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GRAPE GRO WING C This article is excerpted from a chapter written by Richard Mendelson and Robert Steinhauer, two well-known wine industry experts, about modern-day viticulture in the Napa Valley. Their chapter appeared in "The Science of Viticulture: Volume 1" (K.V. Peter, editor), published last year by New India Publishing Agency. The chapter covers a broad range of topics, from the Napa Valley's viticultural uniqueness and the legal protections for this irreplaceable agricultural resource to vineyard develop- ment (site selection, infrastructure and plant material) and farming in an environ- mentally sound and sustainable manner. The excerpt that follows is about cultural practices. The entire chapter and citations are available at winesandvines.com. ultural 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 manage- ment, 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 dor- mant vegetative growth.1 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 Cab- ernet 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 sions, an advective freeze3 On some very rare occa- 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. (Wind machine shown in photo, page 32.) Dur- ing 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. As a result of these com- bined 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 over- head 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, although co-author Robert Steinhauer has observed protec- tion down to 21°F. The overhead sprinkler system con- sumes 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 per- manent sod cover crops that are mowed or cultivated row centers. Currently, many sites alternate every other year between sod cover crop and cultivated row mid- dles. 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 manage- ment practices are site-specific. The benefits of cover crops are reduced tractor wheel compaction, improved physi- cal properties of the soil, increased organic matter, reduced chemical use, habitat for beneficial insects, dust control and reduc- tion in water runoff. However, permanent sod cover crop has been found to compete for water and nutrients, which, over time, potentially reduces vine vigor. Vine nutrition is an important compo- nent of a successful vineyard—both for the particular vintage and for the long- term health of the vineyard. Most grow- ers rely on field observations to determine nutrient status, along with soil chemistry Common tools to combat frost damage include wind machines and overhead sprinklers. Wines & Vines nOVeMBeR 2011 33