Released: March 20, 2003

To Know When to Plant, Take Soil’s Temperature

MANHATTAN, Kan. – Knowing when to plant in spring can be confusing when temperatures have been see-sawing for weeks from the teens to near 70 degrees.

Fortunately, soil – not air – temperature is the trigger for seed germination. And soil temperature changes gradually, said Chuck Marr, horticulturist with Kansas State University Research and Extension.

Beyond that, gardeners can expect particular soil temperatures to bring particular results. In the Midwest, for example, most cool-loving garden vegetables germinate only when the soil reaches an average 45 degrees, Marr said. Seeds for warm-loving plants wait until typical temperatures are 55 F.

Unfortunately, however, the dates and rates for soil’s warm-up can vary each year. They often vary within a single yard or garden, Marr said.

Factors that affect the process include the soil’s exposure to sunlight, its texture, moisture content and surface level (low-lying, flat or mounded).

"That’s why the only sure way to know when it’s safe to plant is to take the soil’s temperature," he said.

Marr recommends gardeners do so by following this process:

* Buy a metal probe thermometer with a flat dial that provides low readings (in other words, NOT an oven-temperature meat thermometer, although it looks similar). These relatively inexpensive items are available at some garden stores, as well as many auto supply stores and discount store automotive sections.

"The car folks use them to check how well air conditioners are cooling," he said.

* Plan to take your garden’s temperature between 10 and 11 a.m. – the midpoint between when daily temperatures are coolest (dawn) and warmest (late afternoon).

* Push the thermometer 1.5 to 2 inches into the soil at several locations. Its metal shaft will keep the thermometer from breaking if it hits a below-ground rock.

* Do NOT leave the thermometer in the ground. Store it indoors between readings.

A strong, long-lasting cold front can take soil temperatures down again, especially just after they’ve finally warmed to 45 or 55 F in spring, Marr said. Because soil temperatures tend to rise and fall so slowly, however, their reaching 45 or 55 degrees usually means it’s safe to plant.

Being planted too early isn’t always fatal for seeds, he added.

"Many will simply sit and wait for the soil to warm up," Marr said. "But with warm-loving crops, in particular, the seeds may not stay viable for very long. If they do germinate, their having waited may affect the plants’ health or output."

The horticulturist said Kansans can get an idea of when to start taking their garden’s temperature by monitoring the Kansas Weather Data Library’s Website (http://www.oznet.ksu.edu/wdl). Each day, State Climatologist Mary Knapp posts information from automated weather stations in Colby, Garden City, Hays, Hesston, Hutchinson, Manhattan, Ottawa, Parsons, Powhattan, Rossville, St. John, Scandia, Silver Lake and Tribune. The data include soil temperature at a depth of 4 inches, as well as the day’s precipitation, minutes of sunshine, solar radiation, wind velocity, and high-low air temperatures for each site.

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Kathleen Ward, Communications Specialist
kward@oznet.ksu.edu
K-State Research& Extension News