºBrix) three days before those pruned in the winter. Grapes of very-late pruned vines reached 19 ºBrix 6 days after those pruned in the winter.
Spur-pruning vines after bud burst significantly reduced crop yield compared to standard winter pruning
Vines pruned at bud swelling growth stage had similar crop weight, number of clusters per vine, and cluster weight than those pruned when buds were still dormant. Pruning vines after bud burst, however, reduced yield as compared those pruned during the dormant season. For example, late pruned vines (spur-pruned when apical shoots were about 1.6″ long; Figure 3A central panel) had 26% lower crop yield as compared to those of the standard pruning control group (spur-pruned before bud burst). Reduction of crop yield was related to lower cluster weight and lower number of berries per cluster. While there is not a clear explanation on why late-pruned vines had fewer berries per cluster, several hypotheses were presented including increased production of gibberellins during the initial flush of growth in the late-pruned vines .
Waiting even longer to prune the vines had a detrimental effect, reducing not only cluster weight but also the number of clusters per vine. For example, vines pruned to two basal nodes when the apical shoots were already flowering had no crop at harvest. When vines were pruned so late the basal shoots did not develop flowers and remained vegetative after pruning. I am not sure why any growers would want to wait until bloom to prune the vines, but it’s still interesting to see how such a drastic treatment may limit sources of carbohydrates for developing cluster primordia .
Although uncertainty still exists, the authors suggested that delaying spur-pruning until after bud burst, but not to extremes, may have the potential for reducing crop yield in high-yielding cultivars such as Sangiovese planted in specific regions of Italy. However, long-term field studies are necessary to assess if it is possible to calibrate winter pruning date for managing yield reductions and/or fruit maturation rate.
Did delaying vine spur-pruning to bud swelling stage or after bud burst consistently impact fruit chemistry?
The effect of the timing of spur-pruning on fruit composition at harvest varied between studies and with the extent of the pruning delay. For example, Sangiovese vines late pruned (apical shoots 1.6″ long) had higher total soluble solids (+ 1 °Brix), total anthocyanins and phenolics than winter-pruned vines. However, vines growing in a commercial vineyard (study 1) and spur-pruned to two basal nodes later in the season, when inflorescences of apical shoots were already swelling (Figure 3C), had lower sugar concentration (-1.6 °Brix) and higher TA (+1.8 g/L tartaric acid) than those pruned during the winter, but at the same time they also had higher anthocyanins and phenolic concentrations. This result suggests that spur-pruning canes after bud burst may decouple the accumulation patterns of total soluble solids and anthocyanins, phenolic metabolites. This could be intriguing for growers trying to delay fruit sugar accumulation and acid degradation, while maintaining wine color, but on the another hand it could also come with a reduction in crop yield, quantified to over 50% in this study .
Did delaying winter spur-pruning have negative carry-over effects on the following season?
Pruning vines at the bud swelling stage did not have negative effects on vine growth in the following year. It did not impact bud fertility (number of clusters per shoot) or winter carbohydrate storage, which is important for winter vine survival and following year resumption of growth. However, pruning the vines to two-nodes after bud burst, specifically when inflorescences of apical shoots were already swelling (Figure 3C), reduced bud fertility by 50% in the following year. Those vines were able to recover once standard winter pruning was applied again at the end of the study.
These studies conducted on Sangiovese vines grown in Italy found that:
- Winter spur-pruning can be applied up to bud swelling without adversely affecting vine yield, grape composition at harvest or bud fertility in the following year.
- Vines pruned after bud burst show pronounced delay in shoot development at the beginning of the season, which increased as the pruning time was further delayed. Under the warm conditions of these studies the delay in phenological growth stage decreased or even disappeared over the season. This could be partially explained by the fact that late-pruned vines needed less time than vines pruned during the winter to reach maximum photosynthesis efficiency.
- Delaying spur-pruning to after bud-burst may reduce vine yield, decrease sugar accumulation and bud fertility in the following year.
Delaying winter pruning of vines located in frost prone areas to the onset of bud burst or shortly after that may be used as frost avoidance technique. However, we need to further understand how a delay in shoot development and potentially a shorter growing season (number of days from bud burst to harvest) may impact fruit ripening, yield component, vine over-winter carbohydrate storages and susceptibility to winter cold temperatures, as well as the following year growth. The studies summarized here were conducted in a warm region with a growing season longer than many areas of Pennsylvania. Further research is necessary to corroborate those results under our regional climatic conditions.
