Growing Degree Days

Growing Degree Days (GDD) is an agricultural metric that quantifies accumulated heat over the growing season, providing a more accurate prediction of plant development than calendar days alone. The principle: plants grow at rates proportional to temperature above a "base temperature" specific to each species; below the base temperature, growth essentially stops; above it, growth accelerates with temperature. Calculation: GDD = (Tmax + Tmin) / 2 − Tbase, where Tmax is the daily maximum temperature, Tmin is the daily minimum temperature, and Tbase is the species-specific minimum temperature (50°F / 10°C for cannabis). Daily GDD values are summed across the growing season to give cumulative GDD. Cannabis GDD requirements by strain: Fast autoflowers (Critical Auto) — 2000-2500 cumulative GDD from germination to harvest; Standard autoflowers — 2500-3000 GDD; Fast photoperiods — 2500-3000 GDD; Standard photoperiod hybrids — 3000-3500 GDD; Long-flowering sativas — 3500-4500 GDD. Practical applications: (1) Strain selection — outdoor growers in cooler climates with limited GDD (Northern US, Canada, Northern Europe) must select fast-finishing strains; growers in warm climates can grow any strain. (2) Harvest prediction — accumulated GDD from germination predicts harvest readiness more accurately than calendar dates, especially in unusually cool or warm seasons. (3) Site selection — within a region, microclimates with higher GDD accumulation (south-facing slopes, urban heat islands, sheltered valleys) enable more strain options than colder microclimates (north-facing slopes, valley bottoms, exposed plateaus). Cumulative GDD by US region (May-October growing season): Pacific Northwest coastal — 1500-2500 GDD (limited cannabis options); Upper Midwest — 2500-3500 GDD (most strains work); Mid-Atlantic and Great Lakes — 3000-4000 GDD (good cannabis range); California Central Valley — 4500-6000 GDD (any strain works); Florida and Gulf Coast — 5000-7000+ GDD (year-round potential). Online GDD calculators (Cornell University, USDA) calculate cumulative GDD for any US location based on weather station data. Cannabis-specific GDD guides match strain catalogs to expected accumulated GDD for any planting date and location.