Seed germination depends on moisture, oxygen, viable seed, and a suitable temperature. Air temperature is useful, but the temperature around the seed is what directly affects germination. Outdoors, measure the soil at planting depth; indoors, measure the seed-starting medium rather than relying on the room thermostat.
This chart lists crop-specific indoor seed-starting estimates from Utah State University Extension. The seed packet remains the best variety-specific reference. Shop seed-starting equipment or browse vegetable seeds after choosing an appropriate sowing window.
Vegetable germination temperature chart
| Crop | Optimum soil temperature (°F) | Typical days to germination |
|---|---|---|
| Broccoli | 50–85 | 7–10 |
| Cabbage | 50–85 | 4–10 |
| Celery | 50–65 | 9–21 |
| Cucumber | 65–85 | 6–10 |
| Eggplant | 65–85 | 6–10 |
| Lettuce | 50–65 | 6–8 |
| Melons | 65–85 | 6–8 |
| Onion | 65–85 | 7–10 |
| Pepper | 65–85 | 9–14 |
| Squash | 65–85 | 4–6 |
| Tomato | 65–85 | 6–12 |
These are optimum soil-temperature ranges for starting transplants, not minimum outdoor sowing thresholds or ideal air temperatures for mature plants. Timing varies with seed lot, variety, moisture, and temperature. Crops absent from the source table are not assigned an estimated range here.
How to measure soil temperature
- Use a soil thermometer at the depth where the seed will be planted.
- Measure at a consistent time for several days. Morning readings are useful for avoiding a decision based on one warm afternoon.
- Check more than one part of the bed if sun exposure, mulch, slope, or soil type varies.
- Use the trend with the weather forecast and crop tolerance, not one isolated reading.
Indoor temperature management
Heat mats can warm germination media, but they should be paired with monitoring. Remove or adjust bottom heat after emergence when the crop no longer needs it, and move seedlings into adequate light promptly. Domes can slow moisture loss before emergence; ventilate or remove them as seedlings appear to reduce prolonged saturation.
Browse seed-starting trays, seed-starting media, and other seed-starting supplies.
If seeds are not germinating
- Confirm the seed's age and storage history with the seed viability guide.
- Check moisture at seed depth; both drying and waterlogging can interfere.
- Confirm that the species does not require light, darkness, scarification, stratification, or soaking.
- Allow the crop's normal emergence window before disturbing the medium.
- Check whether the medium is colder or warmer than the surrounding air.
Temperature questions
Is a heat mat always necessary?
No. It is useful only when the medium is cooler than the crop's suitable range. Monitor the medium so bottom heat does not create excessive temperature or drying.
Should I use daytime or nighttime soil temperature?
Use repeated readings, including cool morning conditions, rather than one warm afternoon. A stable trend is more useful than a peak.
Does warmer always mean faster?
Only within a crop's suitable range. Excessive heat can inhibit germination, particularly for lettuce and some cool-season crops.
Does this chart give the earliest outdoor sowing temperature?
No. It describes indoor germination conditions. For outdoor sowing, follow the seed packet and crop-specific local guidance, considering frost, moisture, and the weather forecast.
Continue planning your seed garden
- When to start seeds indoors
- How to read a seed packet
- How to grow brassicas from seed
- How to grow root vegetables from seed
Return to the Seed Learning Center for crop guides, seed-starting help, and shopping pathways.