November 2011 Issue of Wines & Vines

Napa Valley Viticulture: A Farmer's Outlook

by Richard Mendelson and Robert Steinhauer

    1 Richard Mendelson is a lawyer specializing in vineyard and wine law with the law firm of Dickenson, Peatman and Fogarty in Napa, California.  He directs the Wine Law and Policy Program at the University of California, Berkeley School of Law, and is past president of the International Wine Law Association.
    2 Robert Steinhauer managed vineyard operations at Beckstoffer Vineyards (1971-1979) and Beringer Vineyards (1979-2004) and is now a viticultural consultant with and co-owner of Wineland Consulting, LLC.  He is a past president of Napa Valley Vintners and in 2008 received the Merit Award from The American Society for Enology and Viticulture.
    3 The authors wish to acknowledge and thank Hal Huffsmith and Tucker Catlin for their peer review of this article.
    4 In 1978, the U.S. government began the process of recognizing American viticultural areas (“AVAs”), defined as delimited grape growing regions with geographical features (soil, climate, elevation, physical features, etc.) that distinguish the area viticulturally from surrounding areas [10].  An AVA can be used on a wine label if, inter alia, at least 85 percent of the grapes in that wine were grown inside the AVA.  Two AVAs, Carneros and Wild Horse Valley, are located partly in Napa County and partly in neighboring counties (Sonoma County and Solano County, respectively). 
    5 California has lost some of its historically important coastal vineyards to urban and suburban expansion, among them the vineyards of Santa Clara, Alameda and Los Angeles counties. 
    6 The California State Water Resources Control Board under the authority of the California Water Code Sections 100 and 275 (2008) controls and issues the permits for water diversions from Napa streams and for storage of that water.
    7 If market conditions change or if the grower’s initial selection of variety or clone was inadvisable, the grower has the option, in a viable vineyard, to bud or graft on a new variety or clone.
    8 See
    9 Fish Friendly Farming is a certification program for vineyards that are managed to restore fish and wildlife habitat and improve water quality.  See
    10 The California Department of Pest Regulation (DPR) protects human health and the environment by regulating pesticide sales and use.  Before a pesticide can be sold or used in the state, it must be evaluated and registered by DPR.
    11 A pruning trial conducted in Napa Valley utilizing Cabernet Sauvignon grafted to 110R rootstock resulted in significant differences in sensory attributes. That seemed to defy the conventional wisdom.  The low yields of 1.74 tons per acre (4.3+/ha.) resulted in “veggie aroma and flavors, bell pepper aroma, bitterness and astringency,” while the high yield wines of 8.98 tons per acre (22.2+/ha.) were “higher in red/black aroma, jam aroma and fresh fruit aroma” [8].  Other research has pointed out the need to achieve balance between capacity (yields) and vine vigor [1, 56].
    12 A radiational freeze occurs when a cold but dry (low dew point) air mass moves into the valley.  Temperatures can be in the 60s in degrees Fahrenheit in the afternoon but after sunset radiational cooling of the soil surface causes temperatures in the lower atmosphere to fall, sometimes by 30º F or more.  Temperatures a few hundred feet above the ground remain fairly constant, forming an inversion (temperature increase with elevation).
    13 An advective freeze occurs when a cold air mass from the arctic regions migrates into the Napa Valley.  The temperatures at the vineyard are below freezing and become colder at higher elevations.
This article by Richard Mendelson and Robert Steinhauer, two well-known wine industry experts, appeared as a chapter in “The Science of Viticulture: Volume 1” (K.V. Peter, editor), published last year by New India Publishing Agency.

California’s Napa Valley, situated at the northern end of the San Francisco Bay 30 miles from the Pacific Ocean, is the most famous wine appellation in the United States. This small valley, 35 miles long and five miles wide at its broadest point, has produced some of North America’s finest wine grapes from the 1850s to the present. Today it contains approximately 45,500 acres of vineyards along the valley floor, in the foothills and mountains flanking the main valley and in the smaller valleys in the eastern part of Napa County. [45]

The uniqueness of this vineyard landscape is determined, in large part, by the great richness and variety of its geology, soil types, microclimates and topography, all of which affect viticulture. [19] The recognition of Napa Valley as an American viticultural area in 1981 and the subsequent establishment of 13 sub-appellations wholly within Napa Valley1 are evidence of this diversity.

The Valley’s vineyards and wineries have maintained a remarkable continuity in the face of cyclical market forces, recurrent threats from vine pests and diseases and the unique American experience of Prohibition. These cycles have directly affected viticulture, often adversely, but they also have provided an opportunity for innovation and improvement which are hallmarks of American viticulture generally and Napa Valley viticulture in particular.

