Cardinal Temperature Emergence & Is Your Soil Warm Enough to Sow?

Emergence needs a fixed amount of thermal time — growing degree-days above the crop's base temperature. The tool finds the effective degree-days your soil delivers per day (zero at or below the base temperature, rising to a peak at the optimum, then falling toward the ceiling), then divides the crop's thermal-time requirement by that daily rate. Maize, for example, needs about 60 degree-days; at a steady 20°C soil it earns 10 a day (20 − 10°C base), so it emerges in about six days.

Every species has three cardinal temperatures: a base (minimum) below which germination effectively stops, an optimum where it is fastest, and a maximum (ceiling) above which it stops again. Maize is roughly base 10°C, optimum 30°C, ceiling 40°C; cool-season wheat is about base 2°C, optimum 22°C, ceiling 35°C. The tool stores these for each crop and uses them to flex the days-to-emerge with your measured soil temperature.

When your soil temperature is at or below the crop's base temperature, no thermal time accumulates, so emergence is effectively indefinite — the tool shows a do-not-sow flag. Sowing into below-base soil risks rot, chilling injury and very patchy stands. Wait until the soil climbs into the crop's prompt window before planting.

Aim for the prompt window the tool reports — the band, from about 80% of the way to optimum up to the optimum itself, where emergence is fastest and most uniform. For maize that is roughly 26–30°C, for wheat the upper teens to low twenties. Sowing inside this window typically gives near-95% expected emergence.

Measure at seeding depth — about 5 cm for most field crops — using a soil thermometer, ideally in the morning when the soil is at its daily low, for several days running. Bare, dark, well-drained soil warms faster than wet, residue-covered ground. Use the steady reading you expect over the days after sowing, not a single warm afternoon.

Above the optimum, the effective degree-day rate declines toward the ceiling rather than continuing to rise, because heat begins to stress the germinating seed. So very hot seedbeds can be as poor as cold ones: the tool models this decline, and above the ceiling it flags the soil as too hot with no emergence.

It is a guide to how complete and even your stand should be at the entered temperature. It plateaus near 95% across the favourable band, ramps down as you approach the base temperature and tapers off toward the ceiling. A low percent warns that even if seedlings do emerge, the stand will be thin and uneven.

The model is temperature-driven and assumes adequate moisture and a normal sowing depth — the usual limiting factor for emergence once moisture is sufficient. Dry soil, crusting, deep sowing or poor seed-to-soil contact will all slow emergence beyond what temperature alone predicts, so treat the result as the best case for your soil temperature.

If the soil is below the base temperature, wait — sowing gains nothing and risks the seed. If it is in or near the prompt window, sow now. If it is between base and prompt, you can sow but expect slower, patchier emergence; the decision then weighs the calendar (frost, season length) against waiting a few days for soil that emerges the crop faster and more evenly.

They are representative planning values for each species, compiled from published germination thermal-time studies and agronomy handbooks, not a single named cultivar. Cultivars vary, so use the result to compare scenarios and time your sowing, and refine with local trial data or your seed company's specific figures where you have them.

Sixteen — warm-season maize, sorghum, cotton, soybean, sunflower, rice and tomato, and cool-season wheat, barley, canola, field pea, lentil, chickpea, lettuce, carrot and onion. Each carries its own base, optimum and ceiling temperatures and thermal-time-to-emergence in the reference table below.