The soil temperature is one of the most influential factors affecting vegetable seed germination. Gardeners should keep in mind that bare soils, dry soils and south-facing slopes warm faster when selecting a site.
Soil temperature may be a better measure than frost dates for when to plant, due to the varying conditions of microclimates and yearly weather fluctuations.
Soil temperature charts for seed germination include a minimum, maximum and optimum temperature and an optimum temperature range.
Gardeners can raise the soil temperature through the use of clear plastic or black plastic coverings; these provide insulation and trap solar heat, according to the Virginia Cooperative Extension.
Measure soil temperature with a soil thermometer inserted 3 to 4 inches into the soil. Look for a consistent temperature across several days. Measure in the morning and afternoon and average the two readings for greater accuracy.
According to the Cobb County (Georgia) Cooperative Extension, the "time to germinate doubles for each ten degrees the temperature drops below the ideal level." Some seeds will not sprout in cooler temperatures.
- Purdue University Cooperative Extension Service: Indiana Vegetable Planting Calendar
- Cobb County Cooperative Extension: Vegetable Seed Germination and Transplants
- Virginia Cooperative Extension: Seed Germination and Soil Temperature
- Government of Alberta Agriculture and Rural Development: Soil Temperature for Germination