Soil Temperature

by Sandy Hollingsworth 

In talking with other Master Gardeners, I’ve found some faithfully rely on soil temperature when planting while others take their chances. I know I’ve been guilty of impatience planting root vegetable seeds, then later tomato and pepper seedlings, too early, then watching them sit doing nothing and never catching up to the ones I waited to plant at a happier, warmer time. The warm sun can make the top of the soil feel warm enough, but down several inches it’s another story. This year I decided to invest in a thermometer to poke into my containers, prepped soil and raised beds to check before planting. I found a 5-inch-long metal stem pocket thermometer, with a probe sheath for storage. It happens to be in centigrade but that is okay as it is easy enough to figure out the conversion to Fahrenheit.

When reading up on recommended planting temperatures I found this summary: “Soil temperature is the best indicator of when to plant each type of vegetable, no matter what climate zone you live in”, said Annie Chozinski, Oregon State University vegetable researcher. “Crops that germinate in the coolest soils (down to 40 degrees F) include arugula, fava beans, kale, lettuce, pak choi, parsnips, peas, radicchio, radishes and spinach seed. When the soil temperature reaches above 50 degrees, Chinese cabbage, leeks, onions, Swiss chard, and turnips can join them in the garden.  At 60 degrees you can sow warm-season vegetables such as beans, beets, broccoli, Brussels sprouts, cabbage, carrots and cauliflower.”

If you are not at higher altitudes with a short growing season like 9300’ at the Gilpin County CSU Extension community gardens, or if you have a warm greenhouse, you’ll want to wait until the soil warms to 70 - 80 degrees to plant warm-season vegetables, or start them inside well in advance. These include tomatoes, eggplants, peppers, melons, tomatillos, cucumbers, corn and squash. Some mountain residents with pockets of warmer micro-climates may be able to grow these warmer season vegetables, but most will be better off sticking to cool season vegetables. Even if you do have a warm micro-climate, do be aware that the harvest will be smaller than lower and warmer elevations.

Our beloved mountain grown potatoes need at least 40 degrees to sprout and be productive. In our shorter, higher altitude growing season cool weather vegetables with shorter growing times tend to be most successful.

Although it is rarely an issue in the mountains except in greenhouses, know that some plants have an upper soil temperature that they will tolerate, such as 80 degrees for lettuce and spinach, and 95 degrees for most anything including cucumbers and tomatoes. Of course, you can plant more vegetables when the soil returns to its preferred temperature if your growing season gives you enough time to harvest it, or you use row covers or other season extension techniques to help regulate the soil temperature.

Here is a sampling of Fahrenheit vs Centigrade conversion if needed:

45 degrees F = 7.2 degrees C

50 degrees F = 10 degrees C

60 degrees F = 15.5 degrees C

70 degrees F = 21 degrees C

80 degrees F = 26.6 degrees C

90 degrees F = 32.2 degrees C

Colorado State University Fact Sheets 7.244 Colorado Mountain Gardening, 7.235 Choosing a Soil Amendment, and 7.214 Mulches for Home Grounds provide more information about controlling soil temperature.  They also explain soil amendments and that reducing surface evaporation and conserving soil moisture factor into soil temperature. Raised beds warm faster plus using south facing areas will usually be beneficial to reduce temperature fluctuations if combined with enough protection from wind. 7.248 Vegetable Gardening in the Mountains includes more scientific explanations about plant base temperature “T-base” and GDU Growing Degree Units, plus tips on seed and site selection.

The thermometer will be a welcome addition to your garden tools and fun to poke around with while you wait patiently for the right time to plant.  Happy planting!