Grapevine is a plant of warm climate: it requires a sufficient amount of heat for growth. As its development is strongly influenced by daily temperature, grape growers often use weather-based indicators to predict vine growth stages such as bloom, veraison, and maturity. The growing degree day calculator is one of the most known tools to predict plant development rates.
Growing degree days (GDDs) also help grape growers to predict the development of different grapevine pests. Insects, just like vines, need a sufficient amount of accumulated heat in the season to emerge. With the help of calculated GDDs, vineyard managers can predict when the insects will be active or when they will be in the most vulnerable phase. Hence, the grower can effectively suppress them with a minimum impact on the environment.
Apart from those two reasons, growing degree days are useful also for:
- prediction of the suitability of a region for the production of a particular grapevine variety;
- prediction of the best timing of fertilizer or pesticide application;
- estimation of the heat stress;
- comparing one region to another and one season to another.
How to calculate growing degree days?
Growing degree days (GDDs) are calculated for each day based on the daily average temperature and base temperature of a particular crop or pest.
GDD = (Tmax + Tmin) / 2 – Tbase
The base temperature for grapevines is usually set to 10°C, but it can be more depending on the growing region. Also, each grapevine pest has its own base temperature that is required for them to develop. It is assumed that under the base temperature, vines and insects don’t evolve and grow. In cases when the daily mean temperature is below the base temperature, the added value to the calculated GDDs is zero. Also, the maximum temperature can be set, and it differs for different crops, pests, regions, and models.
Typically, GDD starts to accumulate at the start of growth in spring – for vines in the Northern hemisphere, that’s the 1st of March or 1st of April, while on the Southern hemisphere on the 1st of September or 1st of October and ends at the end of the growing season. This is when the average daily temperatures start to exceed the base temperatures for GDD calculation.
Examples
Different vine varieties require different amounts of summed GDDs for a specific growth stage. For example, early bud burst vine varieties, such as Chardonnay, need about 75 GDD for budburst and 345 GDD for flowering, with a base temperature 10°C and starting date of calculation on the 1st of March. On the other hand, the late bud burst vine varieties, such as Cabernet Sauvignon, need approximately 87 GDD accumulated until budburst and about 375 GDD accumulated for flowering.
For the grapevine pests, for example, in Turkey, the base temperature for European Grapevine Moth is set to 12 °C. The first generation emerges at about 120 GDD, the second at 520 GDD, and the third at 1047 GDD.
Comparison of different wine growing regions -> Winkler index
To compare one wine-growing region to another, A. J. Winkler developed the Winkler index (originally for California) based on the growing degree days summed over the season. He classified the climates of wine-growing regions into five regions or classes and identified typical grape varieties that fit each region. For the calculation, he used the sum of degree days over 10°C from April 1 until October 31. Winkler index has some limitations since, for example, it doesn’t consider soil temperatures, sun exposure, latitude, precipitation, and the risk of extreme weather that can influence grapevine growth. However, it gives a rough overview of the climate of various wine-growing regions and the possibility of comparing one wine region to another.
| Regions | GDD | Wine regions | Grape varieties |
| Region I | <1390 | Tasmania, Champagne, Burgundy Friuli, Chablis | Chardonnay, Pinot noir, Sauvignon blanc, Riesling |
| Region II | 1391 – 1670 | Yarra Valley, Alsace, Boredeaux, Napa | Cabernet sauvignon, Merlot, Semillion, Syrah, Chardonnay |
| Region III | 1671 – 1950 | Rioja, Piemonte, Clare Valley, Capetown, Barossa valley | Tempranillo, Grenache, Barbera, Syrah |
| Region IV | 1951 – 2220 | Langhore Creek, Montpellier, Florence, McLaren Vale | Tempranillo, Mourvedre, Carignan, Cinsault |
| Region V | >2220 | Sicily, Sardinia, Jerez, Swan Valley | Fiano, Primitivo, Palomino |
Conclusion
There are several reasons why grape growers calculate growing degree days for their vineyards. Mostly to predict vine growth stages and the development of grapevine pests, analyze the region’s suitability for the production of desired vine variety, estimate heat stress, and compare seasons and grape growing regions.
Sources:
Growing degree days, Oregon viticulture
Growing degree-day, Wikipedia
Growing degree days, farmwest
Winkler’s climate regions, WineWisdom by Sally Easton MW
Temperature, wineskills
Arnold J. Bloom, UC Davis; Presentation: Using Satellite and Ground Data to Examine Climate Change and Premium Wine Production in California
Personal notes
Updated on 22.3.2023.
