Calculating Degree Days

Degree Days vs. Phenology

Understanding Plant and Insect Development

Have you ever noticed that some years your peas sprout days after planting, while other years they take nearly two weeks? This happens because plants and insects don’t follow a calendar—they follow the weather.

In phenology, we often look for indicator plants (common, easy-to-recognize plants that signal a change in the season by blooming or budding) to tell us when to look for certain pests. Indicator plants are a key component of phenology and can be useful to target the most susceptible life stages of the pest. However, indicator plants aren’t always reliable. You may not have the right plant growing nearby, or the plant may not bloom at the exact moment a specific pest becomes a problem.

That is where degree days come in.

What Are Degree Days?

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. All plants and insects develop in response to temperature. The warmer the weather, the more quickly they develop, and the cooler the temperature, the slower they develop. 

All species have a cutoff temperature below which no development occurs.  This base temperature, or developmental threshold, differs depending on the species.  As the ambient temperature increases above the threshold, development occurs.  The higher the temperature, the faster the rate of development. 

The most common developmental threshold is 50°F.  This is the temperature at which dormancy is broken in plants and when many insects become active.  But there are a number of insects, particularly those that are active early in the spring, that have a lower threshold (38° or 43°F), and many cool-season vegetables such as cole crops and peas have a base temperature of 40°F.  The base temperature is often noted as a subscript number following the abbreviation for degree day (e.g., DD₅₀).

Degree days are accumulated whenever the temperature exceeds the predetermined developmental threshold.  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 mathematics for calculating actual degree days can be very complex (determining the area under the curve for the graph of time vs. temperature), but easy mathematical equations yield approximations satisfactory for practical applications.

The basic procedure for calculating degree days is very simple, and all you need is a maximum/minimum thermometer.  Each day, record the high and low temperatures.  Determine the average daily temperature by adding the daily high temperature and low temperature together and dividing by two:

Next, subtract the base temperature from the average daily temperature to get the number of degree days for that day:

Finally, to keep track of degree day accumulation, keep a running total of all degree days accumulated from the first of the year.  In Wisconsin, we often don’t accumulate many degree days before April 1, so if you wish to take a break from the math from January to April, go ahead.

The equation above is a very rough estimate of degree day accumulation.  There is also an upper threshold for development in insects, above which there is no appreciable increase in the rate of development.  This is obviously of more concern later in the season than in the spring.  To compensate for the reduced growth rate at high temperatures, modified degree days use an upper threshold of 86°F as well as the lower threshold.  Whenever the actual temperature is below the base temperature, the base temperature is substituted as the low temperature for the day.  Similarly, whenever the daily high is above 86°, 86° is used as the high temperature for the day.

Author: Karen Delahaut, UW-Madison Fresh Market Vegetable Program
Revised:  8/6/2012
Item number:  XHT1086