The importance of Rainfall in the Grape Growing process
Rainfall brings cloud cover and moderates the temperature. In California, temperature moderation and cooling are almost synonymous, but not always. Especially on frosty spring mornings, rain can increase the temperature.
One of the reasons that California produces such great grapes is that the state generally has dry weather in late spring and summer. Too much rain during these months can cause grey rot and other diseases. A rainy summer can also push back the harvest date. But some rain is necessary during these months.
There are specific times during the growing season that rain can be disastrous. Rainfall right before fruit set can cause millerandage or coulure. Both occur when the grapes are not properly fertilized due to cold weather or excessive rain.
Precipitation right before harvest is also bad news. The vines eagerly suck up the water and the sugar/acidity balance is thrown off. Mildew and rot is also a worry. Delicate varietals such as Riesling and Pinot Noir are particularly susceptible. The best years are dry in the weeks leading up to harvest.
Successful California Wineries are intimately aware of their property’s climate, including the amount of rain their vineyards normally receive. But Mother Nature is unpredictable and this is far from an exact science. As Mark Twain said, “Climate is what we expect, weather is what we get.”
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