Instructions How to Measure Growing Degree Days (Participation is Optional!) 1. A Soil Temperature Thermometer is Needed If you don't have one you can purchase for $13 (plus $4.50 shipping) from: Weatherama (800-848-4912 X 3187). Request Order #116. 2. Measure the Soil Temperature Every Day 3. Record temperature and date on this Growing Degree Day Chart: Growing Degree Chart 4. Learn how to calculate "Growing Degree Day" units See this Sample Chart for practice and details. Important: Understanding the term "Base Temperature" Everyone knows that plants don't grow when it's too cold. Scientists have found that there's a certain temperature at which a plants can begin to grow. This threshold is called the "base temperature". The base temperature is different for different plants, but preliminary research shows that the base temperature for milkweed is 33F (1C). This means that: Unless it's above 33F, milkweed does not grow. Each day, use the Growing Degree Day equation found on the chart to calcuate the amount of heat your plants have received. Subtract the base temperature from the soil temperature. These daily units of heat are called "Growing Degree Days". Link to SAMPLE Growing Degree Chart Link to Your Growing Degree Chart Copyright 2000 Journey North. All Rights Reserved. Please send all questions, comments, and suggestions to our feedback form