The amount of water that wine grape plants need to maintain a healthy growth cycle largely depends on the grape variety, soil quality,the width of planting rows, and the current life stage of the plant. A lot of grape varieties take on different flavors depending on how much water that they receive. In dry climates, for example, grape yields are characterized by a concentrating of the more sour flavors of plant. Wet climates are known to produce more fruity flavors. That being said, plants in common wine growing areas like Napa valley require about 35-50 inches of water during the grow season.
That said, a lot of vineyards over-water their grapes, according to Mark Greenspan of winebusiness.com. Water can be saved by holding off irrigation until enough moisture leaves the soil to slow vegetation growth, he argues, and that the available moisture in the soil allows vines to withstand heat without supplimental irrigation.
Grapevines as a crop use very little water in the real world. Agneau in the first answer above mentions that Napa grapevines require 35 to 50 inches of rain – this is wrong by a factor of ten, and the confusion probably arises from incorrectly interpreting the information in the article her references concerning the use of plant water stress testing to schedule irrigation.
Based on temperature, humidity, wind speed and other factors, one can calculate how much water a plant can possibly use. The 35 to 50 inches Agneau mentions is the maximum possible water usage of a grapevine during a year – that is, if you fed a grapevine as much water as it could possible transpirate, that is how much it would use.
A grapevine that is given as much water as it can use rapidly grows out of control and becomes a monstrous bush, very difficult to work with, and it’s water use efficiency is very low under those conditions – that is, it becomes fat, happy and stupid. Grape growers aren’t interested in grapevines like that. In fact, we irrigate grapevines far below that level to encourage water stress, to control the annual growth of the plant, and control the quality of grapes. In the real world, grapevines in California survive quite well on 2 inches of water per month during the hot growing season months of June, July and August, or even half that. Many growers don’t irrigate grapevines at all, a practice known as Dry Farming.
Here is a coffee-table factoid: A grapevine given one-half of it’s annual water needs will produce roughly 80 percent of it’s maximum potential yield. Think on that for a moment – cutting the plants basic food supply in half, it can still produce 80 percent of it’s theoretical maximum yield. Clearly these plants have a wide range of water usage ability. And in fact, for premium wine grapes, we operate the plants far below their maximum yield potential – we typically strive for controlled yields of 3 to 4 tons of fruit per acre, while the plants can readily produce 20 tons or more per acre given an infinite supply of water and lots of hot sunny weather.
Anyway, short answer is NO, THEY DON’T NEED A LOT OF WATER. Maybe 2 gallons per mature vine per week in a hot climate.
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