In this article, we examine modern day viticulture in Napa Valley. Following a general introduction to the Valley’s natural conditions, we address the human factors—site selection, planting material, vineyard layout, infrastructure and cultural practices. Although none of these practices is legally prescribed as part of the American appellation system, there is widespread agreement today in the viticultural community of Napa Valley about best farming practices which are chronicled below.

A century and a half of grapegrowing and winemaking
Vineyards were introduced to Napa Valley with the earliest European settlement and within a half century came to dominate the Valley’s landscape, as they do to this day. The first vines were planted in Napa Valley sometime between 1838 and 1845 by settler George Yount. [29] The discovery of gold in the Sierra foothills in 1848 brought a large number of immigrants to California, including many from the wine regions of Europe who were familiar with grape growing and winemaking. Thirsty miners packed into the new city of San Francisco and provided a ready market for domestic wines. By the mid-1850s, there were regular, commercial shipments of wine from Napa to San Francisco. [29] The completion of a railroad the length of the Valley, from the city of Napa to the town of Calistoga, in October, 1868, made Napa Valley wines even more accessible to the growing city of San Francisco.

Vineyard and winery development increased during the 1870s and early 1880s until, in 1884, the relatively small County of Napa surpassed Los Angeles County as the state’s leading producer of dry table wines. [29] By 1886 the Valley had an estimated 175 wineries. The vast majority of the early vineyards in the County were planted on the valley floor, but as early as the 1870s, some vineyardists planted hillside vineyards in the quest for higher grape quality and to avoid spring frosts. The Valley’s reputation for fine wines spread around the country and the world. At the 1889 Paris World’s Fair, the Valley’s wines captured 20 of 34 medals awarded to California wines. [28]

Phylloxera ravaged the Valley’s vineyards in the 1880s and 1890s. Although Phylloxera resistant rootstocks were identified and vineyards were replanted beginning around the turn of the century, the specter of Prohibition presented another threat to the wine economy. In 1919, the Eighteenth Amendment to the United States Constitution was passed forbidding the manufacture, sale or transportation of alcoholic beverages. Interestingly, viticulture continued during Prohibition with grapes sold and shipped by rail to home winemakers; each family was allowed to make 200 gallons for its own use.

The Twenty-first Amendment to the United States Constitution ended Prohibition in 1933. Yet the revival of the United States wine industry was slowed by World War II. Only a handful of new wineries opened their doors in the Napa Valley between the end of Prohibition and 1966. Since that time, however, the Valley’s wine economy has burgeoned. Today the Valley is home to around 400 wineries.

Geographic specificity and viticultural uniqueness
The complex geology of the Napa Valley is determined, in large part, by its location on the seismically unstable western edge of the North American continent. The Franciscan Formation, the Great Valley Sequence and the Sonoma Volcanics make up most of the bedrock elements that determine the chemical and mineral elements of the Valley’s soils. Seismic forces along the San Andreas Fault brought these three elements together and formed the Mayacamas and Vaca Mountains that border the main valley on, respectively, its western and eastern sides. The chemical and mineral content of the bedrock affects not only the nutrient status of soils but also their ability to retain water and the chemical content of the groundwater upon which most of the Valley’s vineyards depend for irrigation. [67]

The extreme diversity of the 33 distinct soils series which have evolved from these bedrock components is attributable to the complex interplay of the Valley’s varied topography, climate and biological resources. [59] The small size of the Valley accentuates the effect of its topography in relationship to the climate.

Climate in the Valley varies along a complex set of variables. These include the marine influences of the San Francisco Bay and Pacific Ocean to the southwest of the Valley, most apparent in the Carneros region, and elevation variations from the Valley floor to the crests of its mountain ranges. In general, warm dry summer daytime conditions are regularly followed by the influx of a cooling marine fog layer in the evening. A dry season typically extending from April to November allows vineyardists to carefully calibrate the water stress of the vines through irrigation and dry farming, adding another tool to determine grape quality. [60]

Because of the wide range of viticultural conditions and the lack of any governmental controls over grape varietal selection, Napa Valley is home to a wide array of grape varietals, including principally Cabernet Sauvignon (19,557 planted acres in 2010) and Chardonnay (7,000 planted acres in 2010).

Legal protections
Napa Valley is directly impacted by its proximity to a major urban center, the San Francisco Bay metropolitan area. It is likely that only the highest quality grapes, capable of making wines of exceptional quality, had any chance of inspiring the political actions necessary to protect this irreplaceable vineyard resource from unchecked suburban development and rampant tourism.2 The Valley’s vineyard and wine industry benefits from some of the United States’ earliest agricultural land protection laws and regulations.

