Growing Degree Days

(This column was published in the Monday, May 26, 1997 Buffalo News.)

For 67 years my father worked for a Rochester fuel company. As a small boy I often joined him on evening trips to his office where one of the things he showed me was how to calculate heating degree days. Those heating degree day seasonal totals -- they accumulate temperatures below 70 degrees -- gave my father comparative data that helped him to determine the amount of coal and oil his company would need for its customers.

It had been years since I thought about that statistic, but when Tom Niziol of the National Weather Service Buffalo Office told me about growing degree days, I guessed immediately that they would be a mirror image of heating degree days. And indeed they are.

Although there are more sophisticated ways of calculating growing degree days using hourly temperatures and even the integral calculus, a satisfactory daily value may be obtained by subtracting a base temperature -- 50 degrees is used locally -- from the average of that day's high and low temperatures, discarding any negative values. For example, if the high temperature on a given day was 70 and the low 50, their average (60) minus 50 would give 10 growing degree days. If, on the other hand, the high had been 50 and the low 40, their average (45) minus 50 would have been negative and no growing degree days would be recorded. Those daily values are then summed from March 1 on to give the accumulated seasonal total.

Growing degree days now serve an important role in agriculture. Previously the development of crops and their associated insects and diseases were estimated only by calendar date with no consideration for colder or warmer seasons. If, for example, a farmer followed guidelines to apply a pesticide to defend against an insect or disease during the first week in June he might be too early in some years, too late in others.

Farmers and gardeners get a better picture by comparing growing degree day totals which take temperature into account. For example, arborists treat for elm leaf beetles, those destructive insects that in some years defoliate many Buffalo trees, when growing degree days accumulate to between 363 and 912. Lawns are defended against Japanese beetles later -- between 1029 and 2154.

You will not be surprised to learn that this is an unusually cold spring. As of May 21 we had accumulated only 48 degree days compared with 133 last year and 163 the year before. Even with a protracted warm spell, it will take weeks to catch up.

Although crop growth and insect life cycles have been retarded, what is more evident is the effect of this cold on many birds. With their natural food supply not yet available, orioles have been forced to visit hummingbird feeders or to feed on orange halves birders set out for them. No such help is available to the warblers migrating through this area. Because they are frantically searching for food, they have been much in evidence and high counts have been recorded. Unfortunately many will die if the cold continues much longer.

That is the bad news. The good news is that some undesirable insects may not only be late emerging but also reduced in numbers.

To see if that is the case with elm leaf beetles, volunteers are being recruited to monitor the beetles in Buffalo. The volunteers will be trained and then assigned to inspect elm trees in various parts of the city. If you can help with this important activity, call Bill Nowak at 851-4361 for details.

Meanwhile let us all hope for warmer -- but not too hot -- weather.