Soil temperature

Soil temperature, not air temperature, governs germination: spinach and peas sprout in the 40s°F, tomatoes and peppers want 60s°F+, and sowing into cold soil rots seed. Frost dates are a proxy — soil lags air by weeks in spring — which is why our windows lean conservative for warm-season direct sowing. A $10 soil thermometer beats any calendar.

See also: Direct sowing · Warm-season crops