When spring is in progress, gardeners converse in Fahrenheit. They cheer when the thermometer hits 50, crow when a sunny day breaks 60, commiserate over a drop to 40. But the number that should be on all lips is the temperature of the soil.
Soil heat rises steadily in spring, but its progress lags behind that of the air. Here’s an odd fact: soil at 25 feet below the earth’s surface takes until midwinter to catch up with the peak of summer heat. It is warmest in February, coldest in August. This yearly fluctuation is only 3 or 4 degrees, and doesn’t affect gardens. But in the top two feet where vegetables’ fortunes are made, even though the lag is much less, it has a big impact.
A balmy February afternoon may get our own sap flowing, but if the soil is still barely above freezing few seeds will germinate. Lettuce, peas, spinach, radishes, carrots and potatoes would be fine at 45. But squash and bean seeds will rot in 60-degree soil. Corn, too, should wait for 65, although if you start it indoors and then transplant it outside, it will withstand cooler temperatures as seedlings.
Most of the warm-weather fruiting crops such as tomatoes, peppers, melons, squash, cucumbers and eggplant are so dependent on soil warmth that it is folly to set them out until the soil is at least 60. Waiting until it’s 70 will ensure stronger plants that will soon overtake any set out too soon.
Old gardeners who know by experience what soil feels like to a stalk or a seed can often time plantings without much thought, but for most of us a soil thermometer is reassuring. Best not to stick a thermometer made for other purposes into hard earth unless you regard broken glass and mercury as soil amendments.
I use a simple pocket model, the Checktemp 1 from Hanna Instruments. You insert its metal probe 2 inches into the soil in early spring (4 inches at tomato time), wait a few minutes for the digital display to stabilize and there’s your magic number.
There are tricks you can play to get your garden to warm up faster, such as siting it on a south slope. A cold frame, or even just a sheet of clear plastic laid on the soil, will trap daytime heat and keep it from radiating back into space at night.
A sandy soil warms up more quickly than a clay one, but loses heat faster in cold, dry weather. Dark soil warms faster than light, and some gardeners even blacken their soil with charcoal in spring. A shallow mulch helps to keep soil temperature constant, but this is more of a help in fall and winter, as an insulating blanket, or in summer, to keep soil cool. Bare earth in spring welcomes the sun. A dose of patience comes in handy too.