May 2018 Issue of Wines & Vines

Cold Hardiness of Grapevines

How growers can encourage it given the limits of nature and nurture

by Paolo Sabbatini, G. Stanley Howell and Josh VanderWeide

In his book on viticulture, Pierre Galet noted that the grape botanical family (Vitaceae) exists in the fossil record in the Early Cretaceous Epoch (66-145 million years ago), with examples of the genus Vitis found in the Early Eocene era 34-56 million years ago.2 Geologists tell us that this was a time of the super-continent Pangaea, whereby plate tectonics subsequently drifted into a broader array of climatic regions, which placed further selection pressures on the offspring of those species.

Vitis survived in nature because natural selection allowed their genotypes to adapt and succeed in their environment. The genotypes best "fitting" that environment of growth produced more viable seeds and favored the resulting offspring. Grapes, as food, attracted animals to consume the berries and deposit seeds in other locations where natural selection continued, and processes of adaptation occurred and resulted in the array of Vitis species.
These species range in diversity from acidic soils (V. rupestris, V. riparia) to alkali flats (V. Berlandieri) and sub-tropical climates (V. vinifera) to the -40° C line (V. riparia; V. amurensis) common to many hardy woody perennials.3 Clearly, grapevines existed alongside dinosaurs and millions of years before our genus of hominids migrated out of Africa and began to play a part in the gathering and culture of the fruit and processing of grapes into wine.
     The evolution of Vitis contributes considerably to our understanding of the series of adaptive grapevine physiological processes we collectively call cold hardiness.4 What is grapevine cold hardiness? Cold hardiness describes the ability of a grapevine to perform the following processes relative to its specific location:
     a) mature and initiate bud, cane, and perennial vine tissue prior to the earliest autumn freeze episodes             (acclimation);
     b) achieve adequate levels of freeze damage resistance during the coldest winter freeze episodes;
     c) display resistance in response to dormant season warm temperature fluctuations that cause reduction           in freezing resistance (mid-winter maintenance); and finally,
     d) slowly decline from maximal mid-winter hardiness as local conditions become more favorable for bud             burst and shoot growth after the final spring frost (de-acclimation).
     Thus, cold hardiness is a localized response of the vine to its natural environment. Despite a cold-hardy vine's ability to withstand low temperatures, severe mid-winter freeze events can injure tissue. In addition, a freeze event occurring outside of the normal winter season, either during the hardiness transitional periods of autumn acclimation or spring de-acclimation, may also lead to damage. Regardless of when they occur, freeze events make the assessment of vines for cold damage an inevitable and common practice in cool-cold climate viticulture.
     Grapevine cold hardiness is comprised of anatomical, cellular and biochemical characteristics that are controlled by the grapevine genome. These intricate factors associated with cold hardiness can lead to elastic and plastic responses to cold stress. The responses vary relative to the vine's annual cycle, the frequency of freeze episodes during the dormant season, and with the degree to which adaptive genetic processes can be fully expressed.
     It is not surprising that attempts to define cold hardiness, whether within the vine, as a response to variation and timing of weather conditions, or as a relation between the two are very complex. A large variation in cold hardiness is seen between different cultivars, as well as within a single vine.6 On the same date, differences as large as 15° C have been measured from the same vine. Thus, methods of evaluating hardiness of vine tissues must be precise. This is yet another reason why there can be wide differences in the hardiness level attributed to any specific cultivar (see the table on page 86).
     The cold hardiness of dormant buds is critical to the development of more viable seeds. Each dormant bud belonging to any species of grapevine has three potential shoots. The primary bud is the largest and most anatomically advanced of the three. However, it is also the least hardy and most susceptible to winter conditions. A healthy primary bud has the greatest capacity to produce berries with seeds (see the photos A to C below). Its loss, whether by winter cold, spring frost or insect predation, shifts production possibilities to the secondary shoot, which bursts later in most seasons and has a reduced capacity for fruit and seed formation. Should the secondary be lost, the tertiary shoot, while typically fruitless, can still produce a canopy and allow the perennial vine to survive another year.

