The Importance of Soil Temperatures in Your Garden

It is the time of year when Georgia gardeners think about their Fall, cool-season gardens.   Leafy greens like spinach, leaf lettuce, and kale are all popular cool-season crops.  They don’t require the time necessary to make a “head”, you can eat the thinnings, and the varieties available are endless.

The Importance of Soil Temperatures in August
Lettuce in the Trustees Garden Savannah, Georgia

Often at the beginning of cool-season planting time, germination rates can be an issue.  “I have purchased new spinach seed and my germination rate is only about 50%.”  Or, “My arugula just did not come up at all.”  The problem might not be the seed quality but the soil temperatures, especially in a hot summer like we have been experiencing.  Seeds require a specific range of soil temperatures for best germination.

This chart from Cornell University shows optimum soil temperatures for germination of popular cool-season crops:

Crop Soil Temperatures needed for Germination Comments
Arugula 40 – 55° F May fail to germinate in warm soils
Lettuce 40 – 85° F Best germination below 70° F
Spinach 45 – 75° F May fail to germinate in warm soils
Kale 45 – 85°F
Collard Greens 45 – 85°F
Mustard Greens 45 – 85°F

If soil temperatures are close to the range extremes, the germination rate will definitely be affected.  These temperatures not only affect the germination rate but how quickly the seeds emerge.  For example, at 50°F spinach seed can take as much as three weeks to emerge.  At 70°F you could see emergence in just days.

Here are soil temperatures being reported by the University of Georgia Weather Network as of Tuesday, August 30th at 9:30 a.m.

Location Soil Temperature at 2″ depth
Ellijay  73.5 °F
Dallas  79.6 °F
Jonesboro  77.3 °F
Statesboro  83.0 °F
Tifton  78.9 °F
Valdosta  80.4 °F

Using the information shown, gardeners will have a difficult time growing spinach at this time in most parts of Georgia.

The lesson, be patient and monitor your soil temperatures.  Cool conditions are coming, I promise!

Happy Gardening!

Becky Griffin

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