Growing Degree Days

Data: Growing Degree–Days

Because temperature plays an important part in the rate of development of plants and many diseases and pests (especially insects), a measurement including the accumulation of heat with passing time is necessary to predict maturation. Growing degree-days provide a measure for calculating the effect of temperature on the development of plants and pests. One growing degree-day is the amount of heat that accumulates when the temperature remains one degree above the base developmental threshold for 24 hours. One growing degree–day is also the amount of heat that accumulates when the temperature remains 24º above the base threshold for 1 hour. (What about Heating and Cooling Degree-days?) Note that there are no negative degree-days. If the temperature remains below the threshold, there is no degree-day accumulation.

Unlike strict time predictions of plant or pest development, growing degree-day predictions hold true regardless of location or temperature fluctuations. As long as you know the number of degree-days necessary for plant/pest development, you may use degree-days as an accurate predictor. For example, you may know that it takes, in general, three weeks for a specific pest to develop. What you will find, however, is that the pest may take 4 weeks to develop in cooler weather and only 2 weeks to develop in warmer weather. The time prediction can be off by up to a week by looking at time alone, while the degree–day prediction should result in far greater accuracy.

Source: Davis Instruments