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Grapes are a unique agricultural product. While more than half go toward the production of wine, they are also grown to be dried into raisins or eaten fresh. Grapes command more return per acre than almost any other plant, and in 2018, a single hectare of grand cru vineyard in Burgundy cost over seven million dollars on average. Further, unlike many crops that are planted each growing season, vineyards are a long-term investment—they require several years to become established and are designed to survive for decades.
Unlike many commodity plants, the profitability of wine grapes is driven by quality, which includes the grape’s ability to convey a unique sense of place. While other agricultural crops look to new varieties for flavor improvement, disease resistance, and adaptations to climate, most wine producers rely on a small number of established cultivars. Site selection and vineyard practices, however, are critical, since improvement is achieved through management of the vine’s environment.
Grapes were one of the first fruits to be domesticated by humans. In ancient times, they were prized for their high levels of sugar, a source of both nutrition and novelty. Most of the grape varieties used in wine production belong to a single species, Vitis vinifera, which was first domesticated from wild grapevines, called Vitis vinifera subsp. sylvestris (or Vitis sylvestris), at least 7,000 years ago in the land between the Black, Caspian, and Mediterranean Seas. As nomadic people settled into an agrarian lifestyle, they carried grapevines south to Mesopotamia. Domestic vinifera grapes were spread from the Fertile Crescent throughout the Mediterranean and Europe, driven by the westward migration of farming communities and, eventually, the expansion of the Roman Empire. Vitis sylvestris is native to Europe and Western Asia, and wild grapevines still inhabit these areas. Some evidence indicates that there may have been other centers of domestication of Vitis sylvestris, including sites in the Iberian Peninsula and Southern Italy.
Over time, a collection of grape varieties was generated through the process of evolution, breeding, and human selection. Today, roughly 10,000 grape cultivars exist, with over 1,400 in commercial production, and grapegrowing has spread to hospitable zones throughout the world.
Grapevines are lianas: unlike trees, they do not produce extensive wooden support systems but are instead “structural parasites,” climbing on trees for support. They are also phototrophs, or sunseekers, and invest most of their energy into producing leaves and tall shoots, since rapid vertical growth is essential for competition with other plants for vital sunlight. In nature, grapevines invest little energy in fruit production, yielding just enough scraggly clusters to ensure proliferation. Wild vines are also dioecious, which means that both male and females plants exist, and successful fertilization relies on wind and insects for pollination. Male plants bare no fruit, and female plants are only fruitful when a male plant is nearby.
By contrast, since domesticated grapevines are cultivated for their fruit, they have been selected and managed to be prolific. Hermaphroditic, self-pollinating varieties were likely chosen initially, since these vines would have reliably produced more fruit. In addition to high yields, the selection of vines suitable for agriculture favored those with other beneficial characteristics, including large clusters, adaptations to the growing environment, and resistance to disease. The ability to attain the high sugar concentrations necessary for wine production, as well as taste, aroma, and appearance, also factored into selection. Analogous to a house cat and a lion, domestic vines have diverged significantly from those found in nature.
While the romantic notion of a vineyard paints it as a natural space with little intervention, it is more akin to a highly cultivated garden, organized for both ease of use and optimization of yields and fruit quality. Rows facilitate management and allow tractors and other equipment to access the vines easily. Grapes are propagated vegetatively, generally grafted to a different species’ roots, and trained into small shrubs to facilitate management. Annual pruning dictates the number of shoots that will form in the following year and where they will be located. Growers often impose moderately stressful conditions, such as limited water availability, to encourage the vine to limit its vegetative growth and concentrate its energy on fruit production. By taming and training them, humans have coaxed vines to defy their nature in order to be cultivated effectively for food and wine production.
Grapevines are perennial, deciduous plants that have a permanent woody frame consisting of a trunk, cordons, canes, and spur positions. Below the ground, an extensive root system anchors the vine and provides an interface with the soil, which supplies water and nutrients to the plant. A vine’s root system is mostly located within the top three feet of soil and consists of mature roots, which survive year to year, and smaller feeder roots, which grow anew each year. Often, Vitis vinifera is grafted onto a phylloxera-resistant rootstock. Grafted vines consist of an above-the-ground portion called the scion, which is joined to the rootstock at the graft union, visible a few inches above the vineyard floor. Some rootstock species develop deep root systems, while others grow more laterally.
