Leaf removal is a practice widely applied throughout most wine-growing regions for various reasons, including improving sunlight penetration to promote bud fertility, exposing bunches to achieve a specific wine style, maintaining an ideal canopy density to ripen high-quality fruit, and manipulating bunch microclimate to lower disease pressure. In New Zealand, researchers investigated whether manipulating the leaf area: fruit ratio by extensive leaf removal can lead to lower total soluble solids and subsequent lower alcohol wines while still maintaining quality.

Project layout
Three Pinot Noir sites, clone 777 grafted onto 3309 rootstocks, were utilised. These vineyards are located in Marlborough, Central Otago, and Canterbury. They are all trained to VSP canopies. The study spanned two growing seasons (2015-2016 and 2016-2017). The trials were conducted using randomised block designs containing five repetitions of combinations of the following treatments.

• H: 50% canopy trimming treatment (trimming the top half of the canopy)
• Q: 75% canopy trimming (trimming the top half of the canopy, followed by further hand trimming)
• V-: trimming at pre-veraison stage
• V: trimming at veraison
• V+: trimming at post veraison stage

Results

• Shoot trimming treatments successfully lead to reduced leaf area: fruit ratios and significantly slower sugar accumulation in berries.
• Early trimming (V-) reduced berry anthocyanin concentrations.
• Later trimming (V and V+) preserved anthocyanin levels.
• Total phenolics and tannins were unaffected by treatments.
• Halving the canopy after veraison (HV+) reduced sugar levels, maintained favourable anthocyanins and preserved favourable root carbohydrates.

Significance of the study

This study shows that post-veraison canopy thinning can potentially be a tool to reduce sugar levels, thereby lowering alcohol levels in Pinot Noir wines from cool climates while maintaining adequate levels of phenolics and colour intensity without affecting vine reserve storage. In hotter climates, vines are more frequently exposed to stresses like water deficit and heatwaves, and similar results may not apply. Grapevine varieties will also differ in treatment responses; however, producing quality wines at slightly lower alcohol levels would be seen as desirable by some producers. Research like this could prove valuable in pursuit of this goal.

Reference:
Assefa, M. K., Creasy, G. L., Hofmann, R. W., & Parker, A. K. (2025). Asynchronous accumulation of sugar and phenolics in grapevines following post-veraison leaf removal. OENO One, 59(2). https://doi.org/10.20870/oeno-one.2025.59.2.9314