In 1965, a California law known as the Williamson Act created a mechanism for individual counties to protect agricultural land through controls on local property taxation. [7] In order to enact these protections, counties were required to establish “Agricultural Preserve” districts, which Napa County did in 1968. [47] The Agricultural Preserve still covers most of the Valley floor, preventing individual parcels of less than 40 acres from being created and allowing property owners to pay taxes based upon the value of the land as farmland, rather than as land available for subdivision and urban development. In addition, most of the foothills and mountain lands of the county are zoned Agricultural Watershed and are subject to a minimum parcel size of 160 acres, which further restricts residential development potential in such areas. [50]

In 1983, Napa County adopted urban growth boundaries which limit the development of urban infrastructure outside established municipal boundaries. [46] In 1990, the voters of the county adopted, by initiative, restrictions on the conversion of agricultural land to urban uses, requiring until the year 2020 a county-wide vote on every proposal to redesignate agricultural lands to non-agricultural purposes or to change the building intensity of such lands. [43] That law has since been extended to 2053. [44] Also in 1990, County government began to regulate more closely the establishment of new wineries and the expansion of existing wineries, restricting their commercial and industrial operations on agriculturally zoned lands, further protecting the Valley from encroaching urbanization and from the pressures of tourism. [48]

In 1991, agriculturalists and environmentalists worked together to put in place regulations designed to minimize the environmental impacts of vineyard development. Inspired by the continuing development of hillside vineyards, these local regulations supplement state and federal environmental laws and mandate the inclusion of erosion control measures, riparian and wildlife corridors, protections for streams and wetlands and other environmental controls in vineyard development and operations. [49] The so-called Conservation Regulations apply to any new or redeveloped site with a slope of more than 5%.

Developing a vineyard: site selection, infrastructure and plant material
The diverse and complex soils of Napa Valley, combined with the wide range of climatic conditions, demand challenging viticultural decisions in the development of new sites and the redevelopment of existing vineyards. Experienced growers recognize that physical features and biological factors dramatically influence the wine’s structure and flavors so they strive to fully understand all the characteristics of the site in order to make the best decisions in planting a new vineyard.

In terms of climate, the breezes from San Francisco Bay make the southern end of the valley the coolest, with a warming gradient to the northwest. However, other climatic conditions also are important viticulturally, including wind direction and speed, slope, aspect, solar radiation, elevation, humidity and rainfall which varies from less than 20 inches per year in Carneros to more than 40 inches per year in Calistoga. Fortunately, there is sophisticated weather monitoring equipment throughout Napa Valley today that helps the grower to delineate weather conditions.

The diverse soils of the Napa Valley are the result of the interaction over time of climate, geology (specifically, tectonic or earthquake moving and shaking), topography and biology. Growers and their viticultural consultants typically conduct inspections to determine the soil characteristics of a particular development or redevelopment site. Soil pit analysis allows them to determine soil texture, physical restrictions, soil structure, mottling (anaerobic conditions indicating poor drainage), effective rooting depth, water holding capacity (including the need for subsurface drainage), the presence of pests such as parasitic nematodes and various chemical parameters. Figures 1 and 2. The chemical measurements generally include nutrient status, salinity and other chemical concentrations that affect soil structure or that may be toxic to vines.

Water availability is a critical site development parameter, particularly in water deficient areas. Both source (stream flow,3 run-off, drain tile and/or well) and chemical composition of the source water are important. For example, some wells are very high in iron which may precipitate and plug drip emitters; other isolated wells have high boron concentrations that can be toxic to vines.

Napa Valley grape growers have long understood that terroir, which is the synergy of the biophysical components of soil, geology, topography and climate, and the human factors of rootstock and varietal selection and cultural practices, gives a sense of origin and uniqueness to the resulting wine. With this knowledge and appreciation, the winegrower (a term of art that recognizes that wine is “grown” in the vineyard) uses his intuition, technical education and historical experiences to make the key decisions for a specific site development. These decisions include the need for subsurface drainage, whether and how to irrigate and frost protect, whether to add pre-plant nutrients like gypsum or lime and the depth and method of ripping (deep cultivation) and soil preparation. The grower also must decide on rootstock, grape variety (also known as scion or cultivar) and clone, row direction, vine spacing, trellis type and block layout to match soil types, including the location of roadways and avenues.