A grapevine’s trunk is analogous to that of a tree; it’s the permanent, vertical structure. Cordons, canes, or spur positions may be attached to the trunk, though the vine’s form will ultimately depend on pruning decisions made during the first few years of the vine’s life.
Canes are shoots grown in the previous growing season that have lignified, or turned brown. After pruning, they are generally one to four feet long. Spurs, however, are canes that have been trimmed to a length of several inches. Cordons are horizontal extensions of the trunk and have a number of spur positions located along them. Along a cane, there are dormant buds, and spurs generally contain between one and three buds. During the growing season, these buds develop into fruiting shoots. Every few inches along each shoot, there are nodes, which resemble knuckles, and the portion of stem between nodes is called the internode. Leaves, buds, clusters, and tendrils are joined to each shoot at the node. Collectively, all of the vegetative green growth that develops during the growing season is called the canopy.
Two types of buds are located at each node, between the leaf and the stem: lateral buds and dormant buds. Each bud contains a highly compressed potential shoot. Lateral buds develop into shoots called laterals during the current growing season. These are side shoots that branch off of the main fruiting shoots. They are typically non-fruiting but may produce small clusters known as second crop. Laterals are often trimmed or removed through canopy management to prevent overcrowding and shading. Dormant buds, also called latent buds, spend the year maturing and develop into shoots in future years. As a rule, the dormant buds that formed last year, on canes and spurs from one-year-old wood, are the most fruitful. Dormant buds on older wood may also develop into unwanted shoots called suckers. Generally, suckers will not produce any fruit, and they are removed while they are small.
At each node, a single leaf develops, adjacent to the buds. Leaves are the powerhouse of grapevines, where photosynthesis takes place, and petioles are their stems, connecting leaf and shoot.
Clusters are located at nodes near the base of the vine. Most shoots contain between one and three clusters, with two being most common, though the typical number varies by grape variety. Carignan, for example, is known for producing three clusters per shoot. The area of the vine where the fruit is growing is described as the fruit zone. Long, thin coils called tendrils support the vine by wrapping around and attaching to trellises, trees, or other supports. Technically, they are modified flower clusters, though the two bear no resemblance to each other. Clusters and tendrils develop along each shoot in a “hit, hit, miss” pattern. The first few nodes closest to the base of the vine typically have neither. Then, every two nodes have either a cluster or a tendril, while every third node has neither, with the pattern continuing up the shoot.
Ampelography is the science of identifying grape varieties based on morphology. Clusters and berries vary in shape and size, and, along with leaf characteristics and the vine’s overall growth patterns, these attributes are used to identify grape species and varieties based on their unique patterns.
Plants create sugar from carbon dioxide and light through the process of photosynthesis, which takes place primarily in the leaves but may occur in any green plant tissue. Afterward, sugar is transported throughout the vine to be used for growth and development, and also into the fruit during ripening. Plants have a fluid transport system akin to veins in humans. The xylem carries water and nutrients from the roots throughout the vine, while the phloem carries sugar from the leaves throughout the plant.
A subsequent reaction, respiration, converts sugar into usable energy called adenosine triphosphate, or ATP. This reaction requires oxygen and releases carbon dioxide and is required for vine growth and development. It occurs in every part of the plant, including the roots, both day and night.
During photosynthesis, carbon dioxide in the environment is taken in through microscopic pores in the leaves called stomates. When the stomates are open, water vapor is released from the plant into the atmosphere through transpiration. During periods of stress, especially hydric stress, the vine will close its stomates to conserve water, halting photosynthesis and slowing respiration.
A vine with ample water and nutrients will develop a large canopy with fast-growing shoots. Vine vigor refers to the amount of vegetative growth produced by a vine, and it is assessed through several markers, including shoot length and diameter, the number of shoots per vine, and the vine’s tendency to produce laterals and suckers. Vigor may be quantified through pruning weight, which is literally the weight of the material that is removed from the vine at pruning, sampled across a selection of vines.
A vine’s capacity is the optimum amount of fruit, or yield, it is able to produce, given its specific conditions. Vines that carry too much fruit for their frame may not be able to successfully ripen it, especially in marginal climates, and will weaken over time, further reducing capacity. On the other hand, if too little fruit is left, the vine will become more vigorous, and the amount of fruit produced will gradually decrease. There is a general belief that balanced vines make balanced wines. Vine balance considers vegetative versus fruit growth. The Ravaz Index, the ratio of fruit weight to pruning weight, is one metric used for assessment. Ratios of 4 to 10 are generally considered balanced. Growers also look at the length of shoots and internodes, targeting three- to four-inch internodes and roughly four-foot shoots.
