Sunlight is probably more important to winegrowing than is both water or nutrient availability, and its availability throughout the year limits the regions where winegrapes may be successfully grown. I frequently see vineyards with leaves stripped from the fruit zone, with clusters hanging helplessly, baking in the midday sun. Perhaps some members of our industry have looked at the cover of Richard Smart and Mike Robinson's Sunlight into Wine¹ book without getting further into the meat of the book. Over the last two decades, the pendulum has swung from vineyards having lush, dense foliage towards very spare canopies. Where we once had to move aside layers of shoots and leaves just to look at the fruit, we can now see bands of perfectly placed fruit even without leaving the comfort of our pickup trucks. I think that we need to let the pendulum swing back a notch or two.

The problem with fruit exposure

Overexposure of fruit to sunlight can create losses in both productivity and wine quality. Productivity losses occur when fruit is damaged by heat and shrivels on the vine. This damage usually occurs after veraison when fruit is pigmented (at least in red grapes) and efficiently absorbs thermal energy from the sunlight. However, damage may also occur before veraison. Such damage may destroy the berries, leaving them to shrivel to tiny pebbles. Or, it may be more subtle, leaving the fruit mostly intact but causing the fruit to mature improperly. Often, it is just the outer berries that shrivel, but the damage may be widespread enough to prevent whole clusters from being harvested, resulting in substantial percentages of yield loss to the grower.

Wine quality may also be hampered by fruit's overexposure to sunlight. The fruit's sensitivity depends upon variety, and there are at least three well-known varieties that are particularly known to have problems with sun exposure. Cabernet Sauvignon, with its small berry size, may be damaged by full sunlight even before veraison. Heat-damaged berries lag in color development at veraison and may never attain more than a pinkish color at maturity. Two other varieties, Grenache and Merlot, are also known to be highly sensitive to sun exposure. Fruit directly exposed to the sun during ripening will not develop full color by maturity. It has been said that the fruit gets "bleached out." While this reflects the appearance of the fruit, bleaching is not really what happens-rather the color never fully develops. During the heat wave experienced in mid-July of 2006, damage to pre-veraison berries was seen in many regions and to numerous different varieties. The damage was restricted primarily to those berries exposed to many hours of afternoon sunlight.

Light is required but in moderation

The industry's move into more "thrifty" canopies was no doubt a move in the right direction. Sunlight is indeed needed to fully develop fruit color and possibly other components of fruit and wine flavor, aroma and mouthfeel. Yet, full sunlight intensity is not needed to achieve the desired result. Bergqvist et al. (2001)² found that only 10 percent of full sunlight was needed to develop full anthocyanin pigment concentration in fruit. Exposure of fruit to direct sunlight raises the temperature of the fruit and affects anthocyanin and other berry constituents in a negative manner. They found that fruit on the South (sunlit) side of the east-west-oriented canopy did not achieve the same concentration of anthocyanin as the fruit on the north (shaded) side, probably due to overheating of the fruit during its development.

Hence, the downside of sun exposure is that solar radiation (infrared, visible light and ultraviolet light) heats up the fruit. The heating of fruit may assist fruit ripening for vineyards in very cool climates, but in most grape growing regions in California, the climate is already warm enough to ripen fruit. Bergqvist et al. found that, in the San Joaquin Valley, Cabernet Sauvignon and Grenache clusters exposed to sunlight were 13°F warmer than ambient air temperature and 18°F warmer than shaded fruit. There have been even higher numbers reported for sun-exposed fruit: Spayd et al. (2002)³ found a temperature elevation of 23°F above ambient in exposed clusters. Bergqvist et al. reported that heating of the fruit beyond some unknown temperature (probably somewhere between 85 and 95°F) not only diminished color and phenolic development, but resulted in higher pH and lower titratable acidity. Shaded fruit was able to be maintained at about 5°F cooler than ambient air temperature and, consequently, had a more favorable fruit composition.

Spayd et al. confirmed that it was fruit temperature, not incident light, that was responsible for the differences in total and specific anthocyanin pigments, and that higher temperatures consistently produced fruit of lower anthocyanin concentration. They also investigated the effect of ultraviolet (UV) light on anthocyanin and flavonol composition of the fruit at harvest and found that UV did not have an effect on anthocyanin production, but stimulated flavonol production significantly. It is questionable, however, whether flavonols have an important role in wine sensory properties.

Leaves provide sunscreen for grapes

The primary take-home message from these two works is that light on the fruit cluster is necessary for anthocyanin and phenolic development, but only up to about 10 percent of full sunlight, beyond which it saturates. Since grape leaves transmit about 5-10 percent of visible light, one leaf layer surrounding the cluster is desirable for optimum fruit and wine composition. Direct fruit exposure to sunlight, especially during the hot afternoon hours, will overheat the fruit and diminish the quality of the fruit and wine.

The desired goal is to develop a fruit zone that receives "dappled" direct light from sunshine but that is shaded by a single leaf layer in most of the zone at any given time. The "flecks" of light will shift as the sun moves across the sky and as the breeze blows the leaves about, which will prevent heat buildup in any one portion of the cluster.

