Garden Thermometers: Which Kind Do You Need?
Three kinds of thermometer answer three different garden questions: is the soil warm enough to plant, how cold did my garden actually get, and is it freezing out there right now. Here's what each does and where to put it.
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Three thermometers, three questions
- Soil thermometer: "Is the soil warm enough to plant?"
- Min/max thermometer: "How cold did my garden get last night?"
- Wireless thermometer with frost alarm: "Is it getting close to freezing out there right now?"
Frost dates tell you what's typical. These tell you what's actually happening in your yard this year.
Soil thermometer: when to sow
Seeds respond to soil temperature, not the calendar or the air. Cold soil delays germination and lets seeds rot; warm soil gets them up fast. A soil thermometer is a metal probe you push into the bed.
University of Wisconsin–Madison Extension suggests measuring at about 2 inches deep for seeds and 4–6 inches for transplants, at the same time of day for three days in a row, since soil warms and cools much more slowly than air. Its minimums for planting include roughly 45°F for lettuce, 50°F for peas, 55°F for corn, and 70°F for tomatoes and squash.
Use it alongside your frost date: frost dates tell you when it's safe from cold nights, soil temperature tells you when seeds will actually grow.
Min/max thermometer: learn your microclimate
Official lows are measured about five or six feet above open ground at a weather station that may be miles away. Your garden can run several degrees colder on a clear, calm night, especially in a low spot. See why your yard may differ from the station.
A min/max thermometer records the lowest and highest temperatures since you last reset it. Set it at plant height in the garden, check it each morning, and compare it with the forecast low. After a few frost seasons you'll know whether your garden runs colder or warmer than the forecast, and whether to shift your planting dates.
Wireless frost alarm: a warning before damage
A wireless indoor/outdoor thermometer puts a sensor in the garden and a display in the house. Many models sound an alarm when the outdoor reading drops below a temperature you set. Set it to 35–36°F and you'll know to throw covers on before frost forms, instead of finding blackened plants in the morning.
It's most useful in the weeks around your first and last frost dates, when a single cold night can arrive between forecasts.
Where to put them
- At plant height, in the garden itself. Not on the house wall, porch, or deck; buildings give off heat and read several degrees warm.
- Shaded from direct sun. A sensor in sunlight reads high during the day. Mount it on the north side of a stake or in a small shield.
- In your coldest spot if you only have one sensor. Low corners and open beds frost first.
When a frost is coming
When the forecast or your alarm says frost, cover tender plants before dark with frost cloth and water the soil. See how to protect plants from frost for the full checklist, and look up your ZIP code for your typical frost dates.
Sources
- When Is the Right Time to Plant Vegetable Seeds? Check Soil Temperature, University of Wisconsin-Madison Extension
- U.S. Climate Normals, NOAA National Centers for Environmental Information