Soil temperature chart for seed germination

With each seed I am amazed that a handful of seeds (part of the size of the pinhead) can be transformed into an entire garden of tomatoes, cucumbers, kale, squash, broccoli, lettuce, basil, parsley, and marigolds.

But germinating all these seeds isn’t necessarily so cut and dried. You need good fresh seeds and you need moisture. It needs to be planted at the right time and temperature is important, but I’m talking about soil temperature here, not air temperature here.

If the temperature of the soil is as warm (or cool) as those particular seeds prefer, you may be seeing seeds that do not germinate at all.

Related: How to Germinate Stubborn Seeds

However, soil temperature is important for successful transplantation, as is the case with direct sowing seeds in the garden. Many warm season plants, especially plants such as peppers and squash, do not easily establish or thrive in the spring unless the soil temperature is warm enough for root development.

The easiest way to determine this is to use a soil thermometer. This is an essential tool that will help you achieve excellent germination rates. The Mine (link below) is a simple, inexpensive analog soil thermometer that has been around for several years. And as long as you keep it indoors from moisture, you can do that too.

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Step 2: If the thermometer is under the sun, cover the thermometer and wait until the temperature stabilizes for the most accurate reading.

Step 3: Read twice a day: once in the morning, again in the late afternoon or evening. Record these temperature measurements for 2-3 days to obtain an average of current soil temperatures.

Next, refer to the soil temperature chart below to see if you are in the ideal window to plant!

For example, tomatoes germinate in cool soil at 50°F, but can take up to six weeks. Germination often occurs within 5-7 days when owned in soil reaching at least 65°F.

However, some tricks can help warm the soil faster. My own garden bed is always covered with mulch to retain heat and moisture. Also, in early spring, low tunnels with rows of plastic or fabric covers can trap heat and warm the soil several degrees.

Certain cool weather species such as lettuce and Mach also become dormant if soil temperatures exceed the recommended range. If you are hoping to plant lettuce in the summer, it is almost always better to start seeds indoors and then transplant those seedlings in the shade of those seedlings and tall plants to avoid bolting them prematurely.

If your soil is usually not warm enough for a particular plant type to mature, or if you want to give your plants a head start, you can start those seeds indoors to extend the growth phase.

However, whether it’s in a greenhouse or in a cool part of a house, such as a garage or basement, some plants can use a little extra heat to maintain an ideal soil temperature. I like to use waterproof seedling heat mats for these plants (which take much longer than sweet peppers, and take much longer to germinate). (The links below are those I owned for several years.)