How plants count time

Every plant has a minimum temperature for development, and every hour spent above that temperature contributes to growth. In a climate that is changing rapidly, these tools will help you predict your harvest — and more

Unlike human beings, plants can’t control or regulate their inner temperature. This has a huge impact on plant behavior: since all chemical activities are faster when it’s hot and slower when it’s cold, a cold day and a hot day are not the same.

Here is a simple example: whether it's winter or summer, a human pregnancy lasts about 40 weeks. The reason: we are warm-blooded creatures, that can regulate body temperature to sustain all bodily activities. For a plant, the time from flowering to harvest depends on several external conditions and cannot be simply counted in days.

A formula for flowering

In 1730, French scientist Rene A. F. de Reaumur was the first to introduce the calculation of Growing Degree Days (or “GDD”). Since then, this method is being widely used in agriculture to predict the development of plants, cold-blooded creatures like insects, and even disease pathogens.

The base calculation is simple: for every plant, there is a minimum temperature for development, what we call the “base temperature“. Daily hours that are spent above this temperature are contributing to its growth. You can calculate their “heat value” using a simple formula:

Degree days are called “days” because we usually calculate them on a daily basis, although we count them in “heat hours” a certain day can add 10 hours to the degree days accumulation, and a certain phenological stage can be reached after 1500 degree days have accumulated since flowering.

The use of degree days is common in field crops and helps estimate the time it will take plants to reach from one stage to another or to predict the harvest. For example, it takes early maturing sunflowers about 1800–1900 GDD (based on °C) or 3200–3500 GDD (based on °F) from deeding to harvest, so a farmer can plan the seeding date according to the weather in his area and make sure he reaches harvest before the rainy season.

The trees count, too

In tree crops, the situation is different, as traditionally the season starts on the same date every year, and farmers know from experience when every stage is supposed to come and when to expect a harvest.

But the increasing effect of climate change is also changing fruit trees. These changes include trees waking up from dormancy on different dates every year, beginning their flowering cycle on different dates, and reaching fruit maturation and harvest readiness later or earlier than expected. As many farmers put it: “there are no normal years anymore”.

Understanding the mechanism of Growing Degree Days and following it along the season with correlation to phenological stages is important. It can help farmers “make sense” of the changes in their trees’ behavior and help them predict how the season will advance. They will have a better estimate of changes in harvest dates and improve their pest and disease management strategies, which often correlate to specific phenological stages.

Degree Day use case

A great example of the effect of Growing Degree Days on plant development and its importance to farming practices can be seen in the picture below:

In this picture we see the development of two macadamia nuts in two consequent years, finishing their rapid growth stage and reaching a “plateau” that marks the beginning of pit hardening and fruit maturation.

We can see that in 2021 this stage arrived about 20 days earlier, and that is in correlation to the faster accumulation of GDD in 2021 compared to 2020.

Knowing when pit hardening starts is very important in macadamia. That is when farmers should start using chemical control of the Macadamia Nut Borer, a major pest in this crop. The Nut Borer has a narrow 2-week period in which it's exposed to chemical applications before it digs into new nuts. If the farmer expects pit hardening to arrive in mid-July and it comes 20 days earlier, he will miss the narrow 14 days window and may suffer damage to his crop. Counting the GDD along the season can alert the farmer that this stage, which arrives at about 1300 GDD, will come earlier this year so he can act in time.

SupPlant will help you count degree days

In SupPlant we understood the importance of Growth Degree Days to our farmers. We added this important feature to our system, enabling our growers to follow the accumulation of GDD throughout the season. alongside it, our plant soil and weather sensors data will give you a better understanding of your crop development throughout the year and an important decision-making support tool in a rapidly changing environment.

Nitzan Shatzkin, Chief Commercial Agronomist, SupPlant