Carbon assimilation and distribution and vine cold hardiness: source-sink interaction
In nature, the key process for perennial plant survival is carbon fixation via photosynthesis, and the distribution of carbon-based molecules to important locations within the vine. This process is called "carbon partitioning," and the specific locations, termed "sources" and "sinks," control carbon partitioning. M. Keller provides an excellent explanation of the kinetics of assimilate partitioning: they are dynamic, sensitive and hierarchical.7
     The influencing factors are:
     a) Proximity: it is advantageous to be close to the source (leaf);
     b) Connection: it is advantageous to be directly connected to the source;
     c) Interference: a loss of vegetative sink (through shoot tipping and hedging) can positively influence                 "cross transfers" advantageous to other sinks;
     d) Communication: vines;
     e) Competition: vines; and
     f) Development: vines.
     As Keller states succinctly: "For an organ or tissue to be a strong sink, it pays to be large, close to the source and to have good connections." Mature leaves and storage tissues such as roots and trunks are sources, which allocate carbon to developing tissues, termed sinks. These include young leaves, shoot and root tips, and fruit-bearing clusters, which utilize this incoming carbon for growth and seed development.
     These sinks are found in various locations throughout the vine, and the strength of a given sink may change during the annual cycle, and by season.1 With this in mind, genes controlling vine hardiness can only be fully expressed after the metabolites and building blocks necessary for continuing the life cycle have been produced at adequate levels. Consequently, a goal of viticulture practices in cool climates must be to meet the needs of the weakest sink in order to achieve maximum gene expression in vines.
     Previous work at Michigan State University has demonstrated that cold hardiness of primary buds was impaired the following winter more by varying levels of leaf removal at veraison than Brix accumulation in the current season or bud fruitfulness the following season. This adds to the concept that bud cold hardiness is a relatively weak sink, and any management practice which lowers the rate of accumulation or distribution of fixed carbon would have a detrimental effect on primary bud hardiness.8 This has important implications for viticultural management in cool-cold growing regions.

As viticulturists, we nurture grapevines to achieve desired economic results: consistent annual yield and desired fruit quality. In cool climates, nurture requires that viticulture be utilized to fully express the cultivar's genome for all the vine hardiness factors. Cool-climate viticulture is a culture of limits, and economic success will depend upon the degree to which these limits may be ameliorated or eliminated through our choices as viticulturists.
     One seldom mentioned valuable choice is rejection. Once a choice is made, it must stand up to continued evaluation and if lacking, be rejected. Some of these nurture components are within the control of the viticulturist, and some are not. In the Great Lakes Region, variation in growing season length, growing degree days (GDD) accumulation, and precipitation amount and timing, differ annually. While these are not within our control, they may be ameliorated.
     An important question for the viticulturist seeking to maximize the expression of innate levels of cold hardiness is: "when do vegetative tissues and next season's buds ascend to apex sink status?" Since the fruiting cluster is the apex sink for most of the growing season, it is crucial that the vegetative tissue not inadequately compete with the cluster for available assimilate.
     If there is one term that underpins successful viticulture, it is "balance." Various authors, beginning with M. Louis Ravaz in 1911 and more recently Richard Smart in 1985 and Stan Howell and R.P. Smithyman in 1998 have discussed concepts of balance. Those discussions have focused on vine cropping, vegetative growth, and fruit composition and quality. The concept of balance may be effectively applied to many aspects of viticulture. The following is a listing of decisions which must be made both before and after planting, along with a comment regarding the impacts of each on vine expression of genes for cold hardiness.
1. Site selection: A potential grower must become aware of the abiotic limits for a specific location. Most important are macro- and meso-climates. Site selection is an especially important issue for regions where freeze damage is an annual concern.
2. Cultivar selection: The process of choosing cultivars for a particular site must first consider the site selection. Annual winter temperatures of -35° C, occasionally experienced in cool-climate growing regions, limit the available varieties to those which are cold-resistant, and can withstand such conditions. In addition, the rootstock selected must consider a given cultivar's susceptibility to phylloxera, and ability to achieve desired vine vegetative vigor.
     The time frame of ripening for a cultivar should also be considered. Late bud-burst and late ripening cultivars are often required in cool-climate regions to avoid late spring frost damage, but often require low cropping levels, which may lead to greater canopy vigor and growth. Resulting canopy shading can delay fruit ripening, increase disease incidence and decrease bud and cane wintering quality.
     The parameters necessary for survival of many cultivars have been established and researched, including growing season length, variability of length and GDD. The desire to plant a given cultivar, then, requires extensive measurements and knowledge about the capabilities of a site selection regarding these parameters.
3. Vine spacing and training system: The space given between vines, width between rows, and training system utilized all have an impact on sun exposure and productivity. Numerous variations exist between the three which can lead to differences in leaf exposure within a canopy. These are also critical components of the yield/unit land area and the berry composition and quality issues required for economic success.
4. Water and nutrition: Excessive water and nutritional stress will reduce photosynthesis while reducing carbohydrate accumulation and consequently cold hardiness. Too much water and mineral nutrition, in particular nitrogen, can extend vegetative growth at the expense of carbohydrate storage and therefore vine cold hardiness. There has been some suggestion in the literature that K and/or Ca applications can improve vine hardiness. To date studies have not shown any benefit of such applications when the elements are in the non-deficient range.

