Soil Temperatures, Frosts and Planting Dates

Posted by Eric Hammond Adams County

As the first week of May comes to a close, impatient gardeners everywhere are asking, “Is it time to plant my tomatoes yet”? The answer to this question is probably, “Not just yet”.   

Appropriate planting times for vegetables are based on both air and soil temperatures.  Air temperature is fairly straightforward.  While cool season crops will tolerate varying degrees of cold weather, most warm season crops, like tomatoes, squash and peppers are tender - meaning they will not tolerate a frost.   It’s not difficult to look at the forecast and see if there is potential for freezing temperatures.  In fact, in my neck of the woods it looks like there is a good chance that we are going to have frost Monday morning.   However, even if the air temperature is not prohibitively cold, you also have to take soil temperatures into account.

The soil in my raised beds was around 52°F
this morning

If soil is too cold seeds may not germinate and transplants may be slow to establish and grow.  Ideal soil temperatures range from 45°F for crops like spinach or Fava beans to 70°F for many squash and melons.   Tomato transplants prefer a temperature between 60 -65°F.   I measured the temperature of the soil in my raised beds all week and temperatures ranged from 50°F to 52°F.  So, at least in my garden, the soil is too cool for tomatoes and many other warm season crops. 

The recent rains which have saturated Front Range soils will mean that they will be slower to warm, as more energy is needed to warm both the solid components of the soil and the water the soil is now holding, than would be required to warm the solid components alone if the soil were dry.





Soil Thermometer

Soil temperature can easily be measured with a thermometer.  They make specialized soil thermometers which you can purchase at many local garden centers or online.   You can also use a meat thermometer, though if you do I would recommend designating it as your full-time soil thermometer, retiring it from duty in the kitchen and buying a new one for your meats.  To measure the soil temperature for plant growth, insert the thermometer around 4 inches into the soil.  Temperature should be measured early in the morning, when the soil is its coolest, and you may want to take measurements from several locations. 

Insert the thermometer to a depth of about 4 inches

As mentioned in a previous blog, there are several ways you can speed the warming of soil in the spring, ranging from building raised beds to covering your beds with plastic.  

It’s not quite time to plant most warm season crops quite yet.  So, all you tomato junkies out there, watch the forecast for potential frosts, check your soil temperature and bide your time.  You’re almost there.