- Centinari M, Smith MS, Londo JP. 2016. Assessment of Freeze Injury of Grapevine Green Tissues in Response to Cultivars and a Cryoprotectant Product. Hortscience 51: 1–5.
- Dami I, and Beam B. 2004. Response of grapevines to soybean oil application. J. Enol. Vitic. 55: 269–275.
- Loseke BJ, Read PE, and Blankenship EE. 2015. Preventing spring freeze injury on grapevines using multiple applications of Amigo Oil and naphthaleneacetic acid. Scientia Hort. 193: 294–300.
- Friend AP, and Trought MCT. 2007. Delayed winter spur-pruning in New Zealand can alter yield components of Merlot grapevines. J. Grape Wine Res. 13: 157–164.
- Frioni T, Tombesi S, Silvestroni O, Lanari V, Bellincontro A, Sabbatini P, Gatti M, Poni S, Palliotti A. 2016. Post-bud burst spur pruning reduces yield and delays fruit sugar accumulation in Sangiovese in central Italy. J. Enol. Vitic. 67:419–425.
- Gatti M, Pirez FJ, Chiari G, Tombesi S, Palliotti A, and Poni S. 2016. Phenology, canopy aging and seasonal carbon balance as related to delayed winter pruning of Vitis vinifera cv. Sangiovese grapevine. Frontiers in Plant Sciences 7:1–14. Article 659.
- Jones GV, White MA, Cooper OR, and Storchmann K. 2005. Climate change and global wine quality. Climatic Change 73: 319–343.
- Mosedale JR, Wilson RJ, and Maclean IMD. 2015. Climate change and crop exposure to adverse weather: Changes to frost risk and grapevine flowering conditions. PLoS One 10:e0141218.
- Lorentz DH, Eichorn KW, Bleiholder H, Klose R, Meier U, and Weber E.1995. Phenological growth stages of thegrapevine (Vitis vinifera ssp. vinifera). Codes and descriptions according to the extended BBCH scale. Aust. J. Grape Wine Res. 1, 100–103.
By Michela Centinari
The last month has provided a temperature roller-coaster going from a very, perhaps exceptionally, warm March to a cold beginning of April. Many grape growers are keeping their fingers crossed hoping to escape frost injury. As far as I am aware, no budbreak has been observed for grapevines grown in central Pennsylvania, but budbreak may be close in other PA locations. It looks like a good time of year to review some basic concepts related to post-budbreak freeze injury and frost protection options available for grape growers.
Freeze and Frost
We often use the terms “frost” and “freeze” interchangeably to describe a meteorological event, specifically related to air temperature dropping below 32°F (0 °C). However, “frost” and “freeze” definitions reported in the literature are variable and sometimes confusing. I personally like the definitions used in the book: Frost protection: Fundamentals, practice, and economics [Food and Agriculture Organization of the United Nations (FAO), 2005; 1]. In this book frost is defined as “the occurrence of an air temperature of 0 °C or lower, measured at a height of between 1.25 (49.2 in) and 2.0 m (78.7 in) above soil level, inside an appropriate weather shelter”, while freeze “occurs when water within the plant freezes”.
In other words a frost becomes a freeze event if ice forms within the plant tissues.
Keep in mind that:
- It is the ice formation inside the plant tissue rather than low temperatures per se that cause the damage. The formation of ice crystals can be either inter-cellular (space between cells) or intra-cellular (within the protoplasm of cells), the latest causing cell death  (Figure 1). The general hypothesis is that during spring frosts, freeze injury is mainly caused by inter-cellular rather than intra-cellular ice formation [1, 2]. The formation of inter-cellular ice crystals produces a water vapor deficit/gradient between the interior and the exterior of the cells. As a result, water migrates from the inside to the outside of the cells and deposits on the ice crystals formed in the inter-cellular spaces. If ice continues to grow, the cells become more desiccated and lose their turgor . Freezing-induced dehydration can also permanently damage the structure of cell membranes and other cellular components. This usually causes a flaccidity and/or discoloration of the damaged tissue . Thus, the current view is that dehydration injury is the main cause of frost damage. .
- Water within plants doesn’t always freeze during a frost event. Plants have developed avoidance strategies to avoid ice formation in the tissues, for example, by supercooling, and tolerance strategies (e.g., solute content of the cells) to survive inter-cellular ice formation without irreversible damage of the plant tissue .
The critical temperature is defined as “the temperature at which tissues (cells) will be killed and determines the cold hardiness levels of the plant” . Many factors affect the temperature at which damage occurs including: type of plant tissue, stage of phenological development of the bud/shoot, dew point and surface moisture, probability of an ice nucleation event and pre-frost environmental conditions .
