DDG or degree days growing is widely accepted as a measure of a locations ability to ripen a crop of grapes. There are few other measures as well including the LTI (latitude/temperature index) and calculation of the mean monthly temperature of the warmest month. However, in most of the reading I’ve done DDG is most widely used and also accessed when comparing what varieties will mature in a given number of degree days. For instance, one document on grapes varieties grown in British Columbia describes that you need at least 850 degree days growing and 135 frost free days to grow the earliest wine grapes in BC. It goes on to state that 1000 degree days are required to grow mid-season varieties.
Degree days for different crops are calculated to different base temperatures. For grapes the base temperature is 10 degrees celcius because generally speaking, grapes begin to grow in the spring once the average daily temperature is 10c. There are some hybrids that begin to grow at temperatures a few degrees cooler but again 10c is the general rule. Other crops like corn use 5 degrees celcius. Degree days for a particular day are calculated by taking the average of the sum of the day time high and the day time low and subtracting 10. If your day time high was 26 and the day time low was 12 the average for the day would be 19 (26 + 12 = 38, divided by 2 =19). Then you’d subtract 10, which leaves you with 9 degree days growing above 10c for that day.
You can easily do this for each month as well. For instance if the average daily high for the month was 24 and the average low for the month was 11, the monthly average would be 17.5 (24 + 11 = 25, divided by 2 = 17.5). Then you subtract 10 from the average which leaves you with 7.5 degree day for each day of the month. So if this were July, with 31 days, you add 7.5 degree days for each of the 31 days in July which would give you 232.5 degree days growing above 10c for the month.
So really what we are calculating is how much heat above the 10c base was accumulated over a particular time period. This is important, specifically “above 10c”, especially if you are in an area that is prone to night time lows below 10 degrees celcius. Temperatures below 10c have a negative impact on the growth, development, nutrient uptake of a grape vine. Research has shown that when a vine is exposed to temperatures in the 5c-10c range the growth slows and has a negative impact on the vines ability to metabolize carbon dioxide and on the photosynthesis ability. The effects will have a further lingering/lag effect even as day time temperatures return to above 10c which in turn causes a lag time in vine development and grape maturity. Photosynthesis is best in the morning, so this lag time inhibiting photosynthesis at this time of day is detrimental to growth.
So if you examine temperature profiles for two locations the degree day accumulations may appear the same however, the locations ability to ripen grapes may be better in one location than the other as the day-night (diurnal) temperature swings are less. For instance a location with a monthly average day time high of 24 degrees and monthly average nigh time lows of 14 degrees would have a monthly average of 19 degrees. This location with the average night time lows of 14 degrees would be expected to have few nights with temperatures below 10c. Another location with monthly average day time highs of 28 degrees and monthly average lows of 10 degrees would have the same monthly average of 19 degrees. However, this location would be expected to have several nights with temperatures below 10c and with it one could expect some negative impact on growth and potentially to ripen grapes a few days later than the other site. In fact, scientists who study the effects temperature on vine development often use degree day hours to calculate the accumulation of hours rather than days above 10c as this can be a more accurate assessment of accumulated heat. Of course there are so many other factors as well, wind, hours of sunshine etc., so this is simply a generalization.
But this does become important when one considers that 850 degree days growing and 135 frost free days are required to ripen the earliest grape varieties in BC. If you get 850 degree days but the development and ripening is delayed most mornings due to cool night time lows then one may need more than 135 frost free days to ripen the grapes - perhaps 140 instead. With this in mind one has to remember the principals of frost avoidance not only to safeguard from frost but to help ensure an optimum vineyard mesoclimate. Clearing trees so cold air can drain away, short mid row cover or none at all, and wind breaks to deflect cold air entering your site can not only stave off frosts but can raise the average night time temperature and possibly improve the vineyard conditions throughout the entire growing season.