Hot takes on ground temperatures

YOUR ARRIVAL

It’s finally over — the winter of 2022-’23, that is. The last date of likely frost for us in the Mankato area is May 15. That feels good!

Of course, I’ve been working in the garden already, finishing up the cleaning I didn’t get done last fall, and planting radishes. My first radishes should be ready to eat any day now.

As we head out to the garden, especially early like this, paying attention to the soil temperature is important. As the saying goes, “Some like it hot.”

Of course, for starters, we’re planting seeds that like it cold, or at least cool. First to go in the ground usually have instructions on the packet telling us to “Plant as soon as the soil can be worked.”

This instruction refers not only to soil temperature, but also to moisture levels. If the soil is still icy at a lower depth, water pools on the surface, and seeds will rot.

Bare soil warms faster than grass-covered areas, but surface temps are not what we need to go by. Temps at a depth of 2 to 4 inches are needed for sowing seeds, and at 4 to 6 inches for transplants.

There are several thermometers you can use for taking soil temperature readings. Some have a long probe connected to a cable that connects to a gauge held in your hand. Others look almost like a kitchen meat thermometer. Some look like digital thermometers found in a bathroom medicine chest.

The one I use is a little more fun. It is a non-contact infrared thermometer. It’s hand-held, and all you have to do is point and shoot. There is a digital readout on the back of the device that gives the temperature in both Fahrenheit and Celsius with the push of a button. (I think it’s more fun because I can also check the temps of my dog, my cats, the fence posts, etc.)

Prices range from about $8 for the lower-end thermometers, to the hundreds, if you choose to spend that much. The one I use costs about $30 and requires a couple of AAA batteries.

The first seeds and plants to go in the ground are cold lovers such as radishes, kale, spinach, cabbage, peas and kohlrabies. For most of these vegetables, the ground can be about 50 to 75 degrees.

Because not all varieties of these veggies are the same, be sure to read the package and follow directions. For example, some radishes won’t germinate and grow unless the soil is much warmer.

Green bean seeds need a soil temp of at least 60 degrees to germinate. Basil, peppers and squash also fall into this temperature range for optimal germination. Okra, tomatoes, cantaloupe and sweet potatoes like temps even warmer to get a good start.

I have never purposely grown tomatoes from direct sowing in the garden, but I have had great luck from many “volunteer” seeds left in the garden from the season before. They never sprout right away, but as soon as the soil warms up, they pop up.

If you want to hurry soil warming along, you can always cover the garden with something like black plastic to gather and trap the heat. To maintain an even temperature, it’s probably best to just let the sun do its work in its own time.

If you use raised garden beds, they will warm much more quickly than the ground. In some ways, raised beds reduce work. Weeding is much easier and the area to weed is smaller. The other side of that coin is that they need watering a lot more. Often, they require watering several times a day in warm weather.

I have made the decision in the past to plant the entire garden all on the same day. If the day and soil were too cold, plants died and seeds didn’t germinate. If the day and soil were too warm, I didn’t get any radishes, for example. They put up foliage and went right to seed.