Growing Degree Days (GDD) serve as a meteorological tool used by gardeners, farmers, and researchers to predict the pace of development in various biological organisms, including plants and insects. This system is founded on the principle that the biological rate of development is directly tied to the accumulation of heat over time, a process often referred to as thermal time. By quantifying the daily heat exposure above a minimum temperature threshold, GDD offers a more accurate measure of a crop’s physiological age than simply counting calendar days. The total number of heat units required for an organism to move from one developmental stage to the next remains relatively constant, making GDD a reliable predictor regardless of year-to-year weather variability.
Defining the Critical Temperature Variables
Calculating Growing Degree Days requires establishing specific temperature boundaries that reflect the biology of the organism being tracked. The most significant boundary is the Base Temperature ($T_{base}$), which is the minimum temperature below which a plant or insect ceases development or growth significantly slows down. This $T_{base}$ is not universal; it is species-specific, reflecting the organism’s unique metabolic requirements.
For example, warm-season crops like field corn, sweet corn, and soybeans typically use a $T_{base}$ of $50^{\circ}F$, as their metabolic processes are largely dormant below this point. In contrast, cool-season crops such as wheat, oats, and peas thrive in cooler weather and often use a lower $T_{base}$ of $40^{\circ}F$ or $41^{\circ}F$. Understanding the appropriate $T_{base}$ is the first step, as any daily temperature contribution below this figure is disregarded in the calculation.
Step-by-Step Guide to the Basic GDD Calculation
The foundational method for calculating daily GDD uses the daily maximum ($T_{max}$) and minimum ($T_{min}$) air temperatures recorded over a 24-hour period. This method relies on the assumption that the average temperature between the daily high and low is representative of the heat available for growth that day. The standard formula for this calculation is straightforward, comparing this average to the organism’s specific base temperature ($T_{base}$).
The formula is expressed as: $GDD = (\frac{T_{max} + T_{min}}{2}) – T_{base}$.
To perform a daily GDD calculation, first record the high and low temperatures. For a day with a high of $75^{\circ}F$ and a low of $55^{\circ}F$, and using a corn $T_{base}$ of $50^{\circ}F$:
Calculate the average daily temperature: $\frac{75^{\circ}F + 55^{\circ}F}{2} = 65^{\circ}F$.
Subtract the base temperature from the average: $65^{\circ}F – 50^{\circ}F = 15$.
The resulting daily GDD is 15.
If the calculated average temperature falls below the $T_{base}$, the daily GDD accumulation is recorded as zero. For instance, if the average temperature was $48^{\circ}F$ for a $50^{\circ}F$ base crop, the GDD value is recorded as 0, signifying that no effective growth occurred that day.
Adjusting the Calculation for Temperature Extremes
The basic GDD calculation can overestimate growth during periods of extremely high temperatures because it assumes a linear relationship between heat and development. In reality, most crops, particularly warm-season varieties like corn, experience a plateau or a decline in their growth rate once the temperature surpasses a certain point. This phenomenon occurs because temperatures above this upper temperature threshold cause heat stress, which slows down the plant’s metabolic and physiological functions.
To address this limitation, a modified calculation method, often referred to as the 86/50 method for corn, is widely used. This adjustment involves “capping” the daily maximum temperature ($T_{max}$) at an upper limit, typically $86^{\circ}F$. If the recorded $T_{max}$ exceeds $86^{\circ}F$, the value of $86^{\circ}F$ is used in the formula instead of the actual high temperature. Similarly, if the daily minimum temperature ($T_{min}$) drops below the $50^{\circ}F$ base, it is “floored” at $50^{\circ}F$.
For a day where the high was $95^{\circ}F$ and the low was $60^{\circ}F$, the modified calculation substitutes $86^{\circ}F$ for the high. The average temperature becomes $73^{\circ}F$, resulting in $23$ GDD ($73 – 50$). This adjustment provides a more biologically accurate measure of the heat units that contribute to crop development.
Accumulating and Applying GDD Totals
The true utility of Growing Degree Days is realized not in the single daily value, but in the accumulation of these units over an entire season. This process involves summing the daily GDD values, typically starting from a specific biological trigger point, such as the planting date or the start of a season for perennial crops or insects. The accumulated GDD total represents the total thermal energy the organism has been exposed to since its development began.
This total is then used to predict key phenological events with a high degree of accuracy. For example, corn hybrids require a specific, known number of GDDs to reach various developmental milestones, such as emergence (100 to 150 GDDs) or physiological maturity (2,500 to 2,800 GDDs). Applying GDD totals allows for improved management decisions, such as precisely timing the application of pesticides. Growers can forecast harvest readiness or assess whether the season is progressing ahead of or behind schedule. Many agricultural extension offices and online calculators provide automated tracking and forecasting tools.