This simulator models crop development with the Growing Degree Day method, the standard agronomic way to track phenology from thermal time rather than calendar days. Each day contributes max(0, Tmean − Tbase) heat units, and the running sum is compared against each crop's stage requirements. The tool also applies moisture and photoperiod stress factors, then plots cumulative GDD against days from sowing to estimate the date of maturity and a rough yield index for wheat, maize, soybean and sunflower.
A cumulative GDD accumulation curve over the season. Daily heat units are computed as max(0, Tmean − Tbase), scaled by a moisture factor (0.3 + 0.7 × moisture) and a photoperiod factor (full above 12 h daylight, reduced below). Coloured bands mark the five phenological stages, and a dashed line shows the predicted maturity day.
Five sliders set base temperature, mean daily temperature, temperature amplitude, soil moisture (0–1) and daylength (hours). The four preset buttons load wheat, maize, soybean or sunflower, each with its own base temperature and stage GDD targets. The graph, days-to-maturity, effective GDD/day and yield index update live as you adjust any control.
Growing Degree Days let growers predict pest emergence and harvest timing far more reliably than the calendar, because insect and crop development both depend on accumulated heat. Wheat uses a base of about 5 °C, while warm-season maize uses around 10 °C, below which growth effectively stalls.
A Growing Degree Day (GDD) is a measure of accumulated heat above a crop-specific base temperature. Each day adds max(0, mean temperature minus base temperature) units, expressed in degree-days. Summing these over the season gives thermal time, which tracks plant development far better than counting calendar days.
Each crop has a total GDD requirement summed across its five stages, such as germination, tillering, heading, grain fill and maturity for wheat. The model divides this total by the effective GDD accumulated per day to estimate days to maturity, then draws a dashed maturity line on the cumulative curve.
Both apply stress factors that reduce effective heat accumulation. Moisture scales the rate by 0.3 + 0.7 × its value, so even dry soil still allows 30 per cent of potential growth. Daylength gives full accumulation at 12 hours or more and tapers below that, reflecting how short days slow many crops.
The core GDD calculation follows genuine agronomic practice and the per-crop base temperatures are realistic. However, the moisture and photoperiod factors, stage GDD targets and yield index are simplified teaching approximations. Real field models add vernalisation, frost, nutrient and cultivar effects, so treat the output as illustrative rather than a planting recommendation.
Plant and insect development are driven by temperature, not the passage of time. A warm spring advances a crop faster than a cold one, so a fixed number of days gives inconsistent results. Accumulated degree-days normalise for this, letting the same GDD target reliably mark a given stage across different seasons and locations.