The water a vine loses through transpiration must be replaced by irrigation or rainfall. Transpiration occurs when leaves use carbon dioxide to produce carbohydrates through photosynthesis. On a warm summer day in California, mature, full-canopy kiwifruit vines can transpire 7,000 to 8,000 gallons of water per acre per day.

Irrigation is necessary to keep vines growing and productive and to maximize fruit size. Water stress can reduce fruit size, limit flower numbers (by reducing flower bud initiation), and cause premature fruit drop. Summer drought can cause leaf drop and premature fruit ripening in A. arguta; however, fruit ripen more unevenly and are of poorer flavor. Severe drought also may cause fall flowering, which reduces the following year's production. Maturity of wood also seems to be delayed by drought, thus increasing risk of cold injury. However, too much water also may cause production problems.

Judging vine water needs is difficult. At this time, it's not known whether irrigation requirements in the Pacific Northwest differ from those in other regions. Based on information from British Columbia, Canada, vine water use ranges from 25 to 35 gallons per vine per day (at 70 to 80°F).

Irrigation for frost protection also is necessary to protect young shoots in spring and potentially fruit in fall (for fuzzy kiwifruit only). See "Establishing Your Kiwifruit Vineyard" for more information.

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