Soil Temperature for Germinating Vegetable Seeds
Most cool-season vegetables will germinate once soil reaches about 40°F, and lettuce, onion, parsnip and spinach can start as low as 35°F. Beans, cucumbers, peppers, squash, melons and eggplant need at least 60°F, and they germinate much faster in soil in the 70s and 80s. These figures come from germination research by J. F. Harrington at UC Davis, republished by Oregon State University Extension. The minimum is where germination starts to be possible, not where it goes well.
Soil temperature ranges by crop
All temperatures are soil temperatures in °F, from the Harrington data as published by OSU Extension.
| Crop | Minimum | Optimum range | Optimum | Maximum |
|---|---|---|---|---|
| Asparagus | 50 | 60–85 | 75 | 95 |
| Bean | 60 | 60–85 | 80 | 95 |
| Beet | 40 | 50–85 | 85 | 95 |
| Cabbage | 40 | 45–85 | 85 | 100 |
| Carrot | 40 | 45–85 | 80 | 95 |
| Cauliflower | 40 | 45–85 | 80 | 100 |
| Chard, Swiss | 40 | 50–85 | 85 | 95 |
| Corn | 50 | 60–95 | 95 | 105 |
| Cucumber | 60 | 60–95 | 95 | 105 |
| Eggplant | 60 | 75–90 | 85 | 95 |
| Lettuce | 35 | 40–80 | 75 | 95 |
| Muskmelon | 60 | 75–95 | 90 | 100 |
| Onion | 35 | 50–95 | 75 | 95 |
| Parsley | 40 | 50–85 | 75 | 90 |
| Parsnip | 35 | 50–70 | 65 | 85 |
| Pea | 40 | 40–75 | 75 | 85 |
| Pepper | 60 | 65–95 | 85 | 95 |
| Pumpkin | 60 | 70–90 | 95 | 100 |
| Radish | 40 | 45–90 | 85 | 95 |
| Spinach | 35 | 45–75 | 70 | 85 |
| Squash | 60 | 70–95 | 95 | 100 |
| Tomato | 50 | 60–85 | 85 | 95 |
| Turnip | 40 | 60–105 | 85 | 105 |
| Watermelon | 60 | 70–95 | 95 | 105 |
A few notes on reading it. The "optimum" is where the most seeds germinate fastest in a lab. It is not necessarily the best temperature for the seedling afterward. Pumpkin's listed optimum (95°F) sits above the top of its optimum range (90°F). That inconsistency appears in the table as OSU publishes it and has been left as is here.
Other extension tables differ slightly. Alabama Cooperative Extension's version gives tomato an optimal range of 70–95°F rather than 60–85°F, though both put the minimum at 50°F and the optimum at 85°F. Treat the ranges as approximate.
Why speed matters as much as the minimum
The more useful half of Harrington's data is how many days seeds take to emerge at each temperature. A seed that would germinate in a week at 77°F might take a month or more near its minimum. The longer it sits in cold, damp soil, the longer it is exposed to rot, seed-eating insects and damping-off.
| Crop | 41°F | 50°F | 59°F | 68°F | 77°F | 86°F |
|---|---|---|---|---|---|---|
| Bean | — | — | 16 | 11 | 8 | 6 |
| Beet | 42 | 17 | 10 | 6 | 5 | 4 |
| Carrot | 51 | 17 | 10 | 7 | 6 | 6 |
| Corn | — | 22 | 12 | 7 | 4 | 4 |
| Cucumber | — | — | 13 | 6 | 4 | 3 |
| Lettuce | 15 | 7 | 4 | 3 | 2 | 2 |
| Onion | 31 | 13 | 7 | 5 | 4 | 4 |
| Pea | 36 | 13 | 9 | 7 | 6 | 6 |
| Pepper | — | — | 25 | 12 | 8 | 8 |
| Spinach | 22 | 12 | 7 | 6 | 5 | 6 |
| Tomato | — | 43 | 14 | 8 | 6 | 6 |
Days to emergence, from the Harrington data published by OSU Extension. A dash means little or no germination.
Two patterns stand out. Cool-season crops like carrots, beets and peas can germinate in 40s soil, but it takes five to seven weeks. Waiting until the soil reaches the 50s cuts that to about two and a half weeks. Warm-season crops are worse: pepper seed produced little or no germination at 50°F and needed 25 days at 59°F.
Heat is a limit too. In the same data, spinach and lettuce produced little or no germination at 95°F. That matters for late-summer sowings of fall greens.
How to measure soil temperature
You need a soil thermometer or a digital probe thermometer used only for soil. The method matters because soil temperature changes with depth and time of day.
- Depth: Ohio State's Buckeye Yard and Garden Line says to measure at the depth you plan to sow for seeds, and at about 4 inches for transplants. UW–Madison Extension Kenosha County uses 2 inches for direct seeding and 4 to 6 inches for transplants.
- Consistency: The Kenosha guide recommends readings on three consecutive days at the same time, between 10 a.m. and noon.
- Trend, not a single reading: One warm afternoon after a sunny day can mislead you. Look for readings that hold steady or rise.
Ohio State gives a working rule for the thresholds: about 40°F minimum for cool-season crops like broccoli, cabbage, beets and carrots, and 55–60°F for warm-season crops like beans, tomatoes, peppers and squash. It warns that planting early "can lead to seed rot, delayed germination, root decay, poor growth and disease."
Warming the soil
If you want to plant earlier, warm the soil rather than hoping. The University of Alaska Fairbanks reported soil 8 to 10 degrees warmer under clear plastic than in bare soil, and 3 to 6 degrees warmer under row covers. According to the Harrington data, that can take corn from 22 days to emerge down to about 12. Utah State Extension says plastic mulch lets most crops go in one to two weeks earlier than usual.
Indoors, a seedling heat mat does the same job. University of Minnesota Extension notes that potting mix can run as much as 5°F cooler than the room air, and that bottom heat helps prevent damping-off.
Soil temperature and frost date answer different questions. Soil can be warm enough for beans while a late frost is still possible, and the reverse happens too. See Your Last Frost Date Is a Probability for the air side. Covers and cold frames are in Cold Frames, Row Covers and Low Tunnels, and the whole spring sequence is in Seed Starting and Season Extension.