Go into any garden shed and you will find a shovel, trowel, rake, starter pots, string, and nifty little gadgets you bought on impulse waiting at the checkout stand. But often missed is a soil or garden thermometer.
I admit that I gardened for years without one until one day when I asked the question,
“Why do we start X days before/after the last frost?”
The historical data for my region may or may not apply to this particular year.
For most vegetables, knowing when to plant has a lot to do with soil temperature. There are detailed charts on germination percentages based on soil temperature. The Iowa State University Department of Agronomy tracks soil temperature for farmers, indicating the significance of temperature for germination. We can control moisture and planting depth, but outside of a cold frame or greenhouse, temperature is left to Mother Nature.
So I got my thermometer. Now what?
The standard for measuring soil temperature is 2 inches (5 cm) and 4 inches underground. At that depth you are not as likely to get faulty readings (e.g. surface temperature from a sunny day). The deeper you measure, the more conservative (i.e. the later) you will be when planting.
We use three different kinds of thermometers (see photo):
- A – This analog thermometer has a long probe and is easy to measure down to 4-6 inches (10-15 cm)
- B – Seems to be the most accurate, but has a short probe and is difficult to read
- C – Digital display is easiest to read, has a long probe but no auto shutoff (risk draining battery). I also question the accuracy.
A soil thermometer is just a tool, and by no means a guarantee that your seeds or seedlings are sure to thrive. But it can give you valuable insight into when to plant that is better than conventional wisdom, outdated or conservative frost dates.
(Disclaimer: Plangarden does not endorse or sell any particular soil or garden thermometer.)