Why budbreak is considered the onset of the most susceptible period for cold injury?
Growing organs have a high water content, which makes them susceptible to the formation of ice at freezing temperatures. Air temperature of –2, –3°C can permanently damage green tissues . Early spring growth is particularly susceptible to freeze injury. Freezing tolerance remains low during the most of the growing season and gradually increases late summer and fall (cold acclimation) and reaches its maximum peak in midwinter . In midwinter grapevines are able to tolerate freezing temperature through a complex process called deep supercooling. For example, the cells within the dormant bud become resistant to lower temperature through dehydration (i.e., movement of water to inter-cellular spaces) and accumulation of so-called cryoprotectant (e.g., soluble sugars and proteins). Those compounds lower the freezing point of the water within the plant tissue and stabilize cell membranes  making the dormant buds able to survive temperatures well below freezing. Also, during the dormant season buds are thought to be disconnected or weakly connected to the vine’s vascular tissues, which limit their potential to take up water .
There are two main types of frosts
Advective frost: an advective frost is usually a regional weather event. It occurs when strong, cold winds (colder than the critical temperature) blow into a region day and/or night. The rapid, cold air movement “steals away the heat in the plant causing freeze damage” . Unfortunately there is very little which can be done to protect against an advective frost. For example, wind machines are useless during an advective frost event.
Radiation or radiative frost: A radiation frost is the most common type of frost for many grape growing regions. Luckily, a radiation frost is also the easiest to protect against during a frost event. It occurs when a dry, cold air mass moves into an area when there is almost no cloud cover and no wind at night. Because plants and soil are warmer than the sky temperatures they will “radiate” heat back to their surrounding space and become progressively colder than the air .
Radiative and advective frosts may occur simultaneously, the classification depends on which is one is dominant (Table 1).What are the options available to protect your vines from freeze injury?
Passive or indirect methods (risk minimization)
Passive methods are avoidance strategies, efforts to reduce the probability and risk of freeze damage.
- Site selection
You have probably already heard this, but it cannot be said too many times: “The best time to protect your vineyard from frost injury is before it is planted” . Cold air flows downhill so mid-slope locations are warmer if there are no obstacles to cold air flow  (Figure2). Thus, when evaluating potential sites for establishing your vineyard, look for a site with good air drainage. Get historic records of low temperatures, number of frost-free days, and accurate information on percent slope, aspect or exposure and elevation. You can contact your local county Cooperative Extension office for information about site suitability for a vineyard, or utilize these resources here: http://bit.ly/VydSelectionTools.
- Cultivar selection
Grapevine cultivars may vary in the average day of budbreak by up to two weeks . To avoid or reduce the risk of freeze injury plant cultivars with early budbreak in the location within the vineyard with the lowest risk of frost.
- Training system choice
Many factors related to fruit quality and economics influence the choice of a training system. With regard to risk of freeze damage, a training system which places the buds high on the trellis may reduce frost hazard (Figure 3). Frost hazard is reduced by up to 0.36 °C each 10 cm (3.94 in) above the soil level .
- Pruning choices
- Delay pruning: Pruning too early may accelerate budbreak. Thus, prune as late as possible in frost prone areas of your vineyard.
- Double pruning: this is another option to delay budbreak for cordon-trained vines. The first step is to prune the canes to long spurs, 5 to 8 buds long . Buds at the end of the canes will open first and suppress the growth of basal buds (Figure 3). After frost risk has passed, do a second and final pruning to cut back the long spurs to two-bud spurs. Likewise, for cane-pruned vines one option is to leave long canes (first step) and cut them back (second step) to the desired bud number later, after the frost risk has passed. Some growers opt to retain extra canes as an insurance measure and then remove them later.
- Delaying budbreak by chemical means
Application of vegetable-based oils (e.g., Amigo oil) at nontoxic rates can slow bud de-acclimation and delay grapevine budbreak anywhere from 2 to 20 days depending on several factors including variety, number of applications and coverage [10,11]. Those oils are called “dormant oils” because they need to be applied when the buds are dormant. If you are interested in trying Amigo oil or a similar type of oil in your vineyard, begin with a small selection of vines. Be sure to record phenology, crop yields, fruit composition (Brix, pH, TA) and quality (fruit aromas and flavors, etc.) data for un-sprayed and sprayed vines. In this way, you can assess the impact of oil application on delaying budbreak as well as potential secondary effects on production and fruit quality parameters.
- Middle-row management
Mowing ground cover short will increase the warming of soil during the day and release slightly more heat during the night . Tall cover crops and weeds may also hinder cold air drainage.
