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Heating Degrees and Phenology

April 17, 2021

By Dr. David Galbraith, Head of Science, Royal Botanical Gardens.

All life on earth is driven by chemical reactions. Chemical reactions in turn are linked to temperature. In cold weather reactions are greatly slowed. For plants, the changing of the seasons is in part about how much light is available from the sun, and in part about how much energy is available for the chemical necessities of life.

A fairly simple way to track how temperature affects living systems in the springtime is to measure accumulation of heat units. Also known as degree days, heat units are a concept developed in agriculture. To calculate growing (or heating) degree days (or heating units) from weather data over a span of time, scientists and agronomists start with a base temperature. In agriculture, 10°C is often used as a base. In its simplest form, the high and low temperatures for a day are averaged, and the base is subtracted from that average temperature to get the growing degree day (GDD) number.  GDDs are then tallied over days, weeks and months. Consider an individual plant. If you monitored the temperature of the plant since the first day of spring, the units of heat-time that accumulate since then will reflect how warm or cool the spring has been. Biological events tend to happen when a certain number of heating degree units have been reached and so this calculation can be used to predict some natural phenomena. For example, we know that Forsythia starts to bloom at between 1 and 27 GDD (calculated on a base temperature of 10°C).

Cherry Blossom - May 2020
The link between accumulated heat and time of bloom is very apparent in the timing of early bloomers like cherries.

This idea has found application in studying natural systems as well as agriculture. Turtles, for example, will lay their eggs when a certain number of heating degree day units have been reached each season. Metamorphosis and other processes experienced by butterflies have been linked to the accumulation of heating degree days. In botany, heating degree days are used to help us understand the changing date of the flowering of individual species or cultivars. In fact, this process is one of fundamental concepts in the study of phenology, the timing of biological events. In turn, phenology is one of the key observational sciences available to study the effects of climate change.

Our horticulture staff are reporting that many plants are as much as three weeks ahead of schedule for blooming this year. In order to see if there was a link to the idea of heating degree units, I obtained the hourly weather records for Hamilton from Environment Canada. I then added up the number of degrees spent hourly above 5° C between January 1 and April 14 for each of the past five years. The weather data bears out the observation of early flowering time. As of the middle of April, 2021 is the warmest year of the past five – and by a considerable margin as shown in the chart below.

Whether this trend will continue into 2021 has yet to be seen, but so far, our spring is off to a very warm start!

Graph indicating the hourly heating degree units 2017 to 2021
Hourly heating degree units 2017 to 2021

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