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 a quality wine. Removing the small, immature, un-ripe grapes while they are still green encourages the vine to put more of its energy into developing and ripening the remaining grapes.

This can result in better overall ripeness and the development of more numerous and complex 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 traditional, renowned wine 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, low rainfall in 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 produce 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 viticulturist has a target yield in mind, measured in tons 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 nothing or 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 has been around for sometime. The practice has increased in recent times, along with vineyards in many regions around New Zealand - and in other wine regions where grapes grow vigorously.