Active or direct frost protection methods (frost management)
Active or direct frost protection strategies are efforts to modify microclimate conditions in the vineyard and increase temperatures above injury levels. Some of the most common active frost protection methods are:
- Wind machines (or fans)
Wind machines are well suited for radiational frosts because they use the inversion of air temperature that develops during this type of frost event. Wind machines pull down warmer air, from above the inversion layer, which may provide from 1 – 3°F of warming . The minimum size vineyard recommended for a wind machine is around 7-10 acres. Wind machines may become profitable on sites where there is a 20% (1 in 5 years) or higher probability of spring frost damaging events . It is worth mentioning that wind machines have been noted to produce a loud noise. Operating costs are higher than for over-vine sprinkling systems, but considerably lower than use of return-stack oil heaters and standard propane heaters .
- Over-vine irrigation
Over-vine sprinkler systems have been successfully used for frost protection since the 1940s . Sprinklers provide a constant amount of water covering the buds and shoots. As water freezes it releases a small amount of heat, which increases the temperature of the plant tissue. The level of protection is proportional to the amount of water applied . If properly used, this method is very effective in protecting grapevines from freeze injury. It is the only active method that doesn’t rely on inversion strength during a frost event . However, on the other hand, keep in mind that it requires substantial water resources, is labor intensive and if the system fails during the night/frost event it can cause more damage than otherwise applying no frost protecting strategy.
Heating the vineyard for frost protection is a very old practice. In ancient Rome (at least 2000 years ago) growers used to burn piles of pruned wood and other waste to heat their vineyard during spring frost events . Fossil-fueled heaters are rarely used these days because of the high cost of fuel and labor, low heating efficiency and contribution to air pollution.
Unfortunately there is not a perfect strategy which can provide complete frost protection in every situation. Quite often the combination of different methods is the best option.
If you are looking for detailed information about active frost protections strategies please check:
Understanding and Preventing Freeze Damage in Vineyards. 2007. Workshop Proceedings. University of Missouri Extension.
Evans, R.G. 2000. The art of protecting grapevines from low temperature injury. Proc. ASEV 50th Anniversary Annu. Mtg., Seattle WA, 19–23 June. p. 60–72.
Poling, E.B. 2008. Spring cold injury to winegrapes and protection strategies and methods. Hortscience 43: 1652–1662.
- Food and Agriculture Organization of the United Nations. 2005. Frost protection: Fundamentals, practice and economics. Vol. 1.
- Wilson, S. 2001. Frost management in cool climate vineyards. Final report to grape and wine research & development corporation. Available at: http://www.gwrdc.com.au/wp-content/uploads/2012/09/UT-99-1.pdf
- Poling, E.B. 2008. Spring cold injury to winegrapes and protection strategies and methods. Hortscience 43: 1652–
- Rodrigo, J. 2000. Spring frosts in deciduous fruit trees—Morphological damage and flower hardiness. Scientia Hort. 85:155–173.
- Evans, R.G. 2000. The art of protecting grapevines from low temperature injury. Proc. ASEV 50th Anniversary Annu. Mtg., Seattle WA, 19–23 June. p. 60–72.
- Keller, M. 2010. The Science of Grapevines: Anatomy and Physiology. Publisher: Academic Press.
- Martinson, T. 2001. How Grapevine Buds Gain and Lose Cold-hardiness. Appellation Cornell, Issue 5. Cornell University Cooperative Extension. Available at: https://grapesandwine.cals.cornell.edu/newsletters/appellation-cornell/2011-newsletters/issue-5/how-grapevine-buds-gain-and-lose-cold
- Hellman, E. 2015. Frost Injury, Frost Avoidance, and Frost Protection in the Vineyard. org Available at: http://articles.extension.org/pages/31768/frost-injury-frost-avoidance-and-frost-protection-in-the-vineyard
- Trought, M.C.T., Howell, G.S., and Cherry, N. 1999. Practical considerations for reducing frost damage in vineyards. Report to New Zealand winegrowers. Available at: http://www.nzwine.com/assets/sm/upload/eb/fl/ot/sp/frost_review.pdf
- Dami, I., and Beam B. 2004. Response of grapevines to soybean oil application. Amer. J. Enol. Vitic. 55: 269–
- Loseke, B.J., Read, P.E., and Blankenship E.E. 2015. Preventing spring freeze injury on grapevines using multiple applications of Amigo Oil and naphthaleneacetic acid. Scientia Hort. 193: 294–300.