While all of these viticultural practices have been modified over time, the selections of rootstock, variety and clone have changed most dramatically in the past 20 years. Prior to 1989, most of the Valley’s vineyards were planted on the rootstocks St. George, 5C and AXR#1. Nearly 70 percent of Napa Valley vineyards were at one time planted on AXR#1 rootstock which eventually proved to be susceptible to biotype B Phylloxera infestation. Phylloxera root damage ultimately led to a decline in vine vigor and required major vineyard redevelopment. [23]

The replanting of the AXR#1 sites started around 1987. [72] A group of winegrowers hired Lucie Morton, a viticultural consultant, to review the French literature on twelve rootstocks. Although this information was never published, it was very useful in the selection of replacement rootstock. Dr. Jim Wolpert of the University of California, Davis, prepared a bibliography containing 200 technical papers on rootstocks from all over the world. [74] With such extensive technical information, supplemented by reports from European and Australian growers on the performance of the various rootstocks, the experienced winegrower today, assisted by trained scientists and consultants, is able to make informed judgments on rootstock selection. [37, 41, 53]

Generally, rootstock is determined based on soil type, soil chemistry and potential irrigation requirements along with the growth characteristics of the scion. The growers seek a balance between capacity (fruit yields) and vigor (vegetative growth). Another criterion for rootstock selection is resistance to disease and pests. For example, in sites where Fanleaf Degeneration, a virus disease, is carried in the soil, Dagger Nematode, Xiphinema index, acquires the virus while feeding on diseased roots and can transmit the virus to new healthy roots. [21] Fortunately, the rootstock 039-16, developed by Dr. Harold Olmo at the University of California, Davis, and released in 1988, has a high resistance to Dagger Nematode feeding and to date has prevented the expression of Fanleaf virus symptoms in the scion.

Other vineyard sites may have populations of other plant parasitic nematodes. These include Root Knot Nematode (Meloidogyne incognita and other species), Dagger Nematode (Xiphinema americanum), Root Lesion Nematode (Pratylienchus spp.), Citrus Nematode (Tylenchulus semipenetrans) and Ring Nematode (Criconemella xenoplax). [21] Considerable research is available to guide the grower in the selection of Phylloxera resistant rootstock that also will provide some resistance to these nematodes. [11] For example, Dr. Andy Walker of the University of California, Davis, and Dr. Howard Ferris have developed new rootstocks with broad and durable nematode and Phylloxera resistance. Five of these stocks are available to nurseries for commercial distribution. [70]

The grower’s selection of variety and clone is partially driven by market demand for various wine types.4 The selection of the best variety for the site is also a critical factor because it affects potential wine quality. Of the total Napa Valley grape acreage of 45,500 acres in 2010, Cabernet Sauvignon is the most widely planted varietal, comprising 43 percent of the total planted acres in the Valley. [45] This figure has risen dramatically since 1997, as the market demand for Cabernet Sauvignon wines has soared. Presently, there are only six red varieties and two white varieties of the 35 varieties planted in Napa Valley that each total over 1,000 planted acres Cabernet Sauvignon, Merlot, Pinot Noir, Zinfandel, Cabernet Franc, Syrah, Chardonnay and Sauvignon Blanc. [45]

Most winegrowers are careful to select varieties and clones that are disease free. Growers have known for many years that viral diseases can be spread by propagation materials so it is imperative that they select plant material that is disease free. [9, 33, 66] Winegrowers frequently consult the National Grape Registry, for the current status of viruses and the history of particular varieties and clones.

In addition, the winegrower must select a trellis system to support the vines and distribute the foliage and fruit so as to expose the leaves to sunlight and provide filtered shade the fruit. The trellis design also facilitates the harvesting operation, whether by hand or machine.

The most common trellis system in the Napa Valley is Vertical Shoot Position (VSP). This system, which is used at many different spacings, can facilitate both cane and cordon-spur pruning. Recently, the VSP system has been modified by some growers to include small cross arms for shade. Other common trellis systems include Geneva Double Curtain (GDC); modified lyre; and different forms of a “T” trellis. The GDC and modified lyre trellis types typically are found where the soil is deep and fertile; some growers prefer these trellis systems for vigorous cultivars like Sauvignon Blanc.

Head trained (Goblet) vines still exist in the Napa Valley. In fact, a few viticulturists prefer the head trained system for varieties like Zinfandel and Grenache.

Trellis systems in Napa Valley
Vine spacing in Napa Valley, both between rows and vines, has evolved to much higher densities than in years past. Row widths from six feet to eight feet and distances between vines from three feet to six feet are quite common. Due to excess shading, trellis height typically does not exceed row width. Planting densities range from 907 vines per acre to 2,420 vines per acre.

