Soil temperature is just as important as proper moisture to achieve optimal germination rate. Without ensuring the right soil temperature range is being met, we can expect germination to take longer, and the percent of seeds that germinate to decrease. Temperatures below the minimum can delay or prevent germination, leaving seed vulnerable to mold, or hungry wildlife like birds, insects, or rodents.
Soil temperature generally lags behind air temperature in spring, which may require you to wait longer to sow, consequently eliminating some longer season varieties from your garden. For example, some pumpkins take 120 days to mature, but also require soil temperatures of 70°–90°F for germination. If your soil doesn't typically warm in time for a variety to mature, you can start those seeds indoors, essentially extending your growing season by giving plants a head start. Even inside, however, some plants will need additional heating to maintain that ideal soil temperature; for those plants, use a waterproof seedling heat mat. Using a soil thermometer will also give you an advantage towards successful germination, as you can monitor and adjust the indoor environment to reach the optimal temperature. Germination temperature requirements are often higher than what the plant needs to grow, so once germinated, most varieties can be transplanted into cooler soils (after hardening off, and your average last frost dates).
How to use your soil thermometer
A soil thermometer is an easy-to-use, indispensable tool that can make gardening from seed more successful, helping you achieve a great germination rate. Here are some tips for using a soil thermometer to accurately measure soil temperature.
- Take soil temperature measurements twice a day for an average, in the early morning and in the afternoon/evening. Taking and recording readings over a couple of days will give you an even more accurate average of your current soil temperature.
- Place the thermometer 1"–2" in the soil for seeds, and as deep as the pot in the case of transplants.Note: A soil thermometer with a metal probe is sturdiest and safest. If your thermometer's probe is glass, create a pilot hole with a tool like a screwdriver that is slightly narrower than the thermometer probe. The slightly smaller hole will ensure the thermometer is still getting good soil contact.
- Shade the thermometer if it is in direct sun.
- Thermometers may read slowly, so wait until the temperature holds steady.
- Store your soil thermometer indoors, away from moisture.
Download our soil temperature guide for vegetables or reference the chart below!
For more seed starting tips, see our other articles on seed starting.