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  • Tulips, Temperatures, and the Arrival of Spring

    Measuring the Amount of Heat Required for Your Tulips to Grow and Bloom

    Ice melts, leaves emerge and tulips bloom--the winter world comes alive as the earth warms. Scientists have discovered that you can actually measure the amount of heat it takes to make some spring events occur. This accumulated heat is measured in units called "Growing Degree Days". This activity will help students understand this concept. As they measure temperatures each day, they can analyze the role temperature plays in setting the pace of spring's arrival.


    1. Ask students to think about the steps in baking a cake. The oven must be set at a certain temperature--and the cake must remain in the oven for a certain length of time--in order for it to bake properly. A cake placed in a cold oven won't bake no matter how long it remains there. Ask students how the requirements for baking a cake might be similar to the needs of growing plants. See if they can develop the analogy to explain that it takes a certain amount of heat, accumulated over time, in order for plants to grow.

    Also, just as a cake requires a certain temperature for proper baking, there's a threshold temperature at which plants begin to grow. This is called the base temperature. The base temperature we will use for tulips is 40 degrees. That is, we only expect tulips to grow on days the temperature is above 40 degrees F. When it's below 40 degrees F, we don't expect any growth to occur.

    2. In order to find out how much heat tulips need to grow and bloom, students will add up the heat received every day--as long as it is warmer than 40 degrees, the base temperature.

    Print out a Growing Degree Day Chart.

    Using the sample chart below, show students how to calculate Growing Degree Days. Follow these steps:

    STEP A: Measure the High and Low Air Temperature Each Day
    You can find the daily high and low readings in your newspaper. Make sure you use the day's actual tempeartures, not the temperatures predicted in the forecast. The actual temperatures for a day are published in the next day's newspaper. If you have a weather station at your school, you can measure the temperatures yourself with a maximum/minimum thermometer.

    STEP B: Find the Average Daily Temperature

    To calculate the average daily temperature, first add the high and low temperatures. Then divide by two. Here is the equation:

    High Temp. + Low Temp. / 2 = Average Daily Temperature

    STEP C: Calculate the Amount of Heat Accumulated Each Day

    Each day, use the Growing Degree Day equation to calcuate the amount of heat your plants have received. Subtract the base temperature from the average daily temperature. These daily units of heat are called "Growing Degree Days".

    Average Air Temperature- Base Temperature = Growing Degree Days

    STEP D: Keep a Running Total of Heat Accumulated During the Season.


    • The sample chart below shows the running total added up over a 5 day period. Have students fill in the last 2 days, for practice.
    • Notice that the Growing Degree Day value for Day 1 is zero. This is because the average daily temperature (39 degrees) is below the base temperature (40 degrees). Thus, it was not warm enough on Day 1 for any growth to occur.
    • Growing Degree Days are cumulative. This means you add them up from day to day. In the example below: Day 1 has zero. Day 2 has 2. We add Day 1 and Day 2 together to get 2 GDD. We continue to add the amount of heat the plants receive each day. After 5 days they have received 17 Growing Degrees Days.

    Growing Degree Days

    Measuring the Amount of Heat
    Required For Our Tulips to Grow and Bloom

    DATE Today's Temperature Base Temp. for Tulips Today's
    Growing Degree Day
    Growing Degree Days
    Hi Lo Av (Av. Temp) - (Base Temp) = GDD
    2/3/97 45 33 39 40 F 39- 40 -1 0
    2/4/97 50 34 42 40 F 42 - 40 2 2
    2/5/97 50 36 43 40 F 43 - 40 3 5
    2/6/97 52 38 45 40 F 45 - 40 5 10
    2/7/97 54 40 47 40 F 47 - 40 7 17
    2/8/97 52 42   40 F      
    2/9/97 60 40   40 F