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Climate Factor · Climate metric

Growing Degree Days

Also known as: GDD, Heat units, Thermal units

Growing degree days (GDD) is a heat-accumulation metric that sums daily temperature above a crop-specific base threshold over time, used as a proxy for crop developmental progress rather than calendar days alone. Base temperatures and calculation methods are crop- and region-specific rather than universal.

Dated referenceLast reviewed: 2026-07-12Updated: 2026-07-12
Illustrative diagram · AgricultureID (original)

Growing degree days (GDD), also called heat units or thermal units, express how much "useful warmth" has accumulated over a period, rather than simply counting calendar days. The underlying idea is that plant development speeds up as temperature rises above some minimum (base) temperature specific to that crop, and slows again as temperature falls back toward or below it, so accumulated heat above that base is often a better predictor of developmental stage than time alone.

Because the base temperature, and in many methods an upper cap, reflect how a particular crop or pest responds to temperature, GDD is calculated differently for different crops and is calibrated using local research and field observation rather than a single universal formula or base value. GDD models are widely used alongside — not instead of — direct field observation.

What growing degree days measure

GDD is a running total of accumulated warmth relevant to plant development, calculated over the growing season or over a defined interval such as sowing to a target stage. Because it reflects biologically meaningful heat rather than elapsed time, two locations or seasons with the same number of calendar days can show quite different accumulated GDD, and crop development typically tracks GDD more closely than the calendar.

How it is calculated

  • Simple average method — using daily mean temperature minus the base temperature for that crop.
  • Modified or capped method — limiting the maximum temperature used in the calculation to avoid overstating heat accumulation on very hot days.
  • Sine-wave and other refined methods — modelling the temperature curve through the day for greater precision, particularly where hourly data are available.

In every method, the base temperature itself is not a single global constant; it is determined per crop, and sometimes per cultivar or region, through calibration against observed development.

Why it matters for crops

GDD is used to estimate planting-to-maturity timelines and compare crop progress between seasons or locations, to help select cultivars suited to the typical accumulated heat of a region, and, in pest and disease management, to predict the timing of life-cycle stages such as egg hatch or adult emergence so that scouting and control measures can be better timed.

Using GDD in practice

Growers and advisers commonly track accumulated GDD through a season and compare it with typical requirements for a given cultivar or pest stage to inform decisions such as scouting timing or estimated harvest windows. GDD is generally combined with direct crop scouting and local experience rather than used as a stand-alone predictor.

Limitations and considerations

GDD models are simplifications. They generally do not fully account for the growth-limiting effect of very high temperatures, moisture stress, day-length (photoperiod) sensitivity in some crops, or other stresses, and different calculation methods can give somewhat different values from the same weather data. Consistent methodology matters when comparing GDD across seasons or locations.

Relationships

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Affects

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Scope & limitations

Geographic scope: A general explanation of the growing degree day concept; base temperatures, calculation methods, and typical accumulated values are crop- and region-specific and are not given here.

  • This entry explains the GDD concept; it does not provide base temperatures, thresholds, or accumulated-value targets for any specific crop or region.
  • Different calculation methods (simple, capped, sine-wave) can yield different values from the same weather data, so consistent methodology matters for comparison.

Sources

This article draws on the following authoritative sources. See our sources & methodology for how they are selected.

  1. [1]NOAA — National Oceanic and Atmospheric Administration (opens in a new tab)

    U.S. National Oceanic and Atmospheric Administration (NOAA)

    Authoritative

    Cited for: Agricultural weather and heat-unit data

    Type:
    Government agency
    Jurisdiction:
    United States
    Accessed:
    2026-07-12
  2. [2]USDA NASS — National Agricultural Statistics Service (opens in a new tab)

    USDA National Agricultural Statistics Service (NASS)

    Authoritative

    Cited for: Crop progress reporting using thermal accumulation

    Type:
    Government agency
    Jurisdiction:
    United States
    Accessed:
    2026-07-12
  3. [3]FAO — Food and Agriculture Organization (opens in a new tab)

    Food and Agriculture Organization of the United Nations (FAO)

    Authoritative

    Cited for: Crop development modelling context

    Type:
    Intergovernmental organization
    Jurisdiction:
    Global
    Accessed:
    2026-07-12
  4. [4]WMO — World Meteorological Organization (opens in a new tab)

    World Meteorological Organization (WMO)

    Authoritative

    Cited for: Meteorological data standards underlying GDD calculation

    Type:
    Intergovernmental organization
    Jurisdiction:
    Global
    Accessed:
    2026-07-12

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