The best soil thermometer for most home gardeners is a simple probe thermometer with a stainless steel stem at least 5 inches long. Dial models are cheap and need no batteries. Digital probe models read faster and are easier to see. Wireless sensors that stay in the bed and log readings suit gardeners who want daily data. Whatever you buy, push the probe to planting depth, about 2 inches for seeds and 4 to 6 inches for transplants, and take readings on several mornings in a row.
Buy a dial thermometer if you check the soil a few times each spring. Buy a digital probe if you want fast, easy-to-read numbers. Pick a wireless sensor if you start many crops in succession or garden in a greenhouse. University of Illinois Extension notes that even “a refrigerator thermometer that has a probe” can do the job.
Why soil temperature matters
Seeds germinate by soil temperature, not air temperature. Plant into cold soil and seeds rot or sprout late. University of Wisconsin-Madison Extension explains that seeds in cold soil will “be slow to do so and likely to produce stunted plants that are more prone to disease.”
A thermometer tells you when the soil has warmed enough for each crop. It takes the guesswork out of spring planting dates. For crop-by-crop numbers, see our soil temperature chart for planting vegetables.
Why depth and timing matter
Soil near the surface swings the most. University of Illinois Extension says “The surface soil fluctuates the most, with daily fluctuations of 10-20 F, while temperature deeper in the soil remains more steady and take longer to change.” A single afternoon reading on a sunny day can run well above the morning low.
That is why the depth and the time of day both matter. Measure at the depth where the seed or roots will sit. Read in the morning, when the soil is coolest, and repeat over a few days. If the numbers keep climbing past your crop’s minimum, it is safe to plant. If a cold night drops them again, wait a few more days.
Types of soil thermometers
Dial (analog) soil thermometers
A dial thermometer has a metal probe with a round gauge on top. It needs no batteries and lasts for years. Most read in both Fahrenheit and Celsius. The needle takes a minute or two to settle, so wait before you read it.
Digital probe thermometers
A digital probe shows the reading on a small screen. It settles faster than a dial and is easier to read in bright sun. It needs a battery. Many kitchen probe thermometers work in soil too, as long as the probe is long enough.
Wireless soil sensors
A wireless sensor stays in the ground and sends readings to a base station or phone app. It records highs and lows over days or weeks. This type costs the most, and it suits greenhouses, raised beds, and gardeners who track conditions day by day.
What to look for in a soil thermometer
Look for a stem at least 5 inches long so you can reach transplant depth. Stainless steel resists rust and bending. A pointed tip slides into firm soil with less effort. Check the temperature range on the package: it needs to read down to at least 32°F (0°C) for early spring use. Weatherproofing matters if you plan to leave it in the garden.
Accuracy matters more than extra features. You can check any thermometer in a glass of ice water, which reads 32°F (0°C).
How to use a soil thermometer
University of Wisconsin-Madison Extension gives the method: “Take temperature readings on three consecutive mornings at the same time … Insert the probe to a depth of 2 inches for seeds or 4-6 inches for transplants.”
In firm or dense soil, make a pilot hole first. Illinois Extension suggests “you can use a screwdriver to make a initial hole to the right depth so that the thermometer doesn’t get bent.” Take readings in a few spots across the bed and average them. Soil is coolest early in the morning, so morning readings give the safest planting signal.
Once the soil reaches the minimum temperature for your crop, sow. Read more on the best temperature to germinate seeds and when to plant seeds outside.
Soil temperatures for common vegetables
University of Wisconsin-Madison Extension lists these minimum and optimum soil temperatures for planting seeds, measured at 2 inches deep:
| Crop | Minimum soil temperature | Optimum soil temperature |
|---|---|---|
| Lettuce | 45°F (7°C) | 75°F (24°C) |
| Spinach | 45°F (7°C) | 70°F (21°C) |
| Carrots | 45°F (7°C) | 80°F (27°C) |
| Peas | 50°F (10°C) | 75°F (24°C) |
| Corn | 55°F (13°C) | 95°F (35°C) |
| Onion | 60°F (16°C) | 75°F (24°C) |
| Cucumber | 65°F (18°C) | 95°F (35°C) |
| Pepper | 65°F (18°C) | 85°F (29°C) |
| Squash | 70°F (21°C) | 95°F (35°C) |
| Tomato | 70°F (21°C) | 85°F (29°C) |
| Beans | 72°F (22°C) | 80°F (27°C) |
Seeds sown at the minimum will sprout, but slower. Closer to the optimum, they come up fast and strong. Transplants follow the same pattern, measured at 4 to 6 inches deep.
Soil thermometer FAQs
How deep should a soil thermometer go?
About 2 inches for seeds and 4 to 6 inches for transplants, according to University of Wisconsin-Madison Extension.
What time of day should you check soil temperature?
In the morning, on three days in a row. Soil is coolest early in the day, so morning readings give a safe planting signal.
Can I use a kitchen thermometer for soil?
Yes, if it has a long metal probe. Illinois Extension notes that a refrigerator thermometer with a probe can measure soil temperature.
How do I check if my soil thermometer is accurate?
Put the probe in a glass of ice water. An accurate thermometer reads 32°F (0°C).
Garden shopping
A few places I send readers when they ask where to buy.
- My Amazon storefront: the tools and supplies I recommend
- Botanical Interests: my favourite seed company
- Gifts for gardeners: my yearly gift guide
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