Trimming height should be decided according to the H/S ratio (foliage height/spacing between two rows). Foliage height is generally considered in relation to row spacing so as to avoid casting shade on the neighbouring row as much as possible. Ideally, the trimming height would not be the same for a spacing of 1 as for 2.5m. It is advisable to leave at least five leaves above the bunches on each of the branches to ensure sufficient photosynthesis. This is obviously only an estimate and each winegrower can and should think about a trimming height based on the plot's characteristics, the weather conditions (e.g. strong winds) and the grape variety.
However, be careful to respect the minimum height imposed by appellation regulations. Some specifications impose, on narrow trellised vines, a minimum foliage height of 0.6 times the distance between the rows, which is the distance measured between the binder wire and the maximum trimming height. This is the case for appellations such as Champagne, Pauillac, Anjou Coteaux de la Loire, Burgundy and many others.
The trimmed foliage thickness should also be determined depending on the density of the vegetation. Here again, the winegrower is free to choose the best trimming for their vineyard. In general, by way of example, it is advisable to leave no more than 20 to 40 cm of foliage. If the foliage is too dense, the inner leaves no longer have sufficient access to light. This can result in a decrease in photosynthesis or an increase in the root absorption of potassium (which is partly responsible for an increase in the pH of wines).