Growing Degree Days

Every type of vegitation requires an input of energy from the sun. This is particularly true of wine grapes, many varieties of which do well in a sunny, Mediterranean climate.

The suitability of a particular locale for grapes can be encapsulated in parameters that describe soil quality and climate. One of the best measures of climate is a quantity called growing degree days. Put simply, growing degree days (or GDD) is a number that tells you whether the growing season contains enough days when the temperature is within a range that is conducive to production of sweet, flavorful grapes.

An excellent reference on this approach to climate suitability can be found here, where Amin Pirzade of the University of British Columbia discusses the suitability of the Napa Valley for wine grape production.

GDD depends on the recognition that growth requires a sustained temperature of at least 10 degrees Celsius (50 degrees Fahrenheit), and that temperatures above 30 degrees Celsius do not usually aid growth. The amount of GDD accumulated during a given day is usually calculated from that day's high and low temperatures as

where the base temperature is 10 degrees. We also replace the daily high with 30 degrees if the high is greater than 30, and the low with 10 degrees if the actual low is below 10 degrees. Note that we are using the Celsius scale here, not the Fahenheit. We will use Celsius temperatures throughout this website.

To find the GDD for an entire growing season, say April through September, you would sum all the daily contributions.

In his research linked to above, Pirzade found that an accumulated GDD of 1390 or more is ideal for viticulture in the Napa Valley. A GDD of less than 945 is indicative of poor conditions.

Can we use these numbers for the Pacific Northwest? The answer is Yes. Although GDD is a fairly rough measure of climate, and is just one one of several conditions that one looks at in assessing the suitability of a location for growing wine grapes, we can safely say that it would be good to find a location where GDD is in the 1200 to 1500 range, and where other factors such as soil composition and drainage are appropriate. Locations where GDD is less than 1000 during the growing season are likely to be poor choices.