The grapevine canopy refers to the collective arrangement of the vine’s shoots, leaves and fruit. Ideally, the canopy is characterized by medium diameter shoots with moderate-length internodes and few lateral shoots. Shoots should be spaced about 3 inches apart and have about 15 normal-size leaves. Desirable shoot length is usually considered to be about 3 to 4 feet long. When trained to an appropriate system, such a canopy will have most of its leaves well exposed to sunlight and air circulation will be good. A well-exposed canopy with adequate leaf area promotes high fruitfulness and good fruit ripening. Air circulation promotes rapid drying after rain or dew and lower humidity, which provides a less favorable environmental for fungal disease development.
Vine Balance. A grapevine canopy is described as being "balanced" (or "in-balance") when it has a canopy of adequate, but not excessive, leaf area to support the intended crop load to the desired fruit ripeness. Acheiving vine balance begins with good vineyard design - appropriate vine and row spacing, and an appropriate training and trellis system. In a balanced vineyard, each vine has enough room to express its capacity for growth and fruit production. Vines can be out of balance in either direction - overly vigorous with a dense, shaded canopy, or excessively weak with inadequate, although well-exposed, leaf area.
Several methods have been developed for assessing grapevine canopies and vine balance, including pruning weights, yield/pruning weight ratio, and point-quadrant analysis. The reader is referred to the book "Sunlight into Wine" by Richard Smart and Mike Robinson for an in-depth discussion of canopy assessment and canopy management methods.
Balanced grapevines do not require much in the way of canopy management. Typically, all that is needed to maintain the desired canopy characteristics is standard pruning, perhaps some shoot thinning, and shoot positioning for some training systems. Vines that are under-vigorous should be examined for the cause of their low vigor. Possible factors are: excessive competition for water from weeds or a cover crop, inadequate irrigation (amount or timing), overcropping, or damage to the root system or trunk from disease (root rot, crown gall, Eutypa), insects (phylloxera), nematodes, or poor soil conditions (shallow soil, poor drainage). Correction of the problems, if possible, should enable vine vigor to be increased to a more desirable level. Crop levels should be kept relatively low during the vigor recovery period.
Excessively vigorous vines can be caused by over-irrigation or abundant rainfall, especially on fertile soils or in combination with high nitrogen fertilizer applications. Vine spacing that is inappropriately close will also create a vine too vigorous for its alloted space on the trellis. Improved irrigation management may improve vine balance in lower rainfall regions. Competitive cover crops in vineyard alleys can be used in higher rainfall areas to reduce soil water availability, but caution is advised to avoid excessive competition with grapevines.
Shoot positioning is a standard practice, associated with some training systems, that arranges the shoots in a manner that displays most of the leaves to sunlight. So, it is not considered a corrective action as are the canopy management practices described below. Overly vigorous vines may require such canopy management practices to create a favorable microclimate in the canopy. Common practices employed to reduce canopy crowding and shading include shoot thinning, hedging (topping), leaf removal, and the use of vigor-diversion canes.
Shoot Thinning and Suckering. Most vines will have shoot growth from buds other than those left intentionally at dormant pruning. These “non-count” buds are at the very base of spurs or canes and the shoots they produce are typically nonfruitful. Sometimes secondary shoots, in addition to the primary shoot, will grow from a "count" bud. Regardless of the source, extraneous shoots should be removed early in the season while they can be easily broken off. In some cases, thinning of weaker primary shoots should be done to help balance the canopy and improve fruit quality. Keep in mind a target spacing of about 3 inches between shoots. Avoid excessive shoot thinning, particularly in highly vigorous vineyards. Removing too many shoots under such circumstances stimulates rapid growth of the remaining shoots during the flowering period, which can reduce fruit set. Suckers arising from the base of the trunk are also removed during thinning because, if retained, they will grow up into the canopy, contributing to crowding.
Some vineyards, in areas that are at risk for hail storms, purposely retain excess buds and shoot growth as a form of "insurance" or protection from hail damage. It is believed that the higher shoot number and cluster count may increase the odds that more fruit will survive a hail storm undamaged. It is also thought that a more dense canopy provides fruit clusters with better protection from hail. This strategy, however, has the negative consequences of disrupting vine balance and increasing canopy and cluster shading.
Hedging. Hedging (topping or summer pruning) may be necessary on non-shoot-positioned training systems (i.e., high cordon) to shorten long shoots or laterals. Excessive shoot growth contributes to canopy shading and shoots that extend into the alleyways make equipment passage difficult. Hedging is also commonly practiced on vertically shoot positioned (VSP) vines when shoot growth becomes too long to be supported by the trellis wires. Hedging VSP-trained vines is usually done when the shoots have grown well beyond the top wire and are just beginning to lean over. Shoots are commonly trimmed back to about 6 to 8 inches above the top wire. Hedging can be done either with a tractor-mounted machine or manually with long knives. Be aware that vines respond to hedging; removing the shoot tip stimulates growth of lateral shoots from the nodes immediately below the cut position. This is particularly true for vigorous shoots and the regrowth of laterals may require repeated hedging.
Leaf Removal. The pulling of leaves in the fruit zone of VSP-trained vines can be done to increase fruit exposure to sunlight and improve air circulation around the clusters. It also facilitates good spray coverage on the fruit for disease control. Leaf pulling is usually done shortly after fruit-set because it is important for fruit to be exposed to sunlight early in their development so they can acclimate to high sunlight conditions. Late leaf pulling can lead to sunburning of fruit which can negatively affect fruit quality. Leaf-pulling is often done only on the east or north side (depending on row orientation) of vines to avoid direct sunlight exposure (west or south) of the fruit during the hottest part of the day.
Vigor Diversion. Another method to reduce canopy density and shading on overly vigorous vines is the use of vigor diversion canes. The practice involves retaining one or more extra or “disposable” canes at pruning time, which greatly increases the total number of buds retained per vine. These extra canes are sometimes called ‘kicker’ canes. The kicker canes are allowed to grow until about mid-season, by which time the extra shoot growth has had an overall vigor reduction effect on canopy shoot growth. At this time, kicker canes are cut off at their base and removed from the vineyard. The most difficult aspect of implementing vigor diversion is to find a way to hold the kicker canes away from the fruiting canopy so they don’t interfere with spray penetration or shade the canopy.
Change Training System. In chronic over-vigor situations that require considerable annual effort to maintain a good canopy microclimate, often the best solution is to convert to a divided canopy training system. Canopies can be divided horizontally, as in the Geneva Double Curtain (GDC) and Lyre systems. Horizontally divided canopies require wide row spacing. Narrower row spacings are better suited for vertically divided canopy systems such as the Scott Henry System and the related Smart-Dyson Ballerina system.
The purpose of all divided canopy systems is to efficiently display a large canopy to sunlight, thus they can enable greater fruit production while still maintaining good fruit quality. Previously “over-vigorous” vines can be brought into balance by retaining more buds at pruning and displaying the additional shoot growth on a more extensive trellis system. Growers should be aware that larger canopies have greater water requirements, so plans must be made to increase irrigation rates.
Smart, R.E. and M. Robinson 1991. Sunlight into wine. Winetitles, Adelaide.