 A commonly used row direction, depending on site configuration and slope, is northeast by southwest. This row direction provides for moderate sun exposure on each side of the canopy and reduces the potential for sun burn and high fruit temperatures. [14, 65, 73]

The Napa Valley wine industry adheres to and engages in sustainable and environmentally sound farming practices. The Code of Sustainable Winegrowing Workbook presents hundreds of sustainable practices and provides a self-assessment tool that allows winegrowers to review their current practices, identify areas where improvement can be made and develop action plans to increase the sustainability of their operations. [13]

There are different but complementary methods of implementing sustainable farming. Many growers have become certified organic or biodynamic. The organic farms in Napa County in 2010 covered some 2,970 acres, representing 6.5% of the total 45,500 planted acres. [45]

Napa Green Certified Land program5 is a voluntary, third party certified program for Napa Valley growers and vintners who seek to restore, protect and enhance the regional watershed. The participants develop and certify plans for their respective properties using Fish Friendly Farming program guidelines6 that include wildlife habitat restoration, healthy riparian corridors and sustainable agricultural practices. The program covers not only vineyard land but also wild land, roadways and stream banks within the property. The Napa Green program is recognized and certified by National Marine Fisheries Service, the Napa County Agricultural Commissioner’s Pesticide Regulatory Program and the Regional Water Quality Control Board. Additionally, the United States National Resources Conservation Service is involved with the program through the development of conservation plans. Currently, approximately 44,966 acres of land are enrolled in Napa Green, representing nearly 10 percent of all the acreage in Napa County. Of the total enrolled acreage, 18,074 acres are planted to vines, 10,680 acres of which are now certified.

River restoration projects are common in Napa Valley as a way to enhance the environment for fish and wildlife. These projects generally follow the Fish Friendly Farming guidelines and are undertaken by individual winegrowers, often under the auspices of public agencies.

Most winegrowers have adopted integrated pest management (IPM) practices that integrate chemical, cultural and biological controls.7 The IPM approach involves basal leaf removal, the use of fungicides and pesticides that are effective but more environmentally friendly, increased farm biodiversity to favor beneficial organisms, vineyard floor management practices, insect mating disruption and biological insect controls such as the release of parasitoids and beneficial insects. [6, 12, 18, 21, 27, 40, 52] These practices are employed for a wide range of pest control.

The grower in Napa Valley, as in many other agricultural communities, must deal with a large number of pests and diseases. A complete summary of these insects and diseases is found in Grape Pest Management, Second Edition. [21] Support for the suppression and control of pests in California comes from licensed pest control advisors, ongoing research and the dissemination of technical findings by country agriculture extension personnel, universities and local colleges.

New invasive pests and diseases are a recurring problem in the Napa Valley. In the current decade, four insects have become a major concern—the Vine Mealybug, the Light Brown Apple Moth, the Glassy-winged Sharpshooter and the European Grapevine Moth. Regarding the first insect, a recently introduced and highly invasive Vine Mealybug, Plannococcus ficus (VMB), is requiring special control efforts. The vine and grape clusters are damaged by a build up of sooty mold on the bugs’ excreted honeydew which can damage grape dusters and berries. The insect is also a vector for grape vine leafroll viruses. The introduction of Vine Mealybug has added additional pressures on the Napa Valley winegrower. It is now well documented that leafroll virus disease is rapidly spreading in the Napa Valley and that mealybugs, which entered Napa Valley on propagation materials, i.e., green growing bench grafts, from nurseries in the San Joaquin Valley of California, have become an increasingly important vineyard pest. Reports in Australia, Argentina, Europe and South Africa substantiate the transmission role of mealybugs. [12, 22, 40, 54, 66, 68]

Light Brown Apple Moth (LBAM), Epiphyas postvittana, is a native of Australia and has been found in isolated vineyards in Napa, Sonoma and other California counties. The California Department of Food and Agriculture has imposed a quarantine that prohibits the movement of plants and plant parts unless the shipment is certified as “free from” the presence of LBAM. If LBAM is not eradicated and becomes established in the vineyards, it will increase control pressures for the growers. The quarantine is designed to prevent the establishment of LBAM as a resident pest in Napa Valley vineyards.

The Glassy-winged Sharpshooter (GWSS), Homalodisca corqulata, was first reported in Southern California in 1994. It is a native insect of the southeastern United States and Mexico that feeds on xylem fluids of a large number of plants. By 1999 it had become established in the vineyards of Temecula, California, where it caused substantial damage. To date, this pest is not found in the Napa Valley.

