How to Make A Vegetable Garden Soil Temperature Chart

December 12, 2024|The Rusted Garden|

(DRAFT EDITING NOW) They key to quicker germination and faster growing & maturing transplants is soil warmth. I talked about that at length in my previous post. Here is a way to collect data on how your soil warms without having to take soil temperature over the weeks and months. It is based on average temperatures. When we talk about the temperature of soil we are generally talking at a depth of 4 in/10 cm. Here is the chart that I use to make my planting decisions.

Collect Air Temperature Averages

The chart is built off the average day and night temperatures of each month. This is the ambient or air temperature, not soil temperature. Simple search Average Day Temperature for (Month) in (Your City) and Average Night Temperature for (Month) in (Your City). My chart is in Fahrenheit and uses Baltimore City. The formulas help create a general soil temperature. Keep in mind going from winter to spring, the soil is in a warming trend and the end of the months will be warmer than the average. Going from summer into winter, the soil is a cooling trend and the end of the months will be cooler than the average.

Soil Temperature Formulas

There are 3 formulas. When temperatures are between 32° F/0° C and 80° F/26.6° C or if you don’t want to use all three equations, simple add (d)ay and (n)ight temperatures together and divide by 2 (Equation 1). That number works to get you started. I created 2 other formulas to take into account nights at or below 32° F/0° C (Equation 2), and when days are at or above 80° F/26.6°C and nights are at or above 60° F/15.5° C (Equation 3). Use these temperature ranges to make your monthly soil temperature estimates.

  • Equation 1 (D+N)/2
  • Equation 2 (D+N+N)/3
  • Equation 3 (D+D+N)/3

Each month will now have its average soil temperature and you can use those temperatures to plan when you will direct seed and put transplants in the ground. Again the soil in the first half of the months will be cooler when moving from winter into summer. The begin of April in Maryland is much colder compared the last month of April. Many of my transplants go in around that time frame. Remember these are soil temperature averages but will help create a planting plan.

(MORE TO COME this afternoon)