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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.
Caption: A small lateral shoot and dormant bud are visible in the “elbow” at the node between the leaf and main shoot (Photo credit: Jennifer Angelosante)
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.
Sidebar: Grapevine Function
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.