Temperature is an exceedingly important factor in seed germination. It directly affects whether a plant can sprout, and, if so, how long it will take to emerge from the ground. Additionally, heat and cold impact other aspects of the seed's environment, such as moisture availability and ideal time to plant. Learn each plant's ideal germination temperature and you'll have the tools to plan for a perfect planting season.
Getting the Temperature Just Right
Each species of seed has an optimal soil temperature at which the highest percentage of seeds sown will germinate in the shortest amount of time. Most seed distributors will print this temperature in the seed catalog or on the seed package.
Timing for Temperature
This ideal temperature, however, may not occur outdoors at the ideal time to plant. For instance, in zone 5, waiting for the soil to reach the ideal temperature for watermelon seeds (95 degrees Fahrenheit) means wasting a lot of the growing season. There won't be enough time left for the fruit to mature. And lettuce sown outdoors when soil reaches its ideal temperature of 77 degrees Fahrenheit will never actually form heads because the weather will be too hot. This difference between ideal germination temperature and ideal planting season may be reconciled by sowing seed indoors.
Consequences of Cool Temperatures
If you plant too early in the spring or too deep in the ground, the cooler temperatures will delay plant emergence or even prevent germination entirely. If germination does not occur, the moisture surrounding the seeds will cause them to rot.
In avoiding or accounting for the lower end of the temperature spectrum, remember that the soil will tend to be cooler than the air. This is because moisture is constantly evaporating from the soil into the atmosphere. Evaporation cools the substance from which it removes moisture. Remember also that soil cools down at night. But if daytime temperatures are in a tolerable range, a nocturnal drop of no more than 10 degrees Fahrenheit should pose no problem for seed germination.
Consequences of Warm Temperatures
If seeds remain at too high a temperature, they'll simply be unable to grow. Even if they can germinate, the high temperatures increase moisture loss, stressing and potentially killing the seedlings.
Consequences of Fluctuating Temperatures
Should temperatures fluctuate, seeds may be exposed to the risks of both extremes. This inconsistent environment translates into high-stress conditions which can potentially cause poor root development.