Degree-Days...Looking Into The Crystal Ball

Through the years I have heard the term "degree-days" but I have not actually looked at the definition of the term until this week.  I was reading an article in the American Nurseryman magazine, specifically an article called Degrees of Separation by Dr. Daniel A. Herms (Vol. 194, Issue 4).  This article goes into great detail regarding this scientific form of measurements for the purposes of planning in horticulture.

    Simply stated, "degree-days" are a temperature related measurement of time with regard to the development of insects, diseases, and plant structure such as flowering time.  Okay, this is not as complicated as it sounds unless you are looking for very accurate results, as opposed to an educated guess.  The benefit of "degree-day" knowledge to the home gardener is the understanding of the life cycles.  This is the purpose of using degree-days.

    The 21st Century gardener desires to eliminate or minimize the use of pesticides.  In order to accomplish this task we must develop a greater understanding of the life cycles of pests, the behavior of our plants, improve our cultural techniques, etc.

    Now back to the "degree-day".  There are various levels of sophistication regarding the measurement of "degree-days".  Dr. Herms describes the simplest method as "the average method".  This method requires assigning a "base" temperature which is usually 50 degrees.  The idea is that an insect is actively developing when the temperature is in excess of 50 degrees.  The greater the temperature, the more rapid the development.  When the temperature is below 50 degrees development can stall, but it does not tend to reverse itself.  This is much like building a house.  It can be done in 3 months or 6 months, but regardless of the pace, the framed walls still stand even if the crew leaves for a week.  The progress is not lost.

    Using the average method, the gardener measures the high and low temperature for each day.  The high and the low temperatures are added together and divided by 2.  The base temperature is subtracted from the averaged "high and low" and the difference is the "degree-days".  Example: Your base is 50 degrees.  The nighttime low is 43 and the daytime high is 67.  The sum of 67+43=110.  Divide 110 by 2 to get 55 degrees.
Now subtract the base of 50 from the average of 55.  The result is 5 "degree-days" for the measured period of 24 hours.  Usually your calculations should start as of January 1st for the coming year.  You should also consider that temperature averages hovering around 50 degrees may yield few degree-days, but development is probably marching onward under these conditions.

    By tracking the cumulative "degree-days", one can project the approximate emergence of a pest.  For example, the Eastern Tent caterpillar will emerge after an accumulation of 97 degree-days, probably in March.  The Japanese beetle will emerge after an accumulation of 978 degree-days, probably in June.  Using the average method for degree-day calculation renders approximate results, due to many other variables.

    The real message here is to pay close attention to your environment and keep a detailed garden journal.  Dr. Herms article points out that the Gypsy Moth eggs begin hatching when the Red Buds begin to bloom.  By keeping a journal, you will make notes describing what is going on in your home environment at any given point in time.  Thus, your notes will remind you to examine your Euonymus for scale activity (405 degree-day insect) a week or two after your azaleas have finished blooming.  Additionally your notes will indicate times to prune, fertilize, etc.  All of these cultural rituals will enhance the health of your plants, thus warding off these problems.

    If you want to know more detail about degree-days, try surfing the web.  If degree-days don't interest you, at least consider keeping a garden journal.  Have a great week.

                                                                                    Andy Lynn