- Wolf, T.K. 2015. Viticulture Notes. Virginia Tech University Cooperative Extension. April 2016.
By Michela Centinari
This past November and December were surprisingly warm months in Pennsylvania with temperatures rising into the 60s and even 70s °F (Figure 1a, b). In January temperatures dropped to single digits in many eastern U.S. regions  followed by a virtual temperature rollercoaster ride in the next months (Figure 1). On a positive note, viticulture specialists from Virginia Tech (T. Wolf and T. Hatch) did not observe any winter injury in Cabernet Sauvignon and Merlot buds and canes collected on January 8 (northern Virginia) .
In many regions of Pennsylvania temperatures in January and February did not reach the critical low threshold (Figure 1B) that tends to injure many of the cultivars grown in PA and we are not currently concerned about winter injury in the majority of the state. However, on February 14 temperatures of -10°F and below were recorded in some areas of the state (report from growers from northeastern PA and Figure 1A). The lowest temperature in Pennsylvania (-19°F) was recorded in Potter County on February 14 .
Although we are not aware of the extent yet, we anticipate winter injury in some of the cultivars grown in those areas.
A few reminders about the cold hardiness process:
In late summer/early fall, cold-tender grapevine tissues produced during the growing season gradually acquire cold hardiness and transition to a cold-hardy stage (known as cold acclimation) as a response to low temperatures and decreasing day length  (Figure 2). Bud (and other tissues) cold hardiness reaches its maximum level in mid-winter (known as maximum hardiness). Later in the winter as temperatures increase, the buds begin to lose hardiness (known as deacclimation) . The deacclimation stage ends in budbreak and active growth.
It is a well-known fact that bud cold hardiness depends heavily upon the grapevine species and cultivar. For the past two winters (2013-2014; 2014-2015) many growers in the eastern and midwestern U.S. have had the unfortunate opportunity to test this in their own vineyards. However, in addition to its genotype, the cold hardiness of a specific cultivar is determined by environmental conditions, such as seasonal temperatures and their variation, and by vineyard management practices . It is important to remember that exposure to decreasingly lower temperatures plays a major role in the ability of the vine to acquire its maximum cold hardiness. In other words:
- The colder the region, the closer a vine gets to its maximum cold hardiness potential . For example, bud cold hardiness of Chardonnay and Riesling in the Finger Lakes region of NY (cooler region) was found to be 2 to 3°F greater than that of the same cultivars grown in Virginia (warmer region)  (Figure 3).
- “The type of winter determines the extent of bud cold hardiness” , thus the absolute cold temperature that injures the same cultivar may vary between winters. J. Londo (Geneticist, USDA, Grape Genetics Research Unit, Geneva, NY) reported that the average mid-winter LT50 (lethal temperature for 50% of the buds) for labrusca varied from -24.71 °F in 2012-2013 (defined as a mild-cool winter in upstate NY) to -26.8°F in 2013-2014 (cold winter but with big swings in temperatures) to -26.5°F in 2014-2015 (sustained cold winter) . However, species and cultivars may vary in their response to different temperatures/winters. For example, Tim Martinson, senior viticulture Extension associate at Cornell University, did not observe a decrease in bud hardiness of Riesling vines this year as compared to last year .
A few of the things that Penn State Extension recommends to growers:
- Plan on pruning the hardiest cultivar first and finish with the least hardy . If a cold event occurs late in the winter, like in the middle of February, and cold-sensitive cultivars were not pruned yet, you can still assess bud cold damage and adjust pruning severity accordingly . Leaving extra buds on the vines, and adjusting shoot number after bud break certainly cost growers more money, but it also increases the chance to produce a crop, hopefully close to normal.
- If your site is an area with moderate to high risk of cold damage events, consider keeping some freeze-tolerant grape cultivars in the mix to reduce the economic downside risk. These are considered cultivars that you can rely upon to “pay the bills.” Cold-hardy cultivars, most of which were released by the breeding program of the University of Minnesota (i.e. Marquette, La Crescent, Frontenac, etc.), are increasing in popularity mostly, but not only, in regions where vinifera or other inter-specific hybrid varieties are not well suited or have struggled to survive and perform well in the long term.
Thanks to The Northern Grapes Project we are gaining a better understanding of how to optimize viticultural, winemaking, business management and marketing practices of these fairly new cold-hardy cultivars. For example, important information on Cost of Production in Cold Hardy Grapes was recently published in the Northern Grapes Project newsletter . The fact that most consumers may still be unfamiliar with those varieties and the wine styles they produce doesn’t necessarily mean that they not will have chance to stand alone as a varietal wine if they produce high quality wines.
