A green harvest is the removal of immature grape bunches, typically for the purpose of decreasing the vines yield. In French it is known as a 'vendange verte'.
Green harvesting is a relatively modern practice most often used to produce fine wine. Removing the small, immature, un-ripe grapes while they are still green encourages the vine to put all its energy into developing the remaining grapes.
In theory this results in better ripening and the development of more numerous, mature flavour compounds and increases the quality in the remaining bunches. In the absence of a green harvest, a healthy, vigorous vine can produce dilute, unripe grapes.
Many traditionally renowned regions have natural conditions that repress excess vigour. Examples include the gravelly soils on the left bank of Bordeaux, the often cool, fragile climate of Burgundy, and the infrequent rainfall of Rioja. In these regions, the vine is prevented from producing too many grapes without human intervention required. However, in regions with fertile soil, abundant sunlight, and irrigation practices, the vine can generate large quantities of uninspiring grapes.
One solution is a green harvest. After fruit set, the quantity of grapes that will result from a vineyard can be projected. Often the wine-grower has a target yield in mind, measured in tons per acre or hectolitres per hectare. A portion of the grape bunches are cut off by hand, to leave approximately the correct amount. In Europe, many appellations restrict the yield permitted from a given area, so there is even more incentive to perform green harvesting when presented with excess crop. Often the excess must be sold for a nothing and used for industrial alcohol production.
While the concept of thinning or sacrificing part of the grape crop, i.e. green harvesting, with the aim of improving the quality of the remaining grapes, predates modern critics, the practice has increased in recent times in vineyards found in many regions around New Zealand and areas where grapes grow vigorously.