Canopy management and leaf removal

But is there still a case for leaf removal in some or most vineyards? It has been my observation that winemakers are the most heinous instigators of excessive leaf removal in vineyards. Forgive me for painting with such a broad brush, but why would a grower want to take on the risk of exposing his crop (and revenue) to the harsh elements if there is only a downside by way of yield loss? The answer is that we are growing wine, not just grapes, and growers need to be willing to take on reasonable risk (and cost) to produce a product that the winery will want to keep coming back for. On the other hand, the winemaker or winery needs to understand that if a little is good, more is not better.

It is impossible to create a rule of thumb or general guideline for how, when and if leaf removal should be practiced in any given vineyard. There are so many factors that influence that decision, including trellis systems, grape variety, climate (temperature, wind, rainfall, fog), row orientation, site vigor, etc., that a simple, best-practice statement would be a gross oversimplification.

Ideally, there is no need for leaf removal if a trellis/training system is used that positions shoots vertically or near-vertically using movable or fixed wires, and that produces (or retains after shoot thinning) only three to four shoots per foot of fruiting wire. Lateral shoots, if any, in the fruit zone should be removed to open up the fruit zone. That approach should work for many vineyards in moderate climates. There are, however, some situations that demand less fruit exposure and others that demand more.

Protecting fruit from direct sun

Less exposure or more shade may be necessary in hot climates, for unsuitable row orientations, or for varieties such as Merlot and Grenache that are extra-sensitive to sun damage. And in case you were wondering, I consider many California coastal regions to be hot, including much of Sonoma, Napa, Lake, Mendocino and San Luis Obispo counties. For hot regions and sensitive varieties, the best solution is proper row orientation.

The hottest time of the day in California coastal regions is usually between 3 and 4 p.m. (during daylight savings time). Rows should be oriented so that the sun is positioned overhead and in line with the row during that time (i.e., a shadow is cast right below the vines), which translates to a northeast-southwest direction. That is not always possible due to topographic or other site constraints. So, the key is to protect the fruit from the afternoon sunlight because, as mentioned, the sunshine adds about 20 degrees to the air temperature as far as the fruit is concerned.

Laterals may be retained on the west (or south) side of the canopy to protect fruit from the afternoon sun. Alternatively, or additionally, growers have been using Surround® (Engelhard Corporation) to coat their fruit on one or both sides of the canopy. The product essentially forms a whitewash on the fruit, thereby reducing the amount of solar energy that is absorbed. I have heard mixed reviews about the effectiveness of this product for sun protection, but the indication is that it might be most effective for the protection of sun-sensitive varieties. Likewise, the product, which is a refined kaolin clay, is similar to Bentonite, which is used to fine wines. As such, some winemakers are concerned about Surround residues in their musts; therefore growers should come to an agreement with their wineries about its use.

There are some wineries and growers that are using shade cloth to cover the afternoon side of the fruit zone, and they claim success. There is little doubt that this method would be effective, although the process is tedious and, therefore, costly. I suspect that only high bottle price wines will ever consider this technique unless it can be automated.

Leaf removal to open the fruit zone

After all of this discussion on the dangers of fruit exposure, it is important to discuss when and how leaf removal may be appropriate.

Most commonly, leaf removal is practiced in true cool climates, for tight-clustered and rot-prone varieties, and for bushy "old school" trellis systems that do not have positioned shoots. The purpose for leaf removal in very cool climates and/or tight clustered varieties is to increase air circulation and spray penetration in the fruit zone. In cool fringe climates, such as the Sonoma Coast or Santa Rita Hills, it may be necessary to increase fruit temperature to accelerate fruit development and maturity. On the other hand, in bushy canopies, wherever they may be, leaf removal is a technique to make denser canopies approach a fruit zone closer to our modern ideal canopy.

Generally speaking, leaf removal, if performed at all, should be done only on the side of the canopy exposed to the morning sun (east side of N-S rows and north side of E-W rows), retaining all leaves on the afternoon side to protect the fruit. This is true even in cooler climates as warm fruit generally produces poorer quality, especially in Pinot Noir. Leaf removal should be done early-shortly following fruit set-so that fruit can acclimate to the light environment. Never remove leaves before fruit set.

As if you couldn't tell from the previous discussion, I prefer to remove leaves very sparingly, removing only about two leaves per shoot (along with any laterals). I like to leave one or two leaves below the fruit zone to shield the fruit from heat and infrared energy that are reflected from the ground. Similarly, I do not like to pull leaves far above the fruit zone. That provides an "umbrella" over the fruit so that they become shaded before the sun gets high in the sky.

To sum it up, canopy management is an important attribute in the vineyard. To increase the chances of producing quality fruit that, in turn, will produce quality wine, take care in protecting fruit from overexposure to the sun and remove leaves sparingly from the fruit zone. wbm

1 Smart, R. and M. Robinson. 1991. Sunlight into Wine. Winetitles.

2 Bergqvist, J., N. Dokoozlian and N. Ebisuda. 2002. "Sunlight exposure and temperature effects on berry growth and composition of Cabernet Sauvignon and Grenache in the Central San Joaquin Valley of California." Am. J. Enol. Vitic. 52:1-7.

3 Spayd, S.E., J.M. Tarara, D.L. Mee and J.C. Ferguson. 2002. "Separation of sunlight and temperature effects on the composition of Vitis vinifera cv. Merlot berries." Am. J. Enol. Vitic. 53:171-182.