- As Zabadal et al.  pointed out “Minimizing winter injury is usually not the primary goal of a grape grower; however it must be given attention because of its huge impact on profitability”. Each grower should carefully evaluate if the cost of vine management practices that reduce vine winter injury can increase the business profit.
- Finally, the most important step, making informed decisions before planting a vineyard and always applying good viticulture practices, which includes keeping the vines healthy and in balance.
Only time will tell what weather conditions the rest of winter and early spring will hold in store for us. However, if your vineyard is located in a frost prone area and you have dealt with spring (post-budbreak) freeze damage in previous years, this would be a good time to review the frost protection practices available and assess if and what options could be used for your specific situation (see for instance: Frost Protection in Orchards and Vineyards by R. Evans, USDA or Methods of Vineyard Frost Protection by P. Domoto, Iowa State University).
A two-year study was conducted by our research team at Penn State to evaluate “low-cost” frost protection practices for their efficacy to avoid/reduce crop losses due to spring freeze injury. We tested the effect of a vegetable-based oil (Amigo oil) to delay budbreak on two vinifera (Lemberger and Riesling) and two inter-specific hybrid cultivars (Noiret and Traminette). We also tested the impact of KDL (Agro-K’s Potassium Dextrose-Lac®), sprayed shortly (»24 hours) before a frost event, on reducing frost damage to young grapevine shoots. The impacts of Amigo oil and KDL applications on yield components, fruit composition and perceived wine quality were also assessed. Results from this trial will be presented at the 2016 Pennsylvania Wine Marketing and Research meeting: http://bit.ly/PAWMRB2016Symposium.
- Jones McKee L. 2016. Cold hardiness and dormancy, pp58-64. Wines and Vines, March 2016.
- Eherts F. 2016. February 2016- Pennsylvania Weather Recap. The Pennsylvania Observer. March 2016.
- Martinson, T. 2001. How Grapevine Buds Gain and Lose Cold-hardiness. Appellation Cornell, Issue 5. Available at: https://grapesandwine.cals.cornell.edu/newsletters/appellation-cornell/2011-newsletters/issue-5/how-grapevine-buds-gain-and-lose-cold
- Zabadal, TJ, Dami, IE, Goiffinet, MC, Martinson, TE, and Chien, ML. 2007. Winter injury to grapevines and methods of protection. Extension Bulletin E2930. Michigan University Extension.
- Londo J. 2015. The Big Chill: bud dormancy and cold hardiness in grape. Northern Grapes webinar, December 8, 2015. Available at: http://northerngrapesproject.org/?page_id=257
- Northern Grapes News. 2016. Vol. 5, Issue 1, February 18, 2016. Available at: http://northerngrapesproject.org/?page_id=213
By Michela Centinari
It seems like yesterday we were looking at the weather forecast and worrying about cold winter temperature events and the potential for grapevine injury. Now that it is finally starting to get warmer here in Pennsylvania, we may be faced with another threat: spring frost. A grape grower is never bored!
It was another cold winter in Pennsylvania, particularly harsh in the Lake Erie region (Figure 1). At the Penn State Lake Erie Regional Grape Research and Extension Center (LERGREC) temperatures bottomed out at about -21 °F (-30 °C) on February 16, 2015. Unfortunately several cold events (-13, -14 and -15°F) were recorded over the following ten days. On a ‘positive’ note, the week before these extreme cold events, temperatures were lower than normal, with daytime temperature highs well below freezing, except for one day (34°F). These temperatures may have provided a positive, reinforcing maintenance of the vines’ mid-winter cold hardiness . Bryan Hed and the LERGREC’s crew have been checking the extent of bud and trunk damage on Concord and other hybrid varieties.
Information available on cold winter injury on grapevine
At the 2015 Mid-Atlantic Fruit and Vegetable Convention I reviewed the factors that can affect grapevine cold hardiness, explained how to assess bud, cane and trunk cold damage, as well as how to manage cold-injured vines. For information on grapevine cold injury you can refer to the Grapevine cold injury, end of the season considerations blog post and references within.
If you are looking for specific information on winter injury to vine phloem you can check this recent and comprehensive review: Viticulture and Enology Extension News, spring 2015, Washington State University written by Michelle Moyer (Assistant Professor and Viticulture Extension Specialist at Washington State University).
