Most people who want to grow a garden or who have grown one realize that temperature has a huge bearing on growing garden plants. For that matter, it has a huge bearing on growing any plants. Most people probably associate this with not planting during the winter if there is snow on the ground or planting tropical plants only when and where the temperatures are reasonably high. What far fewer people consider is that there can be a huge difference between air temperature and soil temperature. The temperature of the ground is extremely important in gardening.
The reason that there can be such a huge difference between air temperature and the ground temperature is that there is an enormous difference in density between the two mediums. That is, molecules of air are much farther apart than molecules of soil. When a substance is heated, the atoms get more energetic and bounce around a lot more. That is really what is measured as heat, the motion of the molecules. The more motion there is, the hotter the substance is.
It takes energy to get molecules to move and every time a molecule slams into another molecule, it loses a little energy and gives that energy to the other molecule. Since the molecules of a dense substance are closely packed, it takes a lot more energy to get the entire mass warm.
This is over-simplifying it, but the point is that it takes more time to heat soil or water than it does to heat air. The opposite is also true. Once it is warmed up, soil and water cool down more slowly than air does. This is what causes the phenomenon of fog rising up from a lake or river into the air when the air just above the water is colder than the water.
So why is this important in gardening? Plant roots are extremely sensitive to the temperature of the soil around them. If the ground temperature is too cold, the roots may die or at least slow down their growth. Some plants are more sensitive to the soil temperature than others, but this is the reason many people start seeds in planter pots that are placed on warming pads.
Many seeds won’t even germinate if the soil temperature is too low. This is the case with beans, for instance. For beans to germinate, the temperature must be at least 60 F or 15.5 C. At temperatures lower than this, a bean seed will lay dormant and if the moisture level is high enough for germination, the bean will rot rather than germinating. Eggplants also require a minimum germination temperature of 60 F/15.5 C. (Oregon State University extension service has a handy chart with the minimum germination temperature for various crops.)
This is the reason the difference in ground temperature and air temperature has been explained. This is also the danger for gardeners. As an example, suppose that the air temperature in your location has been around freezing for a month. This is enough time to cool the soil to around freezing, to a depth of several inches. If there is a sudden spring snap and the air temperatures warm up to 70 F / 21 C for a week and the forecasts are for continued warm weather, too many gardeners rush to plant their gardens.
This is a mistake. It is almost guaranteed that the soil temperature will not yet have warmed up. Planting seeds or seedlings of plants that aren’t cold weather crops will usually result in either total failure of the crop or of at least substantially slowed growth. What is more, it takes a lot of time for the roots to recover from the cold, even if they survive. This is such an important point that soil temperature is much more important than air temperature for plant growth.
Put in another way, if the soil temperature is warm enough for vigorous root growth and an overnight cold snap freezes most of the leaves of the plant, the plant might pull out of the condition. Yet, if the air temperature is warm, yet the soil temperature is cold enough that the roots of a plant die, the plant simply won’t recover. It is dead.
It isn’t just when planting seedlings and seeds that the soil temperature is important. Semi-tropical and temperature-sensitive plants often don’t produce blossoms, fruits, or seeds unless the soil temperature is high enough. Thus, eggplants don’t bloom and set fruit until the soil temperature is at least 65 F / 18.3 C. Gardeners that don’t know how important soil temperature is might not understand why their eggplants aren’t producing fruit. Others might be confused as to why an eggplant might produce fruit with the air temperature at or even below 60 F. The secret, which isn’t a secret, is that the soil temperature is above 65 F.
It isn’t even difficult to tell what the soil temperature is. You can simply use a sturdy thermometer that can be pushed about three inches into the ground. Put the thermometer in place, wait a few minutes, then check the temperature. This should be done in several locations in the garden since not all areas will warm up evenly.