Tick-Tock: Inside a Plant’s Hidden Clock

How Growing Degree Days (GDD) help understand crop suitability, yield potential and in-season development

Humanity measures time by movement. Sundials use the rate of the rotation of the Earth throughout the day. A mechanical watch uses gears that click around at a precise rate. Ancient Chinese and Babylonian water clocks used a series of vessels of water that flowed into each other to tell which hour of the day it was. The rate method is perhaps best encapsulated with an hourglass, the double-bulbed glassware full of sand that allowed the concept of recordable time to be taken around the globe on ships.  

Fundamentally, all of these are a rate, a progression over time that allows a human to work out what period has elapsed. But if you were not a human and instead, say, a plant, how would you know what time it is? 

Plants keep time using two different methods.

They use two of the fundamental drivers of plant growth – light and temperature – but measure them in different ways. Light is measured each day as day-length; temperature is a cumulative quantity since the last time there was a cold/warm change.  

When plants know what time of year it is, they can make life-cycle decisions, like when to wake up from their dormant state, when to flower, and when to fruit. These phenological (life-stage) events are what drive our yield, determine fruit size, and ultimately dictate the size and quality of marketable crop in each season. A plant that grows too early in the season may be hit by frost or excessive rain in its flowering window, reducing fruit load. But a plant that grows too late may be outcompeted, losing light, water and nutrients to its neighbours. 

Notable examples of plants that use light to make their life-cycle decisions include rice, spinach, some major cereals, blueberries, strawberries, hazelnuts and peas. Their day-length clock is molecules called phytochromes that accumulate in the dark and are broken down by light. The rate of their decomposition is constant in light. Until the decomposition products reach a specific concentration (i.e. a certain amount of light has been accumulated in a day), they will not move their clock onto the next stage. In tropical and high-latitude species where the light regime is consistent, so too is their seasonal clock.  

In temperate and Mediterranean regions, by contrast, springtime light is variable. Clouds cover the sky, so light accumulation is an unreliable rate clock. To counteract this, many temperate plants use temperature – either thermal time or vernalisation.  

Thermal time, often expressed in growing degree days (GDD), is the other method of time expression. GDD measures the energy available to the plant for growth above a dormancy temperature threshold. This threshold varies by crop but is most often set at 5 or 10°C. Because photosynthesis is a temperature-dependent reaction, higher temperatures allow for greater energy production by the plant. Broadly speaking, higher temperature means the plant can produce more energy, and more energy increases yields and quality. GDD is important because it correlates strongly with production of fruits, nuts, and vegetation.  

Since its first use in 1735, GDD has been the standard measure of plant energy availability around the world. It forms the basis for bioclimatic classifications like the Huglin and Winkler Indices that suggest suitability for different crops and varieties in a given area. GDD is also the basis for many phenological models in fruits like apples and grapes, nuts like almonds, and the major grains like wheat, rice and corn. All use a version of heat units to determine phenological stages. 

Global Growing Degree Days Average (2025-2024, Base 10°C); Source: Demeter

Vernalisation is a measure of the cold experienced or “how much winter has passed”. It works by genetically preventing the plant from growing or flowering, with each cold snap adding another blocker to the process. The blockers prevent ice from building up and damaging the plant and once temperatures warm up again, the blockers are progressively removed, and growth can commence. Vernalisation is generally measured using chilling units, a measure of the hours that are experienced at less than 5-7°C (depending on species).  

Global Chill Portion Average (2015-2024). Source: Demeter

Plants using thermal time can combine GDD and vernalisation. To end dormancy, fruit and nut trees must first accumulate enough winter chilling. Once dormancy is broken, they accumulate GDD until buds burst and flowers open. To predict flowering for species like almonds, apples, peaches, pistachios and hazelnuts we should first calculate winter chill accumulation and subsequently the growing degree day accumulation up to flowering. 

Calculating and exploring global GDD accumulations gives us a window into crop suitability, yield potential, and how the current crop year is likely to be shaping up. As such, they are an essential part of any toolkit for thesis building, underwriting and monitoring. It’s important to match the predictor to the crop, however. To understand almond yields, for example, GDD accumulation is vital. To understand the timing of rice harvests, on the other hand, we should look to length of days and sunlight availability.

Plants think in terms of energy, they see in terms of light availability, and to understand each of those we need to use and compare temperature and light datasets. Humans do not intrinsically think like a plant, but data can help us to see the world from a plant’s perspective.   

No measure is perfect in isolation, but fine-scale data on temperature and light – whether simple readings or complex indices – are the most closely aligned with how crops see time, and provide a valuable insight into likely performance.