Plant Growth: The Growing Degree Day Model

Degree days: counting warmth, not calendar days

Plants don't develop on a fixed calendar; they develop on accumulated warmth above a species-specific threshold. A Growing Degree Day (GDD) model formalises that: each day contributes heat units equal to how far the average temperature exceeds a base temperature Tbase (the temperature below which development effectively stops), and phenological stages — emergence, flowering, maturity — are reached once cumulative GDD crosses known thresholds.

GDD(day) = max(0, (Tmax + Tmin)/2 − Tbase)
GDDcum(day) = GDDcum(day−1) + GDD(day)

Common base temperatures are around 10°C for maize (a warm-season crop) and around 0–5°C for wheat (a cool-season crop that can develop slowly even near freezing) — the value reflects each species' physiological cold tolerance. A day averaging 22°C contributes 12 GDD for maize (base 10) but 17–22 GDD for wheat (base 0–5): the same weather means different developmental progress for different crops, which is the entire point of using GDD instead of raw calendar dates to schedule planting and predict harvest.

Capping the upper end

Most models also cap the daily temperature at an upper threshold before computing GDD, because development rate for most crops actually plateaus or declines above some optimum — extreme heat doesn't accelerate development further and can damage tissue. A common approach clips Tmax to something like 30°C for maize before averaging, so an unusually hot day still contributes only its capped share rather than an inflated one. Real crop models often use this simple 'GDD with cutoffs' method as a lightweight approximation of what's more rigorously a nonlinear, bell-shaped temperature-response curve.

Soil moisture and photoperiod modulate the base rate

GDD accumulation assumes temperature is the only limiting factor, which is only true when water and light aren't. Soil moisture stress slows or halts development even on warm days — a drought-stressed plant effectively can't use the heat units available to it, so realistic models multiply the daily GDD contribution by a stress factor derived from available soil water relative to the crop's needs. Photoperiod (day length) matters for photoperiod-sensitive species: many wheat and some maize varieties won't transition to flowering until day length crosses a threshold, regardless of accumulated heat, because the flowering trigger is genuinely a separate light-driven signal (mediated by phytochrome photoreceptors) layered on top of, not replacing, the temperature-driven vegetative development clock.

From phenology to yield

Once a model tracks which growth stage a crop is in, yield potential follows from how much time and resource the plant had in the stages that matter most — for grain crops, the grain-filling period is especially yield-sensitive, so heat or drought stress that shortens it (pushing the crop through GDD thresholds too fast without enough resource accumulation) tends to cut yield even if total accumulated heat looks adequate. This is why GDD-based crop models are used operationally: agronomists use them to time planting so that vulnerable stages like flowering avoid predictable heat waves or frost windows, and to forecast harvest dates weeks in advance from a season's accumulated GDD trend.

Frequently asked questions

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Everything above runs in your browser — open Plant Growth Simulator and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.