GWSS can cause serious damage by feeding, but it also is a vector insect for Pierce’s Disease (PD), Xylella fastidiosa. PD destroyed the vineyard industry in Anaheim, California, in the early 1900s and has been a challenge to winegrowers for a number of years. Historically, the vector for PD in the Napa Valley has been the Blue Green Sharpshooter (BGSS), Graphocephala atropunctata, which resides in riparian areas and on ornamental plants used for landscaping. The GWSS is much larger, a much more aggressive feeder and can fly further than the BGSS, thus creating the potential for widespread dissemination of PD. Fortunately, the entire California grape industry, working cooperatively with the United States government, California Department of Food and Agriculture, and County Agricultural Commissioners, have prevented the entry of GWSS into the north coast grape districts, including Napa Valley. To pay for the tasks of survey and detection, containment, rapid response and treatment of GWSS infestations, there is a statewide grower assessment, a local Napa assessment and federal and state funding for various GWSS/PD programs. Some of these funds support major research projects on the control of GWSS and PD and its vectors. The success of these efforts to date provides the Napa Valley winegrowers with a model for future invasive pests.

The first discovery of the European Grapevine Moth (EGVM), Lobesia botrana, in North America was found during the 2009 grape harvest in the Napa Valley. The epicenter was the Oakville AVA in the heart of Napa Valley, but the pest spread to many of the vineyards in Napa Valley along with some small populations in Sonoma, Mendocino and Fresno counties. The EGVM also has been found in Chile, Western Europe, portions of Eastern Europe and portions of Africa.

The EVGM can cause serious damage to the grape crops (table, wine, and raisin) because it attacks and lives in the grape clusters. In addition to damaging the grape flowers and grapes, EVGM can create rot conditions in the grape clusters, making them noncommercial. Fortunately, efforts led by the USDA, California Department of Agriculture, County Agriculture Commissioners and a group of scientists from California and around the world have instituted control measures. In 2010, the Napa Valley population was dramatically reduced and almost no EVGM damage occurred there or elsewhere in the state.

Napa Valley growers have done a great job of controlling the pest. Napa experts report that only 76 moths were trapped in the first generation in 2011, compared to 99,000 moths found in the first generation in 2010. In spring 2011, the experts determined that eradication is possible; they feel confident that the growers of Napa Valley are acting diligently to control EVGM and ultimately to eradicate it.

Cultural practices
Cultural practices tailored to the vintage, along with vineyard designs that augment natural features of the particular site, determine the ultimate wine quality in any given harvest year. The most important cultural practices are pruning, canopy management, irrigation, fertilization, vineyard floor management, yield management and harvesting. Pruning is a critical factor in managing yield and wine quality. The Napa Valley winegrower strives for a balance between yield and vegetative vine growth on a consistent, sustainable basis. Optimal values include 0.66 pounds to 2.20 pounds of pruning weight per meter of canopy length and between 5 and 7 pounds of fruit for every pound of dormant vegetative growth.1 [32, 55] Some growers relate pounds of prunings to retained bud count. For example, an average of seven buds retained per pound of prunings will provide for a balanced vine at most Cabernet Sauvignon sites. The experienced winegrower also will take into account the historical yields at the specific site, vine vigor, wine quality and the amount of summer trimming.

There is no clear preference for cane pruning versus spur pruning in the Napa Valley. Both methods are used depending on grower preference.

The Napa Valley is subject to spring frosts. Once bud break occurs, the green shoots can be damaged if temperatures fall below 32ºF. The most common type of frost event is a radiational freeze.2 On some very rare occasions, an advective freeze3 may occur.

Most vineyards that are vulnerable to frost are protected in some manner. The use of wind machines (one per 10 ± acres) mixes the warmer, inverted air with the colder air on the vineyard floor. During a radiational freeze, a wind machine will raise temperatures by 2°-3°F. The air movement of 2 to 10 miles per hour caused by the wind machine also results in a shallower boundary layer, a larger heat gradient, enhanced heat transfer and a warmer leaf. [63] As a result of these combined effects, wind machines can protect vines against temperatures as low as 26ºF. It should be noted that wind machines will not protect the vine, and they may even cause greater damage during an advective freeze.

Overhead sprinkler systems
Another commonly used method for frost protection is a properly designed overhead sprinkler system. Heat released by the freezing of the applied water, which forms an ice film around the green leaves, clusters and shoots, protects the vine by keeping temperatures near the 32ºF mark. This water application can protect to approximately 24ºF, [57, 64] although co-author Robert Steinhauer has observed protection down to 21°F.

The overhead sprinkler system consumes large volumes of water—52 gallons per minute per acre. For most properties, large reservoirs and large pump capacity are required. In return for this investment, the sprinkler system will protect against much lower temperatures than will a wind machine. It also will provide protection in most advective conditions, and the same system is sometimes used for irrigation and cooling during summer heat spike events.

