MANHATTAN, Kan. -- Even experienced gardeners can miss the gate on spring planting if their timing comes from traditional guides -- including beloved almanacs and the area’s average frost-free date.

Soil (not air) temperature is the trigger for seed germination. The dates and rates for soil’s spring warm-up can vary from year to year. They can vary within a particular yard or garden, said Chuck Marr, horticulturist at Kansas State University.

"All kinds of things affect the process," he explained. "Does the soil have a southern exposure? Is it shaded all or part of the day? Is the soil light or heavy? Wet or dry? Low-lying, flat or mounded?"

In the Midwest, most cool-loving garden vegetables germinate only when the soil reaches an average 45 degrees F, Marr said. Seeds for warm-loving plants wait until typical temperatures are 55 F.

The sure way to recognize those points is to take soil’s temperature between 10 a.m. and 11 a.m. That’s the midpoint between when daily temperatures are coolest (dawn) and warmest (late afternoon), he said.

Marr suggests using a metal probe thermometer with a flat dial on top, pushing it 1.5 to 2 inches into the soil for readings at various spots. A metal shaft is particularly important, so the thermometer won’t break when it hits a below-ground pebble.

"You can’t use a meat thermometer, because its reading scale will be limited to hot oven temperatures," he said. "For under $5, however, you can find similar-looking thermometers that register lower readings. Some garden stores carry them. Auto supply stores and the automotive section of discount stores often sell them, too, to use in checking how well car air conditioners are cooling things off."

These thermometers must not remain in the ground. Gardeners should store them indoors and carry them out for each temperature reading, Marr said.

Less specific readings are available on the Kansas Weather Data Library’s Website (www.oznet.ksu.edu/wdl/kansasag.htm). Although housed in Manhattan with K-State Research and Extension, Kansas Climatologist Mary Knapp monitors daily weather conditions in Colby, Garden City, Hays, Hesston, Hutchinson, Manhattan, Ottawa, Parsons, Powhattan, Rossville, St. John, Scandia, Silver Lake and Tribune.

She then updates the Website to include the day’s precipitation, minutes of sunshine, solar radiation, wind velocity, and both maximum and minimum air and soil (at a depth of 4 inches) temperatures at each site

"These temperature readings give you a general indication of what soil temperatures are doing in your area. They also can indicate when to start sampling in your own garden spot," Marr said.

Being wrong when timing spring planting won’t always lead to disaster, he added.

"Many seeds will simply sit and wait for warmer weather," the horticulturist said. "But sometimes -- particularly with warm-loving crops -- seeds won’t stay viable for long. Or, having to wait may affect their plant’s strength or productivity. So, the most efficient and effective approach will always be to wait until conditions are right."

Soon after they’ve reached 45 or 55 F in spring, temperatures sometimes drop again, due to a strong, long-lasting cold front. In most cases, however, soil temperatures tend to rise and fall very slowly. Thus, the day they reach an average 45 or 55 F, it’s generally safe to plant, Marr said.