The Importance of Soil Temperature

Know Your Soil’s Temperature Before You Plant!

Ensure good seed germination with warm soil

by Paula Mohr, Anoka County Extension Master Gardener

Recent 70°F-degree temperatures have been calling us outside. It’s natural for gardeners to want to get their hands in the soil and their seeds in the ground.

As tough as it is, be patient. Instead, pick up a thermometer and check your soil’s temperature. Different vegetable crops require different soil temperatures for seeds to germinate and to develop healthy root systems. Cool-season crops, such as beets and peas, can be sown earlier. Late-season crops, such as tomatoes and green peppers, need warmer soils. If you plant seeds too early, they can rot. Cold soil also can stunt vegetable transplants or cause them to die off.

I’ve learned from experience about planting seeds and young plants too soon. Where we live in northwestern Anoka County, we have mostly clay-like soil. No Anoka Sand Plain sandy soil for us. In the past, I’ve had pea and beet seeds take a long time to germinate because the ground wasn’t warm enough, even for them. Cucumber seed didn’t germinate one year and I had to resow it. And the tomato plants I set too early in the ground seemed to take longer to adjust, temporarily stunting growth. Over time, I’ve learned which portions of the yard, garden, and flower beds warm up first, and not to rush planting.

It’s simple to check your soil temperature. You could buy a soil thermometer at your local garden center. Or use a meat thermometer from your kitchen. The latter works just fine.

Ideally, take your soil temperature early in the morning before the heat of the day. Insert the thermometer a couple inches into the soil down to the root zone. Wait a minute or so for the thermometer to get an accurate reading. Take several readings in the garden area and repeat the process again for a few days so you can get an average temperature for your garden bed.

The University of Minnesota Extension provides specific soil temperature guidelines to help us determine the best time for planting vegetables.

Soil temperature guidelines are:

-For cold-season crops, such as lettuce, peas, spinach, radish and cabbage, soil in the 40°F to 45°F range is acceptable.

-For cool-season crops, such as beets, chard and parsnips, require slightly warmer soil. Root growth for most cool-season plants is optimal between 50°F and 65°F.

-Warm-season crops, such as green beans, corn and tomatoes, need soil consistently at 60°F. Sweet corn also germinates best when the soil temp is near 60°F.

-Heat-loving plants, such peppers, squash, cucumbers and eggplant, require the warmest soil --65°F to 75°F. Along with warm daytime temps, you also want nighttime temperatures to stay above 50°F before transplanting them

Another way to track when to sow seeds is by knowing your area’s last frost date. In Anoka County, on average, that’s around mid-May. By late May, soil should be warm enough for heat-loving vegetables.

However, to make sure your soil is ready to support growing seed, remember to take its temperature.