“From December to March, there are, for many of us, three gardens —
the garden outdoors,
the garden of pots and bowls in the house,
and the garden of the mind’s eye.”
— Katherine S. White
Suddenly it is winter. The garden outdoors is asleep, or so it seems to the casual observer looking across a landscape of empty beds covered with a thin layer of crusty snow. But in my mind’s eye I see the bumblebee queen sleeping at the end of an old tunnel lined with decaying leaves and mouse hair; microscopic icebergs forming in the intercellular spaces of dormant buds; earthworms wriggling deeper. Once in a while I catch sight of a goldfinch perched atop a dried brown seed head, pecking at all that is left of summer’s sunflower.
Life goes on in the winter garden as plants and animals undergo changes that are part of their annual cycles. Out of sight, far from the thoughts of the gardener planning for next spring, these changes are no less important than the phenological changes that occur during the growing season.
The National Phenology Network is an organization of citizen scientists working to record the influence of climate change on phenology, the study of changes in the annual cycles of plants, animals and landscapes. Gardeners will make significant contributions to this effort.
Gardeners are keenly aware of phenological changes, of how each plant and insect species is unique in the pace of its response to seasonal changes. Forsythia buds break at the first hint of spring warmth while the fringe tree is all but given up for dead before it finally breaks bud. Dandelion flowers greet the emerging bumblebee queen.
I remember past efforts to engage my students as citizen scientists tracking plant phenology. Beginning in early April, we devoted every Wednesday’s class to a campus tour, checking each plant for signs of change since the last visit. Were leaf buds swelling on the lilac yet? Were there flowers on the forsythia where last week there were tightly closed buds? Are any of the native maples showing their flowers this week?
Phenological changes are stimulated by accumulated growing degree days, or GDDs, and 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 are set at 50; temperatures greater than 86 degrees 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 pollinating insects and the insects that feed on plants also are controlled by accumulating GDDs. Thus the hatching of viburnum leaf beetle eggs is synchronized with viburnum leaf development, or so it has been in the past.
One question on the minds of scientists who study phenology is whether plants and insects will adapt to climate change at the same pace. Will the bumblebee queen emerge from her winter nest one year, only to discover that there are no plants yet in bloom to supply life-sustaining pollen?
Gardeners of all ages and backgrounds can join the National Phenology Network of citizen scientists, contributing vital information on the timing of flowering and insect development, serving science and society by promoting broad understanding of plant and animal phenology and its relationship with environmental change.
You can learn more by visiting the National Phenology Network website at www.usanpn.org/about.