Soil Temperature Thermometer

A handy, inexpensive tool to have for a garden is a soil thermometer because, as it turns out, different seeds sprout best at different soil temperatures.  There are many resources on the internet to help you determine what to plant at what temperatures.  For example, the link below takes you to a chart that lists many of the most common vegetables along with the minimum and maximum temperatures at which their seeds will sprout, as well as the optimum temperature.

http://www.aces.edu/pubs/docs/A/ANR-1061/ANR-1061.pdf

We have a soil thermometer that I used last week to determine the temperature of the soil inside our hoophouse (also called a "high tunnel".... basically an unheated greenhouse).  Here's what it registered:

I was amazed to see that the soil temperature was almost 60 degrees, especially since just the week before, we had recorded an all-time record low temperature of -20 degrees!  So, I decided to see what the soil temperature was outside the hoophouse.  Here's what I discovered:

That is 15 degrees difference between the soil inside versus the soil outside. 

Now compare these temperatures to the chart pointed to in the link above and you'll see that many seeds can be planted outside even in mid-February.  Spinach and lettuce seed will germinate at 35 degrees, just slightly above freezing, while beets, cabbage, carrots, radish and swiss chard will germinate at 40 degrees.  However, the optimum range of temperatures for germination for most of these starts at 45 to 50 degrees.  Since the temperature inside our hoophouse is 60 degrees, we are able to jump-start most of the cool season seeds listed in the chart in our hoophouse.

You can accomplish the same thing on a small scale with an inexpensive "cold frame", like those shown below.  (You can see our hoophouse in the background.)

You can find plans for making these on the internet.  But, I'll post directions for building the ones we have here in a later post.