Management issues: establishment and production issues
Management goals regarding cold hardiness should be focused on sunlight penetration to leaves associated with shoots that will be retained as bearers for the following season's production. Cultural manipulations should be introduced to create adequate exposed leaf area to mature the crop, mature the canes that will be the next season's bearers, distribute carbon fixed via photosynthesis to perennial vine structures and for storage in roots to initiate the spring growth flush and complete the differentiation of cluster and floret structures for the coming season. In short, the task of the viticulturist is to manage vines in a manner that allows for maximum expression of the cultivar's genes for hardiness.
     As noted above in the discussion of cultivars, there are a number of cultural conditions that can reduce the vine's ability to express its genetic capacity for hardiness. Excess crop is one of these. Carbohydrates are the energy which drives the metabolic machinery of the vine and that includes cold hardiness. The vine will preferentially send carbohydrates to ripening fruit and the potential for excess crop coupled with a short period between harvest and leaf loss makes careful crop control a major component in any effort to favor vine hardiness.
     We expend considerable money and labor creating and maintaining mechanical systems to which we attach grapevines. Viticulturists do so in order to create an effective "light trap." Training vines to this structure, and the subsequent manipulation of the canopy, are all done with a goal of facilitating the penetration of light into the fruiting and renewal zone. The decisions on which to base this decision for a specific cultivar are related to vine vigor and resulting vine size (as measured by the weight of annual cane prunings).
     These are important bases for determining whether a single or divided (either horizontally, as with Geneva double curtain or lyre, or vertically as with Scott-Henry, Smart-Dyson or Sylvoz) canopy will serve best. The best choice for hardiness is the one that favors photosynthesis in the renewal zone with resultant maximal darkness of cane periderm color for that cultivar (see the photo on page 84).
     Leaf removal and shoot positioning also aid light penetration into the fruiting and renewal zone and will have impact on cold hardiness similar to that of training system choice and for the same reason.
     Pruning - an annual exercise - is the first effort that has impact on the amount of crop that the vine may produce. Pruning has a growth-stimulating influence on vines. This means that viticulturists would prefer to do the practice as late in the dormant season as is possible and still get the effort finished before bud burst. Practical considerations often dictate that the effort begin prior to mid-winter.
     This dilemma can be resolved by selecting the most-hardy cultivars that are least responsive to winter temperature fluctuations and begin the effort with them. Least hardy cultivars will be pruned last as that will also allow a final, pre-pruning assessment of bud and cane survival and can help determine the amount of "spare parts" to retain.

Many factors will influence whether vines will be damaged by winter freeze episodes. Many are beyond the control of the viticulturist once planting has occurred. No single decision by the prospective grape grower can be more important than choice of site, with specific emphasis on macro- and meso-climate. Rationally, we must start there. It makes sense to begin with decisions that will be most difficult to change once the vineyard has been established.
     Increasingly difficult would be the necessity to change cultivar. Replanting means a loss of at least three to four years of production while the conversion occurs, not to mention the cost of vines, plus the labor for planting and training those vines. However, the most expensive change will be the result of a rational rejection of the selected site for culture.
     Vineyard site selection and retention, like all real estate value judgments, revolves around three overriding principles: location, location, location. Spending time to evaluate locations for potential grape production is the single-most valuable effort made in vineyard establishment and indeed for the long-term economic survival of the vineyard. The quality of this initial effort can save or cost much money and can determine whether the enterprise has a long-term future.
     In conclusion, the ideas here reinforce the importance of effective, timely application of cultural practices appropriate for the location of culture and the cultivars being grown. Nurture cannot produce hardiness levels above those made possible by the vine's genome. However, inadequate, poorly-timed application of practices can negatively influence the expression of the genes the vines possess for cold hardiness.

All three authors are located at Michigan State University. Paolo Sabbatini is associate professor of horticulture, G. Stanley Howell is professor emeritus of viticulture and enology, and Josh VanderWeide is a graduate student in the department of horticulture.

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