Soil Temperature for Seed Germination: A Vegetable Sowing Chart

August 15, 2026 Bill Hageman

Introduction

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.

FAQs About Soil Temperature

Is a heat mat always necessary?

No. A heat mat is useful when needed to bring the growing medium to, or maintain it within, the crop's suitable germination temperature range. Monitor the medium's temperature and moisture, and use a thermostat where possible to prevent overheating and excessive drying.

Should I use daytime or nighttime soil temperature?

Use repeated readings of the growing medium, taken at a consistent depth and at representative times, rather than relying on a single warm afternoon reading. For outdoor sowing, look for a consistent temperature trend appropriate for the crop. For indoor seed starting, monitor the medium where the seeds are planted.

Does warmer always mean faster?

No. Germination generally becomes faster as temperatures rise toward a crop's optimum, but temperatures that are too high or too low can slow or inhibit germination. Lettuce and some other cool-season crops can be particularly sensitive to high temperatures. Follow the recommended germination range for the specific crop and variety.

Does this chart give the earliest outdoor sowing temperature?

No. This chart describes indoor germination conditions, not the earliest safe date for outdoor sowing. For direct sowing outdoors, check the seed packet and crop-specific local guidance, including the recommended soil temperature, frost sensitivity and expected weather conditions. Soil temperature alone does not determine whether outdoor conditions are suitable.