This section looks at the importance of soil temperature when growing seedlings and offers a printable PDF chart showing what vegetable seedlings grow best at different temperatures.
The temperature that seeds germinate varies depending on vegetable varieties. For example lettuce seeds will germinate when the soil temperature is just 10 degrees whereas capsicum seeds require 18 plus degrees. If you try and grow capsicum seedlings in soil lower than 18 degrees you run the risk of the seeds failing to germinate. Even if they do germinate the growth rate of the resultant seedlings will be slower than if the soil temperature was 18 plus degrees.
This does not mean that seedlings that germinate in 10 degrees cannot be grown in higher soil temperatures, though seeds that germinate at lower temperatures tend to have a lower maximum soil temperature threshold then seedlings that require very warm soil to germinate.
Seeds that will germinate in cool soil 10+ °C can be grown in early spring * in an unheated seedling box with a simple plastic cover over it. Seeds requiring warm soil 15+ °C can be grown without artificial heating but will do better when grown using artificial heating. Seeds that require very warm soil 18+ °C are difficult to grow in early spring without the use of artificial heating.
* This chart is based on conditions in Ballarat, Australia, which is in a cool mountainous growing zone.
TOP: Styrofoam seedling boxes in my propagation greenhouse. BOTTOM: Single cell seedling trays in two tray aquarium heater propagation boxes. Aquarium heater propagation boxes are a cost efficient way to artificially heat seedlings. For information on aquarium heater propagation boxes see: HOW TO MAKE AN AQUARIUM HEATER SEEDLING PROPAGATION BOX.