What are growing degree units?

Last week I teased you about growing degree days.  Weren't you intrigued and just dying to know more?  Probably not, but that doesn't matter because you're going to find out.

Let's start with the problem.  I have two greenhouses set at different temperatures.  I also planted greens on different dates throughout the fall.  In the chart above you can see:

1. The average temperatures were different in the two greenhouses, especially as it got cooler
2. The temperatures vary depending what time of the season greens are planted 

This is where growing degree units come in!  But what are they?

"Growing degree units are a measure of heat accumulation used by horticulturists, gardeners, and farmers to predict plant and pest development rates such as the date that a flower will bloom or a crop reach maturity."  Wikipedia 

So, they measure heat accumulation.  Luckily they are easy to calculate, you just need to know:

1. Average daily temperature
2. The base temperature, below which your plants won't grow.  For salad greens I use 40 degrees F.  This is called the biofix.

With these two things, and the date you can calculate the number of growing degree units in a table like the one below.  All you do is subtract the biofix from the average daily temperature. The difference is the number of growing degree units accumulated that day.  Then you can add them up over a period of time (like a growing season) and compare differences in temperature!

These two tables show the number of growing degree units accumulated in each greenhouse over a ten day period.  Greenhouse 2, the warmer one, accumulated nearly twice as many growing degree units in this time!

Growing degree units are used to predict flowering blooming, crop maturity or insect emergence.  I've been using them to examine the growth rate data, but more on that in the future

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