Click below to listen to my Garden Bite radio show: Germination rates for plants
Mother Nature will decide when to plant! Oh, we have dates on the calendar of when the “average” time is to plant but the best indicator is soil temperature… yup, stick a thermometer in the soil and check it’s temp.
Here is a list of common vegetables and the minimum and optimal soil temperatures for seed germination:
Crop / Min. Temp (F) / Optimal Temp. (F)
Bean / 60 / 60-85
Beet / 40 / 50-85
Cabbage, Carrot, Cauliflower / 40 / 45-85
Corn / 50 / 60-95
Cucumber / 60 / 60-95
Eggplant / 60 / 75-95
Lettuce / 35 / 40-80
Pea / 40 / 40-75
Radish / 40 / 45-90
Squash, Watermelon, Muskmelon / 60 / 70-95
Now, that said, the OPTIMAL temperature for germination is about 10 degrees warmer while the top end, where the seeds are sweatin’ it out, can be 30 to 40 degrees warmer. Tomato and pepper plants really prefer soil to be around 70 degrees.
Recognize that even if the minimum temperature is met, it may delay the number of days until the seedling appears. For example according to research done by J. F. Harrington, University of CA at Davis, a carrot planted at one-half inch that germinated in 41°F soil took 51 days for the seedling to appear while only 17 days when the soil temperature was 50°F.
You can pick up a soil thermometer at most big box stores, some local garden centers or online for anywhere from 8 bucks to 60 bucks. Personally, the less expensive are just as reliable. Really any thermometer will work so long as it can go down to the lower temperatures.