Growers coax vines toward balance through planting decisions, including choice of rootstock and trellis systems, as well as vineyard operations, such as pruning and irrigation. Balanced vines pay off, maximizing yields and fruit quality. Yet it’s important to recognize that balance can take many forms. Larger vines have more capacity for fruit production, and vines grown on fertile soils will have more vigor and thus more capacity than those grown on weaker soils. In this scenario, the vine may be more balanced carrying five tons per acre rather than half of that. Appropriate yields should not be prescribed without understanding the conditions of the site.
Vitis vinifera, also called the European grapevine, includes many of the wine and table grapes. It belongs to the family Vitacae, along with other common vine plants like Boston ivy and Virginia creeper. Most cultivated grape species belong to the genus Vitis and have 38 chromosomes, while others belong to the genus Muscadinia, formerly considered a subspecies of Vitis, with 40 chromosomes. Beyond vinifera, several other Vitis species are significant in viticulture. Vitis rupestris, Vitis riparia, and Vitis berlandieri are common rootstock species, and Vitis labrusca, Muscadinia rotundifolia, and Vitis amurensis are occasionally used in winemaking. Within each species, there are many cultivars, often called varieties in a wine context.
Both species and varieties have been interbred. The offspring of two varieties belonging to the same species are known as crossings, and examples include Chardonnay, Riesling, Merlot, and almost all other cultivars used in winemaking. The products of interspecies breeding are called hybrids; rootstocks and niche wine grapes like Rondo, Chambourcin, and Vidal Blanc are examples. Crossings and hybrids have been bred to incorporate the desirable characteristics of both parents. For instance, Norton is a hybrid that combines the cold hardiness and resistance to fungal diseases of Vitis labrusca, with a flavor profile more similar to vinifera. (Labrusca varieties are often marked by a grapey flavor described as “foxy” that is generally not preferred in wine.) Throughout history, grape breeding has occasionally been intentional but more often occurs in nature.
Crossings of Vitis vinifera are responsible for the tens of thousands of cultivars that exist. Pinot Noir, Savagnin, and Gouais Blanc are old varieties, believed to be closely related to Vitis sylvestris, and found in the lineage of many common European grape varieties. Most crossings arose naturally, but a few well-known examples are products of breeding. The teinturier grape Alicante Bouschet, whose durability and deep color made it popular with home winemakers during Prohibition, was produced by crossing Petit Bouschet, also an intensely colored teinturier with thick skins, with fruit-forward Grenache. Müller-Thurgau, once a very important variety in Germany, was produced from Riesling and Madeleine Royale in an attempt to develop an earlier-ripening grape with Riesling-like aromatics. South Africa’s signature grape, Pinotage, was bred to combine the elegance of Pinot Noir with the hardiness and productivity of Cinsault.
From an evolutionary perspective, vinifera grape varieties are organized into three proles, indicating the primary center of their cultivation, evidenced by their physical characteristics. Proles pontica, which includes Zinfandel, Furmint, and Vermentino, is native to the Aegean and Black Seas and has more jagged leaf blades; white hair on the underside of the leaves; mid-sized clusters; and small-to-medium, round berries. Proles occidentalis is native to Western Europe and includes most international grape varieties, such as Cabernet Sauvignon, Chardonnay, Pinot Noir, and Riesling. Occidentalis has convex leaves; small, compact bunches; and small, round berries. Proles orientalis is native to the Middle East, Iran, and Afghanistan and has large leaves, bunches, and berries with an oval shape. Muscat, Cinsault, and most table grape varieties are examples.
Grape varieties differ significantly from one another, both in terms of wine flavor and environmental adaptations, as a result of their unique genetics. Every variety is hardwired to produce different amounts of flavor, color, and tannin. The chemical pathways that create each of these may be upregulated, where the production of a compound is increased, or downregulated, where that production is decreased, in response to the environment. For example, grapes under water stress will produce more tannin than those with an ample water supply, even after accounting for the difference in berry size. Varieties also exhibit different behavior. Some go through budbreak a couple of weeks earlier than others, some require more heat accumulation in order to achieve ripeness, and varieties differ when it comes to yield potential, vine vigor, and tolerance to environmental stressors.
