Plant growing through a clock symbolizing thermal time.

Decoding Thermal Time: A Simple Guide to Plant Development Prediction

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Elliot Brynn in Science & Nature August 2025 • 4 min read.

"Unlock the secrets of how temperature drives plant growth and development with our easy-to-understand breakdown of thermal time."


For centuries, predicting how temperature affects plant development has been a key focus for scientists and farmers alike. Knowing when a plant will reach a specific stage is crucial for everything from planting schedules to anticipating crop yields. Early efforts in crop modeling accelerated this field, particularly as the need to predict the impact of climate change on agriculture became increasingly urgent.

Across various crops like wheat and maize, studies have revealed a common temperature response pattern in developmental processes. This led to the concept of 'thermal time,' a way to quantify the heat accumulation needed for a plant to reach a certain stage. Thermal time is considered a valid measure for all developmental processes of a plant and is widely used in crop models.

Thermal time is valuable because it allows researchers to analyze the effects of other environmental factors by accounting for temperature changes. This means understanding how plants grow in fluctuating temperatures as if the temperature were constant. However, the concept can seem complex due to the many ways it's calculated.

What Exactly is Thermal Time and How Does It Work?

Plant growing through a clock symbolizing thermal time.

At its core, thermal time is a measure of the accumulated heat units required for a plant to progress through its development stages. It acknowledges that plant development doesn't simply follow a calendar; it's strongly influenced by temperature. Different plants need a certain amount of 'warmth' to trigger specific changes, such as germination, leaf growth, or flowering.

The basic formula involves calculating the daily average temperature and subtracting a base temperature (the minimum temperature for growth). These daily values are then added up over time. Once the accumulated thermal time reaches a specific threshold, the plant is expected to reach the next developmental stage.

  • Base Temperature: Every plant species has a base temperature, below which development stalls.
  • Daily Accumulation: Thermal time is accumulated only when the average daily temperature is above the base temperature.
  • Thresholds: Each developmental stage has a thermal time threshold that needs to be met.
While the basic principle remains the same, the specific equations and parameters used to calculate thermal time can vary. Some models use simple linear relationships, while others incorporate more complex curves to account for the fact that plant development doesn't always increase linearly with temperature. Some models also factor in things like day length or water availability.

Putting It All Together

Thermal time models can be powerful tools, but it’s essential to recognize their limitations and potential sources of error. By understanding the underlying principles and being mindful of factors like air vs. plant temperature, water availability, and the time step used for calculations, you can get the most accurate predictions and insights into plant development. Whether you're a seasoned farmer or a home gardener, mastering the concept of thermal time can lead to better planning and greater success in your growing endeavors.

About this Article -

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Everything You Need To Know

1

What is thermal time, and how does it help in understanding plant development?

Thermal time is a way to measure the heat accumulation needed for a plant to reach a specific stage of development. It acknowledges that plant development is strongly influenced by temperature, quantifying the 'warmth' needed for changes like germination, leaf growth, or flowering. It's calculated by accumulating daily temperatures above a base temperature threshold specific to the plant.

2

What does 'base temperature' mean in the context of thermal time, and why is it important?

The base temperature is a critical parameter in thermal time calculations. It represents the minimum temperature below which a plant's development stalls. If the average daily temperature doesn't exceed this base temperature, no thermal time is accumulated for that day. Each plant species has its own specific base temperature.

3

What are some limitations and potential sources of error when using thermal time models?

While thermal time models are valuable, they have limitations. The accuracy can be affected by factors like differences between air and plant temperature measurements, water availability, and the time step used for calculations. The relationship between plant development and temperature isn't always linear, so models may need to incorporate more complex relationships to account for this.

4

How can thermal time models be used in practical applications, such as agriculture or gardening?

Thermal time models are used to predict when a plant will reach certain developmental stages, such as germination, leaf growth, or flowering, this is important for planting schedules and anticipating crop yields. By understanding thermal time, farmers and gardeners can better plan their activities and improve their success in growing plants. Thermal time models can also be used in crop modeling, particularly to predict the impact of climate change on agriculture.

5

What factors besides temperature are not directly addressed by thermal time, and how might they affect plant development?

While the concept of thermal time simplifies plant development prediction, it often doesn't account for day length or water availability directly in the basic calculations. More advanced models incorporate these factors, recognizing that temperature isn't the only influence. Additionally, factors like nutrient availability and plant health, which also play a role in development, aren't explicitly addressed by thermal time alone.

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