In nature, seeds fall to the ground and are buried beneath the soil for an entire season or more before they germinate. Seeds need the right conditions in order to sprout into new plants. These conditions include water and the right temperatures. Different seeds sprout at different temperatures.
Seeds germinate when the soil reaches a specific temperature range surrounding an optimal temperature. The closer the temperature is to the optimal temperature for seed germination, the more seeds will germinate in the shortest amount of time. For example, although 90 percent of all onion seeds will germinate if planted at 32 degrees Fahrenheit, they will take up to 136 days to sprout. By contrast, 99 percent of onion seeds will germinate if planted at 68 degrees Fahrenheit and will take less than five days to grow.
When seedlings are grown indoors, they are often placed on a mat that warms the soil so that the seed will receive optimal temperatures. Although a home may be kept in a range from 65 to 70 degrees, the soil in a potting tray may be 5 degrees cooler than the air temperature. Keeping optimal warming conditions for vegetable seeds ensures that the maximum number of seeds grows in the shortest amount of time.
There are several methods for warming soil so that the plants will sprout sooner. As mentioned above, seeds grown indoors are placed on warming trays. Potting trays are also placed under heat lamps or in a warm, sunny window. Potting trays in heated greenhouses boost sprouting rates and times. Cloches, cold frames and floating row covers serve as miniature greenhouses by heating the soil beneath their cover. Finally, a layer of black plastic over plants heats the soil through solar radiation.
Most plants will do well outdoors once the last average frost date of an area has passed. This does not mean that plants will immediately start to grow once the air is warm enough. In many areas, the soil remains cool even after the air has warmed. Heating the soil of your garden so that the temperature is warm enough to grow garden plants is one way to get a jump on the growing season for your plants.
Most plants are divided into two varieties. These are cool-season plants and warm-season plants. Cool season plants such as lettuce, celery and cabbage will grow well when temperatures are cold. For example, lettuce will grow well in temperatures as low as 30 degrees Fahrenheit. Despite this, lettuce seed germinates and sprouts best when temperatures are 75 degrees Fahrenheit. When you choose to grow cool-season plants, germinate and start these plants indoors on a heating mat or using a cold frame or cloche before planting them outdoors.