Canopy management is the attempt to control and direct grapevine vigor and growth in such a way as to maximize both production and fruit quality without negatively affecting vine health or growth. While it may include the impact of factors such as trellising and training systems, pruning severity, fertilization and irrigation, and disease, insect and weed management, canopy management is usually thought of as those manipulations of the canopy during the growing season such as shoot thinning, shoot positioning, leaf pulling, and hedging that directly impact the microclimate within the vine canopy. These practices can have a significant impact on fruit quality and vine productivity both in the year they are applied and in the subsequent year.
Highly vigorous, unmanageable vines have dense canopies that produce undesirable microclimates for high quality fruit production. Low to moderate vigor vines have more open canopies that promote higher quality fruit with less incidence of disease and fruit rots due to a more favorable microclimate within the canopy. In open canopies the majority of leaves and a high percentage of fruit clusters are exposed, whereas in denser canopies a large percentage of leaves and most clusters are in the shade of the exterior leaf layer. This is important because of the need for sunlight by leaves for photosynthesis and by fruit for pigment formation. Grapevines leaves can absorb almost 90% of the sunlight striking their surfaces and reflect around 7%, meaning less than 10% may be transmitted. In dense canopies, then, leaves in the interior of the canopy will not receive enough sunlight to be photosynthetically productive.
Fruit and leaves from open canopies can reach higher temperatures during the day and lower temperatures at night due to radiational heating and cooling, even though the air temperature within the canopy is the same as the ambient air temperature outside the canopy. In dense canopies leaf and fruit temperatures remain near the ambient air temperature. In an open canopy the humidity remains at or near the ambient air humidity, whereas in a dense canopy humidity can increase by as much as 10% as leaves transpire. Air movement within dense canopies is reduced compared to that within more open canopies since leaves block air flow. In very dense canopies wind velocity can be reduced to only 10-20% of that outside the canopy. Less sun exposure, lower daytime temperatures, higher humidity and reduced air flow within dense canopies equal a lower evaporative potential, which means that the surfaces of interior leaves and fruit dry off more slowly after rains and dews than those in more open canopies. This can lead to a more favorable environment for disease development, especially fruit rots. Low light in the basal area of the canopy, where the buds for next year's crop are being formed this year, can result in poor fruit bud development, which leads to poor cluster development, poor fruit set, smaller clusters of fewer berries, or even no clusters at all.
Very few grapevine canopies in Midwest vineyards fall within the preferred range of the parameters that characterize the ideal canopy. In areas with deeper, often somewhat fertile soils, normally abundant rainfall, and a long growing season, these factors combined with the vine's natural vigor may lead to very vigorous vines, often with very dense canopies. Heavy use of fertilization and/or irrigation may also result in excessively dense canopies, even in poorer soils. When this is the case, then vine management practices that help to open the canopy need to be employed to expose the fruiting and renewal zones of the canopy to better illumination and air movement.
The first of the canopy management practices to be utilized during the season is shoot thinning. Shoot thinning allows you to directly determine shoot density by removing excess shoots. Shoot density is defined as the number of shoots per linear foot of canopy and is optimized for most cultivars at about 5 on spur-pruned, low-cordon trellis systems. Thus, for vines on eight-foot spacing on a low-cordon system such as the Vertical Shoot-Positioned (VSP) trellis, there should be approximately 40 nearly evenly-spaced shoots along the cordon length. A lower number of shoots results in an unnecessarily open canopy and a loss of yield potential. Excess shoot number causes shading within the canopy, resulting in poorly illuminated leaves and fruit. On high-cordon trellis systems, more shoots (6-8) may be retained per linear foot of canopy length since they are spread out vertically as well as horizontally and since the canopy is not artificially compressed into a narrow plane as it is in the VSP system. However, for an eight-foot spaced vine shoot number should not exceed 60-70 to avoid excess shading of foliage. Shoots from the base of spurs, multiple shoots from the same node, shoots growing from non-spur positions or originating in the head region or on the trunk are all candidates for removal, unless needed to replace an old or poorly positioned spur or an old cordon. Selecting shoots that are not fruitful for thinning is preferable unless some crop reduction is needed. Shoot thinning can be done anytime after budbreak but preferably before shoots become more than 12 inches long, as longer shoots may be more difficult to remove. The optimum timing for shoot removal is when the shoots are between 3 and 6 inches in length since they are very easily removed at that stage.