Percentage of winter injury does not equal percentage of crop loss
In March, I attended The Northern Grapes Project Symposium in Syracuse, NY. Tim Martinson (Senior Extension Associate at Cornell University) and Imed Dami (Associate Professor and Viticulture Extension Specialist at Ohio State University) highlighted that the percentage of bud cold damage does not always equal percentage of crop loss. The answer often lies in the pruning adjustment strategies adopted by growers. Dami reported that, despite 40% of bud winter damage, Marquette produced about 5 tons/acre in Ohio last year. Those vines were pruned to 5 bud-spurs (‘hedge pruning’) to compensate for winter injury . .
Tim Martinson reported that last year many growers in the Finger Lakes region (NY) left more buds to compensate for winter injury experienced during the 2013-2014 winter. The growers left up to five-fold more buds than they would have done in a normal year. Many cane-pruned VSP vineyards were spurred to 5-6 bud spurs. It was a pleasant surprise that in 2014 widely planted V. vinifera varieties such as Riesling, Chardonnay, and Cabernet franc, came through better than was expected based on bud mortality estimates. I know that many growers prefer cane pruning, and I understand the reasoning behind that, but please take into consideration that cane pruning is not recommended following winter injury .
How to train suckers of cold injured vines?
Imed Dami recommends that growers “actively” train vines back to their original training system in the same season in order to resume production quicker. Therefore, instead of training suckers vertically (they can become extremely vigorous!) they should be trained horizontally along the fruiting wire. With extremely vigorous vines, four shoots should be selected and then two can be laid horizontally on the fruiting wire. With less vigor, two shoots can be selected and laid horizontally, one to each side. Then, shoots should be tipped to stimulate lateral shoot growth. Lateral shoots growing vertically and upward will become the future spurs next season . Latent buds on the lateral shoots will develop like buds from primary shoots. As long as they are exposed to sunlight and clean from disease and insects, they should have the same cold hardiness as any other buds.
Here is a valuable video regarding pruning with regards to cold injured vines: https://www.youtube.com/watch?v=r1Yhv8Rw38o
A few words on spring frost
As we get close to bud-break, the threat of spring frost is approaching. In the spring of 2014, no frost damage was recoded in grapevines in Pennsylvania and hopefully we will have another frost-free spring. If you would like to get information about frost protection strategies you can check the following websites and newsletters. Unfortunately, there is no new exciting or infallible frost protection method. Site selection remains the best way to protect vines from frost injury.
- Frost Injury, Frost Avoidance, and Frost Protection in the Vineyard, eXtension website, by Ed Hellman, Texas AgriLife Extension
- The ABCs of Frost Management by Robert Evans, Supervisory Agricultural Engineer with the USDA-ARS (retired)
- Viticulture Notes Vol.30, April 2015 by Tony Wolf, Professor and Viticulture Extension Specialist at Virginia Tech University
- Comprehensive presentations on active and passive frost protection strategies can be found at the University of California, Davis Cooperative Extension website
To the often asked question: If my vine gets frosted, should I remove the injured shoots?
The answer is: “There’s not much of a point,” according to Tony Wolf, Professor and Viticulture Extension Specialist at Virginia Tech University. A detailed explanation on how to handle damaged shoots and potential consequences on yield production can be found at Viticulture Notes, Vol.25, May-June 2010
Testing the cryo-protectant properties of KDL
KDL (potassium dextrose lactose; Agro-K corporation, Minneapolis, MN, USA) is a potassium based fertilizer. According to the manufacturer’s literature, spraying KDL shortly before a frost event (24-48 hours) would increase the potassium and sugar levels within the plant and reduce the frost injury on young vine tissue. Although attractive to growers, there is not scientific literature that supports the effectiveness of this product in preventing/reducing frost damage. Numerous grower testimonials are available, but growers usually do not leave an ‘untreated’ control area where the material is not applied, which is critical in order to evaluate the efficacy of KDL as cryo-protectant.
A large scale study coordinated by Tim Martinson (Cornell University) and in collaboration with the Agro-K company (KDL manufacturer) has been set up this spring to evaluate the effect of KDL at several vineyard sites located in NY and PA. Penn State is a collaborating university that is helping to work with six commercial growers that agreed to participate in the study in addition to the Penn State LERGREC in North East, PA.
Although, I’m hopeful there will not be a spring frost that growers have to deal with, if we do end up with a spring frost during the 2015 growing season, this study will hopefully provide some useful recommendations for grape growers.
- Wolf T.K., 2015. Viticulture Notes. Vol. 30 supplement, 17 February 2015
- Dami, I.E. , Ennahli S., Zhang Y. (2012). Assessment of winter injury in grape cultivars and pruning strategies following a freezing stress event. American Journal of Enology and Viticulture, 63: 106-111.