The grape varieties are usually divided into red (or black) and white grapes, though pink (or gray) versions exist as well, such as Gewürztraminer and Pinot Gris. White grapes can be further characterized as aromatic, partially aromatic, and non-aromatic, primarily resulting from the grape’s propensity to form monoterpenes, compounds responsible for flavors of rose, lychee, and orange blossom. Red grapes differ in their amount of color and its hue. In both cases, levels of acidity and tannin vary, and each grape has a unique flavor profile.
Clones are variants within a grape variety that differ slightly in terms of morphology or behavior. Grapevines are prone to mutations that arise from errors during cell division, and the genetic variation that results is the major source of clonal differences. Mutations can affect a single bud, leaf, or flower. When a bud is affected, the single resulting shoot may bear some distinction from the parent vine. Cuttings taken from this shoot would constitute a unique clone that may differ from the parent plant in terms of grape color, ripening dates, yields, berry and cluster morphology, and flavor characteristics. Viral infection also influences gene expression and is another source of clonal variation. The Gingin clone of Chardonnay that is popular in Western Australia, for example, was confirmed to have grapevine leafroll virus, believed to be responsible for some of its positive attributes, including low yields. When clonal selection is performed in a nursery, virus-infected vines are heat-treated to remove the virus before they are propagated and distributed.
Old varieties typically exhibit more clonal diversity. Pinot Noir is thought to be at least 2,000 years old, and as a result, many diverse clones exist. As mutations accumulate over time, significant changes may result in the mutant being renamed as an entirely different variety. Pinot Gris, Pinot Blanc, Pinot Meunier, and Pinot Teinturier are considered by many to be separate varieties, but each is technically a clone of Pinot Noir. Similarly, the highly aromatic, pink Gewürztraminer is a mutation of Savagnin Blanc.
Grape breeding is a slow, laborious process that relies on old-fashioned techniques. Parent breeds are selected and hundreds of offspring created through intentional cross-pollination. These new varieties are grown for several years, characterized, and selected for desirable traits.
Today, most breeding programs seek to create varieties that are tolerant to disease and better adapted to the effects of climate change, including drought. While new grapes struggle to gain commercial acceptance, researchers believe they still have a place in viticulture. In 2020, UC Davis released five new Pierce’s disease-resistant varieties with 97% vinifera parentage, created by Dr. Andy Walker. Although these grapes may not be accepted for winemaking, except perhaps as blending grapes, they could be planted around the perimeter of an existing vineyard to shield it from intruders.
Hybrid grape varieties allow for viticulture in environments where grapes would not otherwise grow successfully. In America in the early and mid-1800s, vinifera was interbred with native American grape species like Vitis labrusca and Vitis aestivalis that are better adapted to the cold winters and humid, disease-prone summers of much of the Eastern United States. The resulting hybrid varieties include Clinton, Concord, Catawba, Delaware, Herbemont, Isabella, Niagara, Noah, and Norton. While some of these are used for wine, many are considered better suited to fruit juice and jam on account of their foxy flavors. Hybrids tend to be high yielding, and the resulting wine is generally regarded as inferior to that of pure Vitis vinifera.
After the introduction of phylloxera and powdery and downy mildews to Europe, French researchers looked to hybrids to instill pest and disease resistance until better treatments were found. Beginning in the late 1800s, a large number of French hybrids were generated, including Baco Noir and Blanc, Chambourcin, Chancellor, Couderc Noir, Plantet, Villard Noir and Blanc, Seibel, and Seyval Blanc. These played an important role in European wine production from the late 1800s until the mid-1900s. By the end of the 1950s, hybrid grapes covered one-third of France’s vineyard area. Subsidies encouraged producers to replant vineyards to vinifera grapes, and by the late 1980s, hybrid varieties accounted for only 3% of European production. Today, most hybrids are not permitted by the EU for PDO wine production, though exceptions exist. The German Rondo and Regent, used for their disease resistance and cold tolerance, are most common.
French hybrids like Vidal Blanc, Vignoles, Chambourcin, Seyval Blanc, and Maréchal Foch are more typical in vineyards in Eastern and Midwestern North America, along with newer varieties like Cayuga White, Chardonel, Frontenac, and Traminette, which were bred to withstand winter freeze. Japan’s signature grape, Koshu, is a vinifera-dominant hybrid crossed with the East Asian species Vitis davidii. Hybrid grapes are considered by some to be more sustainable, since many are disease resistant and require significantly less use of fungicides.