Much of the early work in canopy management simply utilized shoot positioning to provide a better environment for quality fruit production. Particularly in low cordon-trained systems such as the VSP, where most of the foliage is produced above the fruiting zone, positioning the shoots into a narrow plane held directly above the fruiting zone by pairs of foliage wires allows greater sunlight penetration and air movement into the lower canopy area. Even in high cordon-trained systems, shoot positioning may help to improve the light environment within the canopy. In these systems the shoots may have a tendency to run along the top of the canopy forming a dense cap of foliage over the fruiting zone. To prevent this situation the shoots are separated and combed into a downward position with minimal overlapping of the shoots. In both low- and high-cordon systems, the goal is to distribute the vine's shoots and foliage uniformly through the vine's available space and to minimize mutual leaf shading. An added benefit of shoot positioning is that it makes other canopy management chores, such as hedging and leaf removal, easier to accomplish. It also improves the efficiency of operations such as pruning. Shoot positioning is easier to do if done once or twice (occasionally 3 times with cultivars such as Norton) as the shoots elongate rather than waiting until the shoots get very long and become entangled. It is also easier to do if the foliage catch wires of low-cordon trellis systems are not in fixed positions but are movable and placed below the cordon level after winter pruning.
Hedging is a very beneficial practice for low-cordon systems such as VSP and its variations. In these systems hedging generally involves removing the tops of shoots once they have emerged through the upper pair of foliage support wires and have begun to arch over, creating shading on the canopy and fruit below. Hedging should be done in these systems at about a foot above the upper foliage wires. This will leave enough leaf area to properly ripen the fruit. It may also be done along the sides of canopies in highly vigorous situations if lateral shoot development is excessive and causes undue shading in the fruiting zone. Hedging should be delayed as long as possible, preferably 3-4 weeks after bloom. Once the shoot tips have been removed through hedging, lateral development will generally be stimulated. In high-cordon trellis systems, hedging normally involves pruning cascading shoots about 1 foot above ground level in order to facilitate air movement within the canopy and weed management.
In the warm, humid climate of the Midwest leaf removal is undoubtedly the most valuable and necessary canopy management task performed. By directly reducing the leaf number in the fruiting zone, leaf removal creates a much more favorable microclimate around that zone. The resulting increase in light penetration and air movement not only provides a better environment for fruit quality enhancement, but also greatly helps in the control of fruit diseases by reducing the favorable conditions for fungal development and enhancing spray penetration into the fruiting zone. (I have seen severe powdery mildew problems on susceptible winegrape cultivars on grapevines with poor fruit exposure while vines that had leaf removal performed had fewer or less severe infections.) Typically, only 3-4 leaves need to be removed per shoot in order to improve the exposure of the fruiting zone. The goal is not to completely strip the foliage from around the fruiting zone, but to provide between 40 and 60 % exposure of the clusters. This can be accomplished by removing a relatively small number of leaves. Also, it is only necessary to remove the leaves from one side of the canopy. In north-south oriented rows it is beneficial to leave some foliage on the western side of the fruiting zone to help prevent sunburn. Likewise, in east-west oriented rows leave some foliage on the south side of the fruiting zone. Leaf removal can be done anytime after fruit set and before veraison, but is best done by the time the individual berries are around pea-sized, usually about two weeks after fruit set. Waiting until later in the season to remove leaves will increase the risk of sunburning the fruit.
Cluster thinning is the removal of excess fruit clusters to help manage crop load and prevent overcropping. While not a direct manipulation of the canopy, it is discussed here as a canopy management technique since excess crop load can have a suppressive effect on canopy development. As with leaf removal, fruit cluster thinning can be done at any time at or after fruit set; however to get the most benefit from cluster thinning it should be done at fruit set. Small-clustered cultivars such as Vignoles or Norton are generally not cluster-thinned unless they are young and still undergoing canopy development. With moderate- to large-clustered cultivars it is recommended to follow the 2-1-None rule: At fruit set, if the shoots are greater than 20 inches in length, retain 2 clusters. If shoots are between 8 and 20 inches in length retain 1 cluster. If they are less than 8 inches in length, retain None.
REVISED: October 5, 2015