- Dami, I.E. 2009. Ohio Grape-Wine Electronic Newsletter Vol.3: 2-5, 6 Feb 2009.
- Dami I.E., 2014.Ohio Grape-Wine Electronic Newsletter, Vol.25, 3 July 2014.
By: Maria Smith
Since I arrived in August, we have been busy at work continuing to collect field data on our current studies. In addition, we have been developing a new study which takes an in-depth look at the role of canopy management practices have on vine cold hardiness.
After weeks of intense anticipation, monitoring berry development, and fretting over weather, harvest has finally come for us at the Centinari lab. As of last week, our commercial plots with Lemberger, Riesling, Noiret, and Traminette used to study frost tolerance and avoidance have all been harvested (Fig 1, See: “Evaluate cost-effective methods to decrease crop losses due to frost injury”). If you recall, these plots were sprayed with KDL, a commercially available cryo-protectant, and Amigo oil, a product used to delay bud break.
Our preliminary observations show that the Amigo oil treatments had noticeably lower yield in both Lemberger (Fig 2) and Riesling (data not shown) grapes harvested. Despite the lack of frost this spring, we did see a 2 week delay in bud break in Lemberger and a 1 week delay in Riesling with Amigo oil treatments. We are currently checking to see if these differences can be attributed to factors such as the number of shoots per vine between the treatments and/or cluster weight. However, if Amigo oil is in fact the cause of a decrease in yield we should see similar results over multiple years. In the case that Amigo oil has an actual affect on yield, it would be prudent to perform an economic analysis to decide if the use of Amigo oil as a frost avoidance mechanism is worth the cost in decreased yield.
Winemaking is a crucial component in determining what sort of impacts delaying bud break can have on the final product. Thanks to the help of Denise Gardner and a group of students in a winemaking and enology independent study class, we have had the opportunity to crush, press, and ferment our harvested research grapes into wine (Fig 3).
In the initial juice, we have noticed an increase in Brix and TA between the Amigo oil and control treatments in Lemberger, though the Brix of all treatments in Riesling were similar (Table 1). We will be running additional detailed chemical analyses on frozen berry samples taken throughout ripening in the upcoming months.
Future work planned for analyzing the effects of Amigo Oil and KDL include:
- Grape Brix, TA, pH on samples from veraison through ripening
- Wine chemistry – Alcohol, SO2, and color density
- Sensory attributes of wine
- Prepare for year 2 KDL and Amigo Oil application
Winter 2014: New work on canopy management and cold hardiness:
We have recently begun work on a second study in cold hardiness. Canopy management practices are often used to improve the microclimate of grapes within the fruiting zone by exposing clusters to increased light and airflow. Early leaf removal (ELR) has been shown to successfully reduce cluster compactness in tight-clustering varieties, thus reducing incidents of bunch rot (Hed et al., 2014). Another common canopy management practice, cluster thinning (CT), is used to reduce fruit load of overcropping varieties to improve grape and wine quality. However, these practices may also change carbon source-sink relationships with cluster thinning removing carbon sinks and ELR removing photsynthetically active leaves (source), which could potentially alter cold acclimation and hardiness in the vine. Using ‘Chancellor’ vines under 3 imposed canopy management treatments – low-intensity leaf removal, high-intensity leaf removal, and CT – and an un-treated control at the Lake Erie Regional Grape Research and Extension Center, we plan to:
- Quantify non-structural starches and sugars associated with bud cold hardiness between Nov. 2014 and March 2015
- Assess how canopy management practices affect grape and wine chemistry and wine sensory perception
- Perform an economic analysis on the cost and return of canopy management practice implementation
Head B, Ngugi HK, Travis JW (in press). Short- and long-term effects of leaf removal and gibberellin on Chardonnay grapes in the Lake Erie Region of Pennsylvania. American Journal of Enology and Viticulture. doi: 10.5344/ajev.2014.14034
The investigation of grapevine frost treatments is made possible by the John H. and Timothy R. Crouch Program Support Endowment, an endowment founded and funded by the Crouch brothers, original owners of Allegro Winery in Brogue, PA. Additional financial support is contributed by the Pennsylvania Wine Marketing & Research Board (PA WMRB). It is with great appreciation to investigate these projects based on funds and interest provided by the Pennsylvania wine community.
A big thanks to Don Smith for his technical support in our lab and Denise Gardner for her assistance in winemaking. Also, thanks to our wine grape grower collaborators. _________________________________________________________________________________________________
Maria Smith is a viticulture PhD student with Michela Centinari, specializing in cold stress physiology of wine grapes.