Relating harvest timing to growing degree accumulation in grapes

Both the grape grower and the winemaker can benefit from the assessment of grape fruit maturity, potential wine quality at harvest, and the estimated optimal timing for harvest. Our studies of the timing of harvest over a period of 16 years for numerous grape cultivars adapted to the Nebraska climatic and geophysical conditions have been related to the accumulation of growing degree days (GDD, base 50°F=10°C) across three research sites. Results are reported for 16 cultivars evaluated for up to 15 years. The mean harvest GDD are noted, including the extremes for earliest and latest harvest (data not shown). The mean GDD for 'Marquette' was 2749, with extremes of 2322 and 2887. If the unusual extreme low is removed from the data set, the mean GDD is 2806 and nearly 80% of harvests fell between 2803 and 2887 GDD. Similarly, we found that the mean GDD for 'Frontenac' was 2880 with a range of 2676 to 3281, while for 'Vignoles', the mean was 3030, with a range of 2710 to 3374. Although harvest parameters such as total soluble solids, pH and titratable acidity (TA) all influenced harvest timing in some years, the data accumulated over 16 years have enabled us to provide growers with a rough estimation of proposed harvest times for these cultivar examples and other hybrid grape cultivars grown in Nebraska vineyards.