Part of the trend to sustainable practices in Napa Valley is the management of the vineyard floor, both in-row and under vine. Best farming practices generally have moved away from pre-emergent herbicides under the vine row to mechanical tilling or applications of glyphosate for weed control. Generally, between rows are permanent sod cover crops that are mowed or cultivated row centers. Currently, many sites alternate every other year between sod cover crop and cultivated row middles. Some growers plant a winter cover crop that is incorporated into the soil as a source of nutrition as well as erosion control. Other growers utilize compost for the same purpose. Typically, these management practices are site-specific.

The benefits of cover crops are reduced tractor wheel compaction, improved physical properties of the soil, increased organic matter, reduced chemical use, habitat for beneficial insects, dust control and reduction in water runoff. However, permanent sod cover crop has been found to compete for water and nutrients, which, over time, potentially reduces vine vigor. [4, 24, 38, 39, 42, 62]

Vine nutrition is an important component of a successful vineyard—both for the particular vintage and for the long-term health of the vineyard. Most growers rely on field observations to determine nutrient status, along with soil chemistry analysis, bloom time petiole nutrient status and leaf blade nutrient analysis. The Napa Valley winegrower has at his disposal many advanced nutrient and soil amendment application capabilities such as fertilization by drip irrigation, mechanical soil incorporation and materials that can be sprayed on grapevine leaves.

The ultimate goal of the winegrower is to produce the largest quantity of the very highest quality grapes that are in the greatest demand in Napa Valley. To achieve this goal, the viticulturist must have a solid scientific understanding of the effects of cultural practices, the vintage and terroir, plus the historical perspective of farming on the particular site. The grower must be flexible in adjusting cultural practices throughout the growing season from site to site, block to block—and even within a block.

Canopy management
While many factors influence the selection of grape variety and the ultimate wine character and quality, site (terroir) and season (vintage) are perhaps the most dramatic. In terms of farming practices, canopy modification and irrigation strategies are demonstrated mechanisms for manipulating wine quality. The response to these and other management practices is dependent upon variety, row orientation and trellising. [2, 16, 31, 55]

Canopy management is important because filtered light has a positive influence on fruit composition. However, too much light and high fruit temperatures are deleterious to wine quality; therefore, the foliage must protect the fruit from sunburn and dehydration. [15, 65, 76] Additionally, sunlight on the shoots will improve bud fruitfulness the following year. [5, 17]

Most Napa Valley winegrowers retain about five shoots per foot of canopy. [61, 77] The spacing of the shoots generally allows for optimum sunlight on the leaves. The shoot length on a balanced vine should be uniform and grow to between 3.5 feet and 5 feet by veraison (the point at which the grape berries turn from green to their final color, representing the change from berry growth to berry ripening.) The shoots should stop growing about two weeks before veraison and retain about 12 to 15 active leaves that remain green and healthy until harvest. The grape clusters typically are protected by filtered light through one or two layers of leaves. Growers may decide to remove basal laterals or laterals plus a few leaves based on observations of the site and leaf cover. Row direction and leaf size (Merlot and Zinfandel are varieties that have large leaves) will influence the decision about leaf removal to obtain the desired amount of fruit exposure. North-by-south row direction may require leaf removal on the east (morning sun) side and no leaves on the west side of the canopy to shade the fruit.

The water status of the vine has a major effect on the canopy and ultimate fruit and wine quality. As such, irrigation management is one of the most important viticultural tools. The Napa Valley generally receives enough rainfall from November through April to fill the soil profile. Normally, very little rain occurs during the summer and fall months. Almost all Napa vineyards are either dry farmed or equipped with drip irrigation systems. Following the advent of drip irrigation, water use by Napa Valley growers has become very efficient and prescribed. A tremendous amount of research on irrigation treatments and their effects on vine physiology and wine quality have been completed. [3, 20, 30, 35, 36, 58, 69, 71, 75]

There are sites with deep loam or clay loam soils that require no irrigation or are intentionally dry farmed. Many growers advocate deficit-irrigation strategies. These irrigation strategies maintain the vine water status post-veraison at 70%-80% of the evapo-transpiration rate. Most growers understand spatial variations in the vineyard and adjust irrigation within blocks or sub-blocks of the vineyard. Along with constant visual inspection to determine the water status of the vines, many growers rely on soil moisture measurements and gauge the midday leaf water potential utilizing a pressure chamber. Many growers will not start irrigating until the vines get to -10 bars of midday leaf water potential. Along with the soil and plant water measurements, the field observation of vine status, especially observing shoot tips, is critical.

Cluster removal, known as thinning, occurs just prior to veraison or at veraison in almost every ultra-premium vineyard in Napa Valley. Typically, the grower removes the clusters that are touching or are compacted together in order to provide light to the berries and also reduce the potential for fungal disease.

