The timing and method of harvesting have a major impact on quality. A grower or producer's decision as to when to harvest will be decided upon with respect to many factors, including seasonal parameters, the weather forecast, the risk of botrytis, the balance of sugar and acidity in the grapes, the maturity of skin and seed tannins and the formation of aromas and aroma precursors. Of course, as the harvest cannot be organised at a moment's notice, careful analysis and sophisticated prediction tools are employed.
Historically, especially in some regions of the Old World, many growers would pick early - in fact pick too early for the grapes often lacked phenolic ripeness or even the desired levels of sugars. Growers also lived in fear of autumn rains spoiling the harvest with bunch rot and dilution and so undertook early 'insurance' picking. In order to ensure that the grapes have an acceptable, minimum level of ripeness, the rules of Appellation d'Origine Protégée (Appellation Controlée) include, in most regions, a 'ban de vendage', a date variable each year before which the harvest may not take place. Nowadays, growers more often wait for phenolic ripeness which, in warm and hot climates this may not come until after the desired levels of sugar and acidity are present. This can result in over-alcoholic and over-loud wines. However, due to climate change and alterations in canopy management, harvesting in now taking place earlier in the season