By Spencer Leidl (Summer Student) and Kathryn Carter (OMAFRA)
In our previous article on Early Season Canopy Management: Shoot Thinning, we discussed early season canopy management and its benefits. We also introduced a technique known as shoot thinning, which involves decreasing the shoot density of the vine to create a more well-balanced canopy. In this article, we present two additional techniques for early season management in grapes: early leaf removal and shoot positioning.
Early Leaf Removal
Early leaf removal involves discarding the basal leaves of the main shoots and, if desired, from the lateral shoots developed from the basal nodes to decrease crop yield. Early leaf removal is generally performed pre-bloom or at trace bloom, however it can be performed as late as fruit set (Smith & Centiari, 2017). Before bloom, these basal leaves provide most of the necessary carbohydrates for the plant while the other leaves are still developing. Removing these leaves will reduce fruit set (and the number of berries at harvest) due to the decrease in available carbohydrates (Smith & Centiari, 2017). Removing the leaves from around the clusters reduces the availability of photosynthates, causing a smaller yield from the vine. Early leaf removal also causes a smaller berry size. Early leaf removal also improves the microclimate inside the canopy, as it allows for more sunlight and airflow reducing disease pressure. Research has shown that conducting early leaf removal one week after fruit set did not affect cluster weight or compactness, nor did it have an impact on rachis length (Martinson & Particka, 2015). Early leaf removal can be an effective tool to reduce cluster compactness in tight-clustered varieties, thereby reducing their susceptibility to rots later in the season.
Additionally, early leaf removal appears to have a positive impact on fruit composition. Smith and Centiari (2017) reported that in good growing conditions, Vitis vinifera varieties had higher levels of total soluble solids, anthocyanins, and phenolic compounds when the vines underwent early leaf removal. Levels of methoxypyrazines, compounds responsible for the “green, herbaceous or vegetative” aromas found in immature grapes at harvest, also decreased after early leaf removal.
One of the challenges of early leaf removal, however, is that there is no generic recommendation for how many leaves to remove. The number of leaves to remove varies depending on several factors including shoot length and leaf area. Vine shoot length differs between vines within and between vineyards. So, pulling off the same amount of leaves on shoots with different lengths can result in excessive leaf removal on short shoots depriving them of carbohydrates, while longer shoots may remain unaffected. Some research conducted in Pennsylvania suggests that the removal of 4-5 basal leaves no later than trace bloom can be effective in reducing fruit set and making clusters less compact. Another challenge with early leaf removal is the cost, as it is done manually and can be time consuming (Smith & Centiari, 2015).
Shoot positioning involves changing the orientation of the shoots to create an open canopy that is both uniform and less shady. Ideally, shoots should grow vertically up or down (parallel to the trunk). However, shoots tend to naturally grow sideways and attach to the cordon wire or neighbouring shoots using their tendrils. As a result, shoot positioning is needed to discourage lateral and horizontal growth, minimize shading, and improve air flow in the canopy (Striegler & Jones, 2019). Shoot positioning is important for reducing disease pressure in the current growing season, improving spray coverage and increasing sunlight penetration to improve fruit quality. Shoot positioning also makes it easier to do hedging and leaf removal, increasing labour efficiency. This technique is ideally started around bloom and is needed through the pea-size berries and bunch closure growth stages, before the tendrils become firmly attached to avoid breakage and optimize efficiency (Striegler & Jones, 2019). Shoot positioning is completed by hand, one to two weeks after bloom. The method by which the shoots are positioned depends on the trellis and training systems used in the vineyard.
While shoot positioning is important in all training systems, proper positioning is particularly critical in divided canopy systems (ie. vertical shoot positioning or VSP). In Vertical Shoot Positioning Systems (VSP) and other low cordon trellis systems, canes are pushed upward and tucked between a set of catch wires (Paolo,2020). This positioning is sometimes referred to as “tucking”, and this process will need to be completed several times (up to three) during the growing season as shoots develop. Cultivars with few tendrils may need extra tying to keep the shoots in their proper position (Minnesota Grape Growers Association, 2015). On the other hand, in high cordon trellis systems such as Geneva Double Curtain (GDC), the shoots need to be positioned so they grow down and away from the cordon. This process, also referred to as “combing”, reduces the vigor of shoots to assist in obtaining the preferred canopy density (Paolo, 2020). Shoot positioning is necessary to reduce shading, but buds need to be firmly attached before combing occurs to reduce the risk of shoot breakage. This usually occurs after bloom, but growers should remain vigilant and start combing with cultivars least prone to breakage (Minnesota Grape Growers Association, 2015). Shoot positioning is labour intensive and it is important to consider the benefits and downfalls of shoot positioning prior to its implementation.
Shoot thinning and early leaf removal play important roles in opening the vineyard canopy to improve air circulation and increase sunlight penetration. These techniques can be used to help to enhance the quality of grapes grown in any vineyard, reduce disease pressure and increase fruit bud formation for the next season’s crop. Each vineyard will vary in its canopy management needs based on cultivar, rootstock, and soil type.
Chien, M. (2019, June 20). Fundamentals of canopy management. National cooperative extension. https://grapes.extension.org/fundamentals-of-canopy-management/.
Goldammer, T. (2018, March). Vineyard canopy management. Grape growers handbook: a guide to viticulture for wine production. Retrieved from http://www.wine-grape-growing.com/wine_grape_growing/vineyard_canopy_management/vineyard_canopy_management_techniques.htm
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Martinson, T. & Particka, C. (2015, November). Manipulating cluster size at bloom. Grapes 101 Newsletter. Retrieved from https://grapesandwine.cals.cornell.edu/newsletters/appellation-cornell/2015-newsletters/issue-23-november-2015/grapes-101/
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Smith, M. & Centiari, M. (2015, September 11). An overview of cluster- zone leaf removal strategies for cool climate vineyards. Penn State extension wine & grapes U. https://psuwineandgrapes.wordpress.com/2015/09/11/an-overview-of-cluster-zone-leaf-removal-strategies-for-cool-climate-vineyards/.
Smith, M. & Centinari, M. (2017, June 2). Early season grapevine canopy management, part II: Early leaf removal. Penn State extension wine & grapes U. https://psuwineandgrapes.wordpress.com/2017/06/02/early-season-grapevine-canopy-management-part-ii-early-leaf-removal-elr/.
Striegler, K. & Jones, T. (2019, June 20). Canopy management: Shoot positioning. National cooperative extension. https://grapes.extension.org/canopy-management-shoot-positioning/.