Soil temperature is one of the three critical factors that affect seed germination. To germinate, seeds need the correct soil temperature, enough moisture and the right exposure to the light. Soil that is too cool will slow germination; damp seeds will rot in the soil. Vegetable seeds have varying soil temperature requirements.
Cold-weather crop seeds germinate in cold soil and grow best in cool weather. Plant lettuce, pea, radish, kale and spinach seeds in the spring when the soil has heated to 4.44 degrees Celsius. Test the temperature of spring soil with a soil thermometer. Submerge the aluminium tip at least 3 inches into the soil, and leave it there for at least a minute to allow the thermometer to register the soil temperature. Cold-weather seeds will germinate and thrive in cold soil and will do progressively less well as the soil and air temperature get warmer. These seeds should be planted early enough in the spring to allow the plants to grow to maturity during the cool spring.
Onion and leek seeds will germinate when the soil temperature rises above 10 degrees C. Carrots, beans, broccoli, cabbage and cauliflower seeds germinate in soil that is 15.6 degrees C or above. Onions, broccoli and leeks need a long growing season in cool weather; they will bolt to seed when the weather and soil get warm. The other vegetables in this group grow and thrive as the spring turns into summer.
Warm-season vegetables such as tomatoes, corn, squash, cucumbers and peppers should not be planted until the soil temperature heats to 21.1 degrees C. They require warm soil and warm air to grow and thrive. Even when seeds are germinated indoors and the seedlings are planted in the spring garden, they will shock and grow slowly if they are exposed to cool soil and cool spring nights.