Temperature is an important factor of soil conditions to consider when planting vegetable seeds. Without proper temperatures, seeds will fail to germinate or will germinate poorly. Soil temperature should be taken by inserting a soil thermometer 3 to 4 inches into the ground.
The optimal germination temperature range for most vegetable seeds falls between 23.9 to 35 degrees Celsius. Some vegetables' optimal temperatures are on the lower side, such as onions, lettuce, and parsley, while other vegetables are on the higher end, such as corn, cucumbers and summer squash.
The minimum germination temperature for most vegetable seeds is between 4.44 to 15.6 degrees Celsius. Some vegetables, such as peppers and beans, require at least 15.6 degrees Celsius. A few vegetables, such as onion and lettuce, can tolerate minimum temperatures as low as 1.67 degrees Celsius.
The maximum germination temperature for most vegetable seeds is from 29.4 to 37.8 degrees Celsius. Some vegetable seeds, such as corn and cucumber, can tolerate temperatures above 37.8 degrees Celsius. Generally, the optimal temperature is about -9.44 degrees C less than the maximum temperature.