Interannual Effects of Early Season Growing Degree Day Accumulation and Frost in the Cool Climate Viticulture of Michigan

Michigan daily climatic data and seasonal vine performance and phenological data (budburst timing) were analyzed to establish relationships between temperature (e.g., growing degree days or GDD) and juice grape yield and quality in Vitis labrusca grapevines. In viticultural regions such as Michigan, early season vine growth is highly important: Vines coming out of their winter dormancy need to withstand any potential bud-killing frosts after budburst. The temperatures during the months of March, April, and May are highly variable from year to year in Michigan, however. The average GDD accumulation at the time of budburst (average date is 27 April) from 1971 to 2011 was 158 (base 10°C) with a coefficient of variation of 45 percent. Seasonal GDD deficit or surplus at the midpoint of a growing season (as compared to an average year) was correlated to grapevine performance and the accumulation of GDD on a yearly basis was found to occur at a highly variable rate. Early season GDD accumulation was found to be a relative indicator of the end season total, where an early season deficit (or surplus) was able to predict whether the season would still be in deficit (or surplus) at the end of 80.5 percent of all seasons studied. Finally, a statistical model based on historical temperature data was created to calculate the date of budburst. Michigan's warming trend will likely continue in the future, which should bring positive effects to the region. Early season variability and post-budburst frosts are likely to still be a concern in the near future, however.