When a vine is grafted, some characteristics of the rootstock are conferred to the scion. While rootstocks were first developed for phylloxera resistance, today, they have other adaptations that may be beneficial to a vine, as they differ in terms of vigor, drought tolerance, resistance to pests and diseases, and adaptations to various soil conditions.
Most rootstocks are hybrids of non-vinifera grape species, especially North American varieties. Three species are frequently encountered: Vitis riparia, Vitis rupestris, and Vitis berlandieri. Other examples include Vitis champinii, Muscadinia rotundifolia, and Vitis solonis. Offspring of these species usually demonstrate characteristics inherited from both of their parents. By knowing the general attributes of each, the behaviors of their offspring can be better understood.
The environmental conditions within the vineyard play an important role in shaping wine expression. This phenomenon has been described as terroir. While terroir has been interpreted literally to refer to vineyard soils, most definitions have expanded to encompass the influences of climate, topography, human practices, and sometimes other external biological factors such as microorganisms and virus. Whatever the precise definition, terroir is broadly understood as the elusive quality that gives a wine a sense of place and makes it a more intriguing, unique product.
The vine’s environment fosters its growth and development. Because a vine is not able to move, it must instead adapt. These adaptations often manifest themselves as differences in fruit characteristics. As an example, water-stressed vines will develop a smaller canopy that provides less shade to the fruit. Along with a host of other differences in fruit composition, berries that develop in the sun will produce more “sunscreen” phenolic compounds. Fruit ripening dynamics, and the amount of sugar, acid, tannin, and flavor, are all impacted by environmental conditions. It is for this reason that wine is often said to reflect the place in which it’s grown.
Climate refers to the patterns and overall amount of heat, sunlight, precipitation, and wind that characterize a region. A related and often confused concept is weather, which describes these properties over a short period. Climate is the long-term average of weather over time. It is often separated into three spheres of influence: macro-, meso-, and microclimate. Macroclimate describes the climate of a larger region, spanning tens to hundreds of miles. While not well defined, mesoclimate identifies a smaller area, a single vineyard or a region that perhaps spans tens of miles and might be impacted by local geographical features like smaller bodies of water, topography, and soil conditions. Microclimate describes the environment directly around the vine and fruit. While this is influenced by the vineyard site, human practices such as trellis systems and canopy management play an important and often underappreciated role in shaping the environment.
Climate classifications consider patterns of temperature and precipitation to give a high-level synopsis of weather patterns and potential hazards. They are a convenient means of comparing regions to one another. While grapes are often associated with Mediterranean climates, some wine regions are better described as maritime, continental, or even subtropical. Mediterranean and maritime climates are moderate, with a small range between summer and winter temperatures. Continental climates have a more dramatic temperature swing throughout the year and experience the classic four seasons. Mediterranean climates have wet winters but receive little rain during the growing season, while maritime and continental climates receive rain year-round. Burgundy, Austria’s Wachau, and Mendoza are typically considered continental; Bordeaux, New Zealand’s Hawkes Bay, and Oregon’s Willamette Valley are maritime; and Tuscany, the Barossa Valley, and Stellenbosch, South Africa, are best described as Mediterranean. While labels are convenient, it’s useful to think of these classifications as a spectrum, with most regions falling in between specific definitions.
Vines are temperate plants and require a dormant season prior to budbreak. As a result, climates without sufficiently cold winter temperatures are not suitable for wine grape production. Tropical climates, for instance, have little temperature variation throughout the year and are not suited to wine grapes, but there are subtropical regions where grapegrowing occurs, including parts of eastern Australia, Madeira, and the Canary Islands. Often, grapes grown in these climates are made into fortified wines, where the effect of the vineyard site is arguably less important than the impact of winemaking.
In the EU, wine regions are classified into zones depending by climate, and certain practices including chaptalization, acid adjustments, and minimum potential alcohol requirements are governed by zone. Germany, the Loire, Champagne, Alsace, and Austria belong to Zones A and B, which are permitted to enrich wine by 3% ABV and deacidify, but not acidify. Portugal, Southern Spain, Southern Italy, and parts of Greece belong to Zone CIIIb. They may acidify, but not deacidify, and enrich to a lesser extent.
