Leaf Removal for Better Grapes
In Niagara and other cool-climate growing regions, growers have developed strategies to maximize the amount of sunlight that reaches the fruit during the region’s comparatively short growing season. Thinning the canopy and removing basal leaves is a well-known technique for improving grape quality.
In 2008, Lakeview conducted a research study on the benefits of leaf removal, comparing mechanical thinning with that done by hand. We have compiled a summary of that research study and some helpful best practices that we hope you will find useful for your own vineyard management planning.
Why remove leaves?
- Increased air flow around the fruit reduces humidity in the cluster zone, which in turn minimizes fungus and reduces Botrytis bunch rot
- Improved spray coverage and efficacy
- Increased exposure of the grape to sunlight thickens the skin’s wax cuticle, improving the grape quality and protecting against infection
- More energy for the grape, resulting in an increased concentration of phenol and polyphenols, natural chemical compounds that affect the taste, colour, and flavour of wine.
How much, and when?
Thin the canopy too soon, and you might negatively impact fruit quality by reducing the fruit set or berry size. If removal is done too late in the season, your efforts are wasted because they will have no effect on the amount of carbohydrates stored in the grape. Removing too many leaves at once can hinder photosynthesis and decrease your overall yield.
Producers typically choose a date for selective leaf removal that falls somewhere after fruit set and before veraison. The selection of a precise date is often made with the goal of preventing the spread of disease.
For the study, we designated three sections of Pillitteri Estate Winery for our control groups. In the first section, leaves were removed manually. For the second section, the leaves were removed mechanically, and for the third, no leaves were removed. For the first and second control groups, leaf removal was done once at pea size, and a second time at veraison.
We inspected the vines over the course of 17 visits, noting the following performance indicators:
- damage to the vines and berries
- vine vigour
- no active shoot growth after veraison
- Brix values
- total yields for each group at harvest.
No disease was noted during the study, so the presence or absence of disease was not considered a performance indicator.
The mechanical and manual leaf removal groups performed better in all indicators than the non-removal control group, with mechanical leaf removal producing the highest yield. Manual leaf removal is a more aggressive method than mechanical removal, resulting in the most cluster exposure and lower yields. Damage to vines and fruit was comparable between mechanical and manual removal and within acceptable tolerances. Brix values were comparable across all three groups.
Tips for Optimal Mechanical Leaf Removal
One of the reasons that manual leaf removal doesn’t stack up to mechanical is human variance. It’s almost impossible to expect one person to remove the same number of leaves from both sides of a vine across an entire vineyard over many hours or even days. When you factor in multiple people working together, you can have significant variances in a vineyard, with some vines thinned too much and others not enough.
Passive mechanical leaf removers that rely on operator skill have the same limitations. It’s difficult for an operator to maintain concentration over prolonged periods. Oddly-shaped vines and uneven rows can limit the machine’s effectiveness, resulting in damaged fruit or inadequately thinned sections.
When purchasing a mechanical remover, look for one that features a sensitive vine contact sensor head. The head floats along the vine canopy and regulates the pressure exerted using an ultrasound sensor, maintaining a steady, operator-defined pressure regardless of vine variability, ground conditions, or operator skill/fatigue.
Other tips for maximizing your yield:
- Prior to removal, tuck in vines and remove any suckers.
- Closely follow leaf reduction with spray application for disease/pests. The improved airflow increases spray penetration into the fruiting zone. Any fruit damaged by the leaf remover will dry up and will be unable to spread rot.
- Reduce the number of leaves removed during very hot, dry growing seasons when too much sun exposure can burn the fruit.
- In our study, mechanical leaf removal worked best with VSP trellising systems.