Many growers, especially the producers of red grape varieties, “color thin” at 80%-95% veraison by removing any green or partially green clusters. This process is undertaken in an attempt to achieve uniform maturity at harvest. Some growers will remove clusters in excess of one cluster per shoot. This process is gaining favor among some ultra-premium producers of Cabernet Franc, Cabernet Sauvignon, Pinot Noir and certain other varieties.

Growers understand that thinning early, while the berries are in the growth phase, will increase the size of the remaining berries. For this reason almost all growers wait to thin until veraison.

Winemaker demands for concentrated flavor and supple tannins at harvest have led to the practice of leaving grapes on the vine for days or weeks after the traditional harvest date. This practice is commonly referred to as “hang time.” This requires continued maintenance irrigation to keep leaves green until just before harvest.

Viticulturists in Napa Valley are divided on the effects that yield (expressed as tons per acre) has on wine quality. Undercropping (too few clusters) can lead a vine to excessive vegetative growth, vegetative wine flavors and high total pH. At the opposite extreme, overcropping (too many clusters) a vine can lead to low alcohol and light, poor-quality wines. A majority of growers believe that precision farming for small yields produces the highest quality ultra-premium wines. The success of these growers makes it difficult to argue with this position. Another group, however, believes that precision farming for balanced capacity and vine vigor is the best avenue to produce ultra-premium wines. Whichever approach is correct depends ultimately on a multitude of factors. Dr. Mark Matthews reports, “Prior research has shown no effect of yield on wine quality.” He also states, “Quality cannot be measured by a scientist because it is so subjective.” [34]

Most viticulturists agree that for the ultra-premium wine market, small yields that are balanced with the vine’s vigor result in superior wines. [1, 8, 51, 56, 61, 69] The Napa Valley average yield for Cabernet Sauvignon from 1986 to 2004 was 3.8 tons per acre. By contrast, the California average for Cabernet Sauvignon for this same period was 5.55 tons per acre.

The Napa Valley climate allows for the full maturity of even late-ripening varieties like Cabernet Sauvignon and Cabernet Franc. For many years, harvest date was determined by measuring sugar, acid and pH. These measurements are still made today, but the winemaker also tastes the fruit to determine when to harvest. For red varieties, most winemakers look for ripe, intense flavors with supple tannin structure. They also want the grape seeds to be brown and mature. Bibiana Guerra describes an Australian research project about Syrah confirming that seed color is a good indicator of physiologically ripe berries, ready for harvest. [25] A given vineyard may be harvested block-by-block or even sub-block-by-sub-block on different days, based upon the flavor profile of the fruit.

Most ultra-premium fruit is harvested by hand. However, the mechanical harvester technology is so advanced that a substantial amount of Napa Valley fruit is now machine harvested.

Harvest is the culmination of growers’ efforts. Before and during harvest, the winegrower and the winemaker (sometimes one and the same) observe the conditions of the vines and determine how best to meet the winemaker’s parameters for the fruit. They observe conditions down to the level of the blocks and sub-blocks (precision farming), and the grower then adjusts the cultural practices accordingly, including irrigation, fertilization and canopy management.

When harvest concludes, the winegrower and winemaker focus on the wine to determine if modifications to the cultural practices would improve yields and quality. The grower then considers whether to adjust those practices in future vintages, cognizant that each vintage year will present its own unique conditions and challenges.

The winegrowers and winemakers of Napa Valley understand that they are fortunate to grow grapes and make wine in such favorable terroir. Winegrowers and winemakers work collaboratively to ensure that their grapes meet the wine quality parameters and will vinify high-quality wines. This strategic alliance of winegrower and winemaker is a key component of the success or failure of a vineyard.

The grower and winemaker also constantly work together to improve their know-how and stay at the forefront of technology. They recognize, respect and appreciate that each winegrowing region of the world has its own valuable history, tradition, science and practices that may differ from those of the Napa Valley, and they remain open to learn and adapt their skills and practices to improve the quality of Napa Valley wines. This commitment to excellence has been evident in the Napa Valley for many generations, resulting in the well-deserved international reputation that Napa Valley has long enjoyed.

Richard Mendelson is a lawyer specializing in vineyard and wine law with the law firm of Dickenson, Peatman and Fogarty in Napa, Calif. He directs the Wine Law and Policy Program at the University of California, Berkeley, School of Law and is past president of the International Wine Law Association. Robert Steinhauer managed vineyard operations at Beckstoffer Vineyards from 1971 until 1979 and Beringer Vineyards from 1979 to 2004 and is now a viticultural consultant with and co-owner of Wineland Consulting LLC. He is a past president of Napa Valley Vintners and in 2008 received the Merit Award from The American Society for Enology and Viticulture. The authors acknowledge and thank Hal Huffsmith and Tucker Catlin for their peer review of this article.

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