The Köppen-Geiger climate classifications divide regions into five main groups—tropical, dry, temperate, continental, and polar—and then further into subgroups based on temperature and precipitation patterns. Under this scheme, most winegrowing regions are categorized as temperate. While this is a very precise and well-defined index, it is seldomly referenced in regard to wine.
Differences in climate can be distilled into a few key properties that are fundamental to a vine’s development: heat, light, water, and nutrients. Without sufficient amounts of each of these, a vine will not be fruitful and, in extreme cases, cannot survive.
It has been said that temperature is the metronome of plants. Heat drives vine growth and development, and many of a plant’s metabolic processes are temperature dependent. In warm climates, vines grow and develop more quickly, and fruit ripens earlier. Vine growth occurs between 50 and 95 degrees Fahrenheit, where mid-70s Fahrenheit is optimal. At lower temperatures, vines are dormant, and at temperatures over 95 degrees, vine growth and fruit ripening may shut down to conserve water. In hot weather, the microclimate around the canopy may actually be significantly cooler than the ambient temperature, as the vine cools itself through transpiration provided that it has a sufficient supply of water. Frigid temperatures can lead to injury and even vine death if precautions are not taken.
Temperature affects both the quality and quantity of grapes. At bloom, it impacts the number of berries that will develop in the current growing season as well as the number of clusters that form the following year. Warmer temperatures result in higher yields, and vines will develop more capacity to support the additional crop.
Understanding a growing region’s temperature profile, which includes both the overall amount of heat and patterns of accumulation, helps growers predict which grape varieties will be most successful. Varieties differ in the amount of heat needed to ripen. It is often claimed that the best quality wine comes from marginal climates where heat accumulation is just sufficient to ripen the grapes, with classic illustrations being Pinot Noir in Burgundy and Riesling in the Mosel. Flavor profiles are impacted, too. Warmer climates tend to yield fruitier wines, with higher alcohol, lower acidity, and softer tannins, but overripeness is a risk if harvest occurs later in the season. In cooler climates, wine may have lower alcohol, higher acidity, more astringent tannins, and fresh fruit and savory flavors; in some years, however, wines may be underripe and lacking flavor.
Heat indices are used to guide varietal selection and to compare climates, estimating the amount of heat that accumulates throughout the growing season as the product of temperature and time. In the United States, the Winkler Index is frequently used to categorize viticultural areas with similar accumulation of “growing degree days” from April 1 to October 31. A region’s degree days are calculated by taking the average daily temperature minus 50 degrees Fahrenheit from every day within this range and summing them. The correction of 50 degrees is used to acknowledge that below this temperature, little shoot growth takes place. The Winkler Index is easily employed, but because it does not account for day length, it is not applicable to all regions. Elsewhere, the Huglin Index, which accounts for latitude, is more common.
While heat summation is a useful metric, the pattern of heat accumulation is also important. A moderate climate with a long growing season may experience the same overall heat accumulation as a warmer climate that has a short season, but each will impact the fruit differently. Two important concepts related to heat accumulation patterns are continentality and diurnal shift. Diurnal shift describes the difference between day and nighttime temperatures. In warm climates, a large diurnal shift is often thought to be important for wine quality as it seems to preserve acidity and flavors. In marginal climates, warm nights may assist in developing acid and flavors.
Continentality is the difference between summer and winter temperatures. Continental climates have wide temperature swings throughout the year and are more prone to spring and fall frost. Continentality can be assessed by comparing the average temperature during the warmest and coolest months of the year.
Sunlight is essential for plant growth. Light in the canopy fuels photosynthesis, which drives plant growth and development and creates sugar that facilitates fruit ripening. The number of sun-exposed leaves on a vine will determine its photosynthetic capacity, where more leaves results in a higher capacity for development, as well as greater water use. About 12 to 16 leaves are required to ripen a cluster.
Metered, or dappled, sunlight improves fruit quality and quantity. Shaded buds are less fruitful, but ample light exposure on the shoots increases yields in the following year. Shaded berries ripen more slowly, while berries with direct light exposure can reach high temperatures, which may interfere with ripening. During a heat spike in 2017, one Napa Valley vineyard observed temperatures in excess of 140 degrees Fahrenheit in sunlit berries. Though light determines the rate of photosynthesis and therefore sugar accumulation in the fruit, other features of ripening, like acid degradation and tannin ripening, may be more tied to temperature.
