Each day, record the high and low temperatures. The basic procedure for calculating degree days is very simple, and all you need is a maximum/minimum thermometer. temperature), but easy mathematical equations yield approximations satisfactory for practical applications. The mathematics for calculating actual degree days can be very complex (determining the area under the curve for the graph of time vs. A certain number of units are added each 24 hour period, depending on how much the temperature is above threshold, to produce a cumulative total of degree days. The base temperature is often noted as a subscript number following the abbreviation for degree day (e.g., DD50).ĭegree days are accumulated whenever the temperature exceeds the predetermined developmental threshold. But there are a number of insects, particularly those that are active early in the spring, that have a lower threshold (38° or 43☏), and many cool-season vegetables such as cole crops and peas have a base temperature of 40☏. This is the temperature at which dormancy is broken in plants and when many insects become active. The most common developmental threshold is 50☏. The higher the temperature, the faster the rate of development. As the ambient temperature increases above the threshold, development occurs. This base temperature, or developmental threshold, differs depending on the species. All species have a cutoff temperature below which no development occurs. The warmer the weather, the more quickly they develop, and the cooler the temperature, the slower they develop. All plants and insects develop in response to temperature. Degree days (also known as heat units or thermal units) are a way of incorporating both temperature and time into one measurement to quantify the rate of plant or insect development. Another way of achieving the same result is to use degree day calculations. You may not have the critical indicator plant nearby to time local activities, or there may not be a good indicator plant for a particular pest’s crucial life stage. Indicator plants are not always suitable for the timing of pest management practices. Karen Delahaut, UW-Madison Fresh Market Vegetable Program
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