Tomatoes need soil at least 60°F about 4 inches deep before transplanting, and they grow best when it reaches 65 to 70°F. In colder soil, roots stall and leaves often turn purple. Enter your soil thermometer reading below to see whether it is time to plant, wait, or warm the bed first.
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What Your Result Means: the verdict tells you whether the soil is warm enough for tomato roots to grow. A single reading can mislead, so check on a few mornings in a row, and also check your frost date and night temperatures with the Planting Date Calculator and Temperature Checker.
Is the Soil Warm Enough to Plant Tomatoes?
Check the soil, not just the air. Tomato roots barely work in cold soil, so plants set out too early sit still for weeks while later plantings in warm soil catch up.
- Push a soil thermometer about 4 inches deep into the bed where the tomatoes will go.
- Read it in the morning, when soil is coolest, on a few days in a row.
- Enter the reading and choose transplants or seeds.
- Plant at 60°F or above, or warm the bed first if it is colder.
Tomato Soil Temperature Chart
| Soil at 4 Inches | Transplants | What Happens |
|---|---|---|
| Below 55°F (13°C) | Too cold: wait | Roots stall; risk of root decay and purple leaves |
| 55 to 59°F (13 to 15°C) | Still too cold | Slow growth; phosphorus uptake is poor |
| 60 to 64°F (16 to 18°C) | OK to plant | The minimum; growth starts slowly |
| 65 to 70°F (18 to 21°C) | Ideal | Roots establish quickly |
| Above 70°F (21°C) | Warm enough | Plant; water in well and mulch |
How Does the Soil Temperature Checker Work?
It converts your reading to Fahrenheit if needed and compares it with the soil temperatures extension services give for tomatoes.
Transplants: under 60°F wait; 60 to 64°F minimum; 65 to 70°F ideal; above 70°F warm enough
Seeds: under 50°F too cold; 50 to 59°F very slow; 60 to 69°F slow; 70 to 85°F best; above 95°F too hot
Worked Example
Say your soil thermometer reads 54°F at 4 inches on a sunny May morning. That is too cold: roots would stall, and plants would likely turn purple from poor phosphorus uptake. Cover the bed with clear or black plastic, check again in a few days, and plant once mornings read 60°F or more.
Factors That Change the Result
- Depth: the surface warms fast on a sunny day, but roots sit deeper; measure at about 4 inches.
- Time of day: soil is coolest in the morning, which is the reading that matters.
- Bed type: raised beds and containers warm faster than in-ground soil.
- Cover: plastic warms soil; straw or other organic mulch keeps it cooler.
- Wet soil: soggy soil warms slowly, and cold wet soil encourages root decay.
When to Use It
Use it in spring once your frost date is near, before setting out transplants. For the date itself, use the Planting Date Calculator; for air and night temperatures, the Temperature Checker; and for indoor seed trays, the Seed Germination Calculator.
Common Mistakes to Avoid
- Going by warm afternoons: the soil can still be cold underneath.
- Measuring the surface: push the thermometer about 4 inches down.
- Straw mulch too early: it keeps cold soil cold; add it once the soil has warmed.
- Feeding purple seedlings: in cold soil, the fix is warmth, not more fertilizer.
- Racing to plant first: tomatoes planted into warm soil a week later often catch up and do better.
Accuracy and Limitations
This is a guide for one reading at one spot.
What It Does Well: tells you whether your soil has reached the temperatures extension services recommend for tomatoes.
What It Does Not Do: check frost risk or night air temperatures, which matter just as much. Soil temperature varies across a garden and from day to day. This is general guidance, not professional advice.
Methodology
Method: compares one soil reading with tomato soil-temperature thresholds. Transplants: at least 60°F, ideally 65 to 70°F (Oregon State University Extension); 60°F minimum, warmer is better (Ohio State University Extension, Franklin County); keep soil above 60°F before transplanting (University of Delaware). Depth: about 4 inches for transplants (Ohio State BYGL). Cold soil: poor phosphorus uptake and purple lower leaves; warm the site with clear or black plastic a few weeks before planting (Ohio State). Seeds: minimum 55 to 60°F (Ohio State BYGL); fastest at 70 to 85°F and too hot above 95°F, matching our Seed Germination Calculator. The under-55°F band splits the “too cold” range for clarity.
Sources: Oregon State University Extension – Let Soil Temperature Guide You, Ohio State BYGL – Is It Time to Plant Tomatoes? Check the Soil Temperature, Ohio State Extension – Planting Tomatoes When the Soil Is Too Cold, University of Delaware – Wilting Vegetable Transplants. Last reviewed: September 28, 2026 · Reviewed by Aiza Anwar.
Frequently Asked Questions
What soil temperature do tomatoes need?
At least 60°F at about 4 inches deep for transplanting, and ideally 65 to 70°F. Oregon State University Extension gives 60°F as the minimum for warm-season crops and 65 to 70°F as ideal for tomatoes.
How do I measure soil temperature for tomatoes?
Push a soil thermometer about 4 inches into the bed, the depth Ohio State University Extension recommends for transplants. Read it in the morning, when soil is coolest, on a few days in a row.
What happens if I plant tomatoes in cold soil?
Roots work poorly in cold soil, so plants sit still, may wilt, and often turn purple on the lower leaves because they cannot take up phosphorus, even when the soil has plenty.
Why are my tomato seedlings purple after planting?
Usually cold soil. Below about 60°F, tomatoes struggle to take up phosphorus, and the lower leaves turn purple. They normally green up as the soil warms.
How can I warm soil faster for tomatoes?
Cover the bed with clear or black plastic a few weeks before planting, as Ohio State Extension suggests. Raised beds also warm faster than in-ground soil in spring.
Is air temperature enough to decide when to plant?
No. Warm spring days can leave soil cold underneath, so check the soil itself as well as the frost date and night temperatures.
What soil temperature do tomato seeds need to germinate outdoors?
Tomato seed can germinate from about 55 to 60°F but is slow and uneven in cool soil; it sprouts fastest around 75 to 85°F, which is why most gardeners start seeds indoors.
Can I plant tomatoes if the soil is 58°F?
It is better to wait a few days or warm the bed first. If you must plant, use row covers or cloches and expect slow early growth.
Does mulch keep soil warm?
Plastic mulch warms soil. Straw and other organic mulches insulate the soil and keep it cooler, so many gardeners add them after the soil has warmed.
Related Tomato Tools
See When to Plant Tomatoes
This checker gives general guidance on soil temperature for tomatoes, not professional advice. Check frost dates and night temperatures too. Spotted an error? Tell us here.