Sunlight is typically considered important for flavor development. Light stimulates the production of phenolic compounds, like anthocyanin and tannin, that are considered key for red wine quality, as well as 1,1,6,-trimethyl-1,2-dihydronapthalene (TDN), the petrol flavor observed in aged Riesling. It also encourages the breakdown of pyrazine, the green bell pepper flavor associated with grapes such as Cabernet Sauvignon and Sauvignon Blanc. While sun exposure upregulates the production of certain flavor compounds, the increase in temperature can result in flavor loss, acid degradation, and sunburn.
The duration of sunlight during the day (sunshine hours) and its intensity influence vine and fruit development. Vitis vinifera is said to require at least 1,250 sunshine hours to ripen fruit. Higher-latitude regions have longer days and receive more sunshine hours, while the sunlight intensity is greater nearer to the equator. Sunlight intensity also increases with elevation, but cloud cover, pollution, and smoke can reduce the amount of sunlight that reaches the vine.
Too little water can stunt growth and development, limit yields, and delay ripening. Under severe water stress, vines close their stomates to conserve water, halting photosynthesis and plant function. Extreme drought conditions result in defoliation and, eventually, vine death. Yet wet conditions can result in excessive yields and slow ripening and encourage the vine to produce a big, vigorous canopy.
Timing is also important. An adequate amount of available water is desired early in the season so that shoots reach their full height prior to veraison, when berries change color. Once berries have formed, mild water stress helps to maintain a moderate berry size and promotes the production of phenolic compounds, which are considered integral to red wine quality. Near the end of the season, water deficit can cause dehydration, but rain near harvest can cause berries to swell and split, resulting in dilution and increased disease pressure. Late-season rain frequently reduces wine quality.
The amount of water available to the vine depends on soil conditions. Soil has a limited capacity to hold water. Heavy rainstorms can deposit a lot of water, but it may not be accessible to the vine, as some is lost through drainage or run-off. On deep soils, well-developed and deeper root systems allow a vine to source water from a larger volume of soil; shallow soils are more limited in their capacity.
The impact of water availability on vine and fruit development, and especially on wine quality, is one of the most important topics being studied in viticulture today. By providing just enough water when the vine needs it, viticulturists hope to improve wine quality and conserve precious resources.
Along with windbreaks, other protective measures can be taken in windy climates. In Provence and the Southern Rhône, the vineyard rows may be planted parallel to the prevailing wind, with vines trained low to the ground, in order to minimize damage. In parts of coastal California, some producers have observed that rows planted perpendicular to the wind will “self-shelter,” resulting in higher sugar accumulation. Regions like Greece’s Santorini and Lanzarote in the Canary Islands have developed novel vine training systems for wind protection.
Wine grapes generally grow between 30 and 50 degree in latitude. In lower latitudes, the vines don’t experience a dormant season, while higher latitudes are often too cold for grapes to attain ripeness, or vines may be threatened by winter freeze. Both temperature and sunlight intensity are generally higher for regions closer to the equator. Those regions further from the equator often have shorter growing seasons but longer days, which accelerates growth and development. Marginal climates may also rely on other influences that increase their viability. For instance, higher latitudes often rely on warming from bodies of water, favorable orientations, and warm air currents, while lower latitudes may benefit from cooling influences like high elevation.
Hills and mountains can result in significant climatic diversity. The first relevant factor is altitude, which tends to reduce temperature but also increases sunlight intensity. Roughly, for every 300-foot gain in elevation, the temperature will decrease by about 1 degree Fahrenheit, and for every 1,000-foot increase, there is a 2% increase in sunlight exposure. Because cold air sinks, lower-elevation bowls trap cold air and may be more frost prone. Gravity causes soil and water to run downhill, so the bottom of the hill typically has deeper soil and more available water. Mid-slope sites are often considered best for wine quality, as they seem to have an ideal balance of soil and water conditions, along with favorable airflow to prevent frost and disease.
Some parts of a hill are warmer and sunnier than others. Slope, or the degree of incline, is an important factor. In Côte Rôtie, inclines can exceed 55 degrees. Vineyards in the Mosel reach 70 degrees—these are considered the steepest in the world. While the sun’s position changes throughout the day and year, steeper slopes will intercept the most sunlight on average and tend to be earlier ripening. (Solar panels are positioned at an angle for the same reason.) In Burgundy, grand cru vineyards are often located mid-slope, on the steepest and earliest-ripening part of the hill.
