The spring warm-up begins
SDSU Extension Horticulture Specialist
We are finally experiencing some nice warm, almost early summer-like temperatures. It is amazing to watch the transformations that occur with the warmer weather. What looked like barren soil for the last few months suddenly shows signs of life emerging from below.
Temperature is what drives that transformation. Soil temperature is slow to catch up
Air temperatures and soil temperatures are linked, but as you would expect, soils warm up much more slowly than the air and show little variation over time while air temperatures can easily vary by 30 degrees or more in a single day.
There are many factors that influence how quickly soils warm in the spring. Dry, sandier, more exposed soil will warm more quickly than wet, clayey and shaded soils. Mulches and turf grass shade the soil and keep it cooler longer into the spring. This can be beneficial in keeping some plants dormant a little later into the spring to avoid spring freeze damage to tender shoots.
If you look around, you will probably see where tulips have been planted up close to a home, particularly on the south side of the home, the tulips are growing quite rapidly. Other sites that are more shaded or mulched will see later tulip emergence and growth.
Soil temperature can play a more important role in the early spring growth of many different kinds of plants than air temperature. This is particularly true for seed germination of vegetables as well as weeds. Radish seed can germinate in soil as cold as 40 degrees while watermelon seeds need soil temperatures close to 75 degrees. Prostrate knotweed can germinate at a range of soil temperatures ranging from 35 degrees all the way up to 85 degrees. Crabgrass, a common lawn weed, will start germinating when soil temperatures reach about 62 degrees in the top 1 to 2 inches of the soil, which usually coincides with the time when lilacs begin blooming.
Vegetable seedlings and transplants also need certain temperatures to germinate as well as grow and produce fruit later in the season. Spinach, peas, radishes, cauliflower and head lettuce can be grown with minimum soil temperatures of 40 to 45 degrees but have an optimum soil temperature of 60 to 65 degrees.
They will not tolerate high air temperatures that can occur in mid-summer so they need to be planted early in the spring or in some cases, planted in late summer for a fall crop. Broccoli, onions, cabbage, chard, leaf lettuce, potatoes, carrots, turnips, beets and kohlrabi are in the next group that can be planted and will tolerate warm summer temperatures. Warm season vegetables like beans, sweet corn, squash and pumpkins need at least 50-degree soil temperatures while cucumbers and muskmelons need at least 60 degrees, and tomatoes, eggplant, pepper and watermelons need 65 to 75 degree soil temperatures.
Measuring soil temperature
It is quite easy to measure the soil temperature in your garden and around your yard. You can purchase an actual soil thermometer or just go to your local grocery store or kitchen aisle and look for an instant read thermometer, which is ordinarily used to check if your steak is done to the right temperature. These inexpensive thermometers range in price from about $5 to $30 and are also available online. Generally you will want to stick the probe about 2 inches down into the soil to get a reading. Take readings at several locations to get a good average of what the soil temperature is for your garden. Just clean it up well before you put it back in the kitchen drawer or your steak might have a little bit of a gritty taste the next time you use it.
Gardeners can also check their state’s climate data to get some idea of what your soil temperature might be. There are a number of different websites that you can use to check. For South Dakota try the SDSU climate webpage. The National Weather Service is another good place to look for this kind of data. The soil temperature at various depths is generally reported, often at 2, 4 or 8 inches. The 2-inch and 4-inch soil temperatures are probably the most important ones to watch since that will be the zone in which your seed is planted and the seedling’s roots will grow.