Water Moderates Both Summer and Winter Temperatures
Water moderation (less continentality, lower MDR) increases the length of the growing season. In the fall, continental sites have greater temperature fluctuation meaning higher day temperatures and lower night temperatures. This aids fruit maturation, but should the temperature drop below 28°F, foliage is killed and the season is abruptly terminated. This not only interferes with fruit maturation, but it can prevent the achievement of potential grapevine cold hardiness.
Where MDR is high, the probability of spring freeze increases. In the spring when the temperature is 50°F or more, buds begin to develop. High MDR sites experience more frequent high temperatures which promote early bud growth and more frequent low temperatures which may cause freeze injury. The primary benefit of water moderation to NY sites is reduced winter cold hazard. The water moderation provides an additional benefit of reduced low spring and fall freeze hazard. More continental sites may require freeze protection (sprinkle irrigation, wind machines or heaters) in the spring, early summer and fall to prevent freeze injury to green tissues.
Low temperature hazard is primarily related to latitude. At northern latitudes water moderation is required for winter cold moderation. Northern European vine production benefits from the moderating influence of the Atlantic Ocean and especially of the Gulf Stream. (Note relatively high December, January and February expected monthly low temperatures in Bordeaux France relative to the North American sites, and that the very continental Fairmont, MN, also stands out as having more extreme temperatures. In the latter case the problem is both variable temperatures and low total heat supply.) Water moderation that reduces continentality facilitates grape production at higher latitudes.
Read more about this topic: Continentality
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