Aspect, or orientation, is the cardinal direction that the vineyard faces. In the Northern Hemisphere, south-facing vineyards intercept the most sunlight during the day, are warmer, and usually ripen earlier than north-facing vineyards, which tend to be the coolest sites. East-facing vineyards get more morning sun, reducing early morning humidity and thus minimizing disease pressure, while west-facing vineyards are exposed during the most intense part of the day, making them more prone to sunburn. Historically, south- and southeast-facing vineyards were preferred, as these conditions facilitate ripening. Mountain ranges tend to have a windward side that experiences more weather and precipitation, with a leeward side that is drier and more protected. Alsace and Mendoza, for instance, are both located in the rain shadows of nearby mountain ranges and receive relatively little precipitation.
Fog is a hallmark of many classic winegrowing regions, including Piedmont, Napa and Sonoma Valleys, and Chile’s Casablanca Valley. It generally results when warm, humid air encounters cooler air. Fog moderates temperature and can reduce the amount of sunlight reaching the vines. It also increases disease pressure. Regions that are known for botrytized wines production, such as Sauternes and Tokaj, rely on humid morning conditions for the development of noble rot.
Many historic winegrowing regions are situated along bodies of water, as this positioning provided a means of transport as well as groundwater or a source of irrigation. As with hills, where a vineyard lies in relationship to an ocean, lake, or river will affect its climate. Water has a large heat-holding capacity and changes temperature slowly. As a result, proximity to bodies of water results in a more moderate temperature range on both a daily and annual basis. Water can also reflect sunlight onto the vines, helping vineyards to ripen earlier.
Air currents that move along water can bring cold or warm air into a region and create fog and mist that reduce the amount of sunlight reaching the vines. Viticulturally important examples include the cooling Humboldt Current off of Chile and Benguela Current in South Africa. The Gulf Stream warms much of Northern Europe, allowing grapes to ripen in locations where they otherwise might not.
The wine business tends to be fixated on the weather due to its profound impact on vintage variation. Because of this, grapes are considered a more sensitive barometer of climate change than other crops. The industry has collected detailed records that illustrate changes over the past 50 years, including in heat accumulation, temperature extremes, and rainfall patterns. In some areas, drought and fire are more rampant than in the past. Changes in climate could redefine quality potential and wine style in classic regions throughout the world. Thus far, they have benefited some areas. Southern England was once considered unsuitable for grapegrowing but is now showing promise for sparkling production. Classic regions in Italy, France, and Germany are producing great vintages more consistently.
Alongside any effects of climate change, significant adjustments in viticultural practices over the past 50 years have also played a role in shifting wine styles. Not long ago, many regions struggled to adequately ripen grapes. As a result, viticulture developed practices specifically intended to accelerate the ripening process, and these were widely adopted. Producers developed more efficient canopy architecture, reduced yields, irrigated less, adopted earlier-ripening clones and rootstocks, and removed diseased vines that delayed ripening. The effect of these changes on grape ripening and wine style should not be underestimated.
Today, producers are looking to viticultural practices to slow ripening. Cooler sites that were historically less desirable, like those with a north-facing aspect, may be preferred in the future. In some cases, producers are even looking to new varieties; in Bordeaux, a proposal to allow seven new grape varieties in AOC wines was put forward in 2019.
Spring frost can kill young shoots, and while new shoots will often replace the lost ones, their development is delayed and they are typically less fruitful. Frost that occurs in the fall prior to harvest will kill the leaves, which prevents the vine from being able to ripen fruit further. In this case, the fruit will not improve and should be picked right away. These frost events are often the result of an inversion layer, where cold air near the ground is trapped under warmer air.
There are several means of frost mitigation:
Winter freeze can cause damage to dormant vines if temperatures fall below 5 degrees Fahrenheit. The methods described above only increase the temperatures slightly so are insufficient for protecting against winter freeze. Most vinifera vines can survive until 0 degrees Fahrenheit, but much below this, they risk death. In regions where winter freeze occurs, cold-tolerant varieties like Riesling and select hybrids may be planted. Otherwise, vines are buried each year for insulation and uncovered the subsequent spring. Recently, some producers have begun covering the vines in geothermal, geotextile blankets as an alternative means of freeze protection.