Plant by soil temperature.

Buy an inexpensive soil thermometer and push it a couple of inches into the bed where the seeds will go. Check it in the morning, around 9 a.m., for a few days in a row. You want the temperature to hold steady at or above the crop's number, not just touch it once on a sunny afternoon.Use a probe thermometer in the seedbed, not an air reading. Take it at seeding depth, about 2 inches for small seeds, 4 inches for larger ones, 6 for beans, in mid-morning around 9 a.m., when the reading best approximates the daily mean, and average it across several consecutive days before you commit. A single warm afternoon means nothing; a stable multi-day average at or above the crop's minimum is the signal to plant.

Seeds wake up based on how warm the soil is, not what the calendar says. Planted too early into cold, wet ground, they just sit there and often rot before they sprout, a problem called damping-off. Warm soil sprouts seeds faster and gives you sturdier seedlings. The frost date tells you the rough window; soil temperature tells you the day to actually plant.Germination is a soil-temperature response, not a date. Below a crop's base temperature, seed sits dormant and exposed; sown into cold (under about 55°F), wet ground, slow-germinating seed is prey to damping-off, the seed-rot and seedling-collapse complex driven by soilborne Pythium and Rhizoctonia. Warm soil germinates seed faster and yields more vigorous seedlings that outgrow disease. The air temperature and the last-frost date frame the season; soil temperature names the actual day.

To plant a little earlier, warm the bed up first. Raised beds warm faster than flat ground. Laying clear or black plastic over the soil for a week or two before planting heats it up, and a cloche (a clear cover over the plant) holds daytime warmth. Most important, make sure the bed drains well, because cold and soggy together is what makes seeds rot.To plant ahead of nature, warm and dry the bed first. Raised beds drain and warm earlier than flat ground; clear or black plastic laid over moist soil a week or two ahead lifts bed temperature several degrees; and cloches or low tunnels trap daytime heat. Prioritize drainage, because the lethal combination for seed is cold plus wet, not cold alone.