2.3.8.b What criteria should be applied when making leaf thinning decisions?

How vigorous is the plot?

Implementing leaf thinning every year does not appear to impact the vigour or fertility of the vine the following year. However, the vine's initial vigour should be taken into consideration. If the vine is very vigorous, leaf thinning is advisable because it reduces the risk of cryptogamic diseases. Conversely, if the vine is not very vigorous, one must be careful not to remove too many leaves, since each leaf is proportionally more valuable, and reducing the number of leaves also reduces photosynthetic activity and therefore the synthesis of sugars.

How are the rows oriented?

Row orientation should also be considered. When the row orientation does not allow the sun to reach the bunches, it is advisable to carry out partial or total leaf removal in order to maximize the bunches' exposure to sunlight.
In general, leaf removal is done on the side facing the rising sun, or on the side opposite the side receiving most of the sun, to the east or to the north depending on the orientation of the vines. Removing the leaves on the rising sun side allows the grapes to be in direct contact with moderate sunlight. It is important to avoid removing leaves on the side which receives most of the sun in the afternoon. This is when the sun is at its hottest and can damage the grapes.

What is the grape variety?

The grape variety must be taken into account when implementing leaf thinning. Some grape varieties need intense, long-term exposure to light. Other grape varieties, are fragile and risk showing symptoms of being scorched if they receive too much direct contact with the sun. Viognier, for example, browns easily when exposed to the sun, as does Riesling. IBMP, the main molecule responsible for green bell pepper aromas in wines from certain grape varieties such as Cabernet Sauvignon and Sauvignon Blanc can be reduced by leaf thinning. This is because the molecule is broken down during ripening by exposure to light and heat.

Are there any recurring physiological problems with the plot?

Taking into account the plot's physiological particularities enables better adaptation and timing of leaf thinning operations. If the plot regularly has a problem with fruit shatter, it is preferable to limit or avoid early leaf removal, which may accentuate this phenomenon by creating more ventilation. Similarly, if the plot is often subject to attacks of Botrytis, leaf thinning can be considered. However, when the leaves are thinned, the bunches are more vulnerable to humidity in cool climates, and condensation on the bunches can increase the risk of Botrytis development. Leaf thinning on both sides can be advantageous late in the season, as can an early, gradual leaf thinning.

What are the weather conditions?

The weather conditions for the plot and/or more generally of the vineyard should be taken into account when considering leaf thinning. If the plot is located in a humid area, close to a watercourse, or in a basin, the risk of cryptogamic disease is higher since the ambient humidity is favourable to certain fungi such as downy mildew.
In such cases, it is essential to carry out leaf thinning early. Knowing the soil's extractable water reserves and monitoring the water available to the vines is advantageous to understand the roots' absorption capacity and the necessity or not of carrying out leaf thinning. When a plot is vigorous and located in a windy area, too much leaf area will result in higher evapotranspiration. In very dry years, this can increase the vine's water stress; leaf thinning can reduce this phenomenon. Similarly, if a plot is protected from the wind, poor ventilation can increase the risks of shatter and disease. Early leaf removal can reduce this risk.

The soil type must also be taken into account: when the soil is very stony, or light in colour, and if the plot has intense sun exposure, the microclimate can expose the bunches to excessive heat; leaf thinning must then be carried out with care to avoid any scorching of the grapes.

What type of wine is being produced?

The type of wine is also a factor when considering leaf thinning. Early, unilateral leaf removal and/or later leaf removal around véraison will improve the quality of the grapes and enable better ripening. Prolonged exposure of the grapes to the sun allows better accumulation of phenolic compounds and anthocyanins for red wines and aromatic compounds for white wines. Leaf thinning and its timing can and should be considered in relation to the harvest date, on the basis of the following question:

Is the objective to harvest early with fresh fruit or later with ripe fruit?
Exposing the bunches to the sun by removing the basal leaves also has an impact on the wine developing any undesirable vegetal notes. Boosting the effect of the microclimate's heat and light on the bunches by leaf thinning reduces concentrations of the IBMP type molecules responsible for the characteristic "green bell pepper" aroma in red grapes like Cabernet Sauvignon.