In my Eastport high school classroom, botany students are monitoring the changes in campus plants as spring advances and correlating these changes with accumulating Growing Degree Days, or GDDs. We devote every Wednesday’s class to a campus tour, checking each plant for signs of change since the last visit.

Are leaf buds swelling on the lilac yet? Are there flowers on the forsythia where last week there were tightly-closed buds? Are any of the native maples showing their flowers this week? Questions like these are the focus of our walks. They are questions central to the science of plant phenology, the study of changes in the annual cycle of plants, especially those changes related to climate.

Gardeners are keenly aware of these changes and of how each plant species is unique in the pace of its response to seasonal changes. Forsythias jump at the first hint of spring warmth, while the Chionanthus (fringe tree) is all but given up for dead before it finally breaks bud. I would have each of my students appreciate such diversity; I would make gardeners of them all.

Phenological changes are stimulated by accumulated GDDs. The first class in this project was devoted to understanding how to calculate each day’s contribution to total GDDs. You begin by averaging the high and low temperature for the day. Temperatures lower than 50 degrees F are set at 50; temperatures greater than 86 degrees F are set at 86. The assumption here is that 50 degrees is too cold and 86 degrees too hot for most plant growth. This method is often referred to as the (50, 86) method of calculating GDDs.

From the average of high and low temperature, you subtract 50 and the result is the number of GDDs for that day. As an example, consider an early April day in which the high temperature was 54 and the low 40. Setting the low to 50, the average would be 52. Subtracting 50 gives a total of 2 GDDs for the day. In early April, GDDs accumulate slowly. Still, some phenological changes, such as bud break in pussy willows, do occur at minimal accumulated GDDs (11.5 according to our study).

As spring advances into summer, GDDs accumulate steadily and at an increasing rate. By the middle of June, when we will have to abandon our project, as many as 600 GDDs may have accumulated.

Plants are not the only organisms that respond to GDDs. The emergence and developmental stages of insects that feed on plants are also controlled by accumulating GDDs. Thus the hatching of viburnum leaf beetle eggs is synchronized with viburnum leaf development.

Landscape managers and nursery growers now realize that planning their insect control strategies using GDDs as benchmarks is far more effective than the old calendar date approach to pest control. Equally effective is the use of landscape plant phenology to dictate pest monitoring. After all, phenological changes are reflections of accumulated GDDs.

On the classroom wall is a large poster, a drawing of a thermometer, its basal bulb colored deep red. Instead of degrees, the long column extending upward from the bulb is marked in GDDs from zero to 600. Slowly the red color will rise upward, tracking the accumulation of GDDs as we post our weekly observations of plant phenology. The growing season advances for both the plants and my students.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to reesermanley@shead.org. Include name, address and telephone number.