Estimate your growing season length.
The growing season is the frost-free window between the last spring frost and the first autumn one, expressed in days between those dates. Growing degree days go further by counting warmth rather than time: each day contributes the amount by which average temperature exceeds the crop's base temperature, which is why a cool long season can accumulate less heat than a short hot one. Seed packets quote days to maturity, so the season length is what decides which varieties can finish.
Season length and degree days
Growing days = first autumn frost day - last spring frost day; growing degree days = (average temperature - base temperature) x growing days
Local meteorological services publish average first and last frost dates by station or postcode. Use the average rather than the earliest, and add a safety margin for tender plants.
Predicting crop and pest development. Many varieties and insect life stages are tied to accumulated heat far more reliably than to calendar dates.
Because species differ in the temperature at which growth begins — around 10 C for maize and many warm-season crops, closer to 4-5 C for cool-season cereals.