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Soil temperature and vegetable seed germination

Sprouted peanutIn the past, I've planted crops a certain number of days before or after our traditional frost-free date.  But the more I think about it, the more the date-based approach feels like eating your meals at set times with no wiggle room if you've been chopping wood and are starving early or have been loafing around all day and aren't really hungry at all.  By planting seeds on set dates, I'm trying to estimate times at which the soil temperature is warm enough to let the seeds germinate and the air temperature is in the right range to let the seeds grow well.  So why not measure soil temperature and plant seeds when I know it's warm enough?

The table below tells the minimum and optimum soil temperatures for germination of most common vegetable seeds.  For spring planting, you can probably get away with planting at the minimum temperature (especially if you soak the seeds to get them off to a quick start), but for many of the summer vegetables it's often best to wait until you reach the optimum temperature range.

Vegetable
Minimum temp. (degrees F)
Optimum temp. (degrees F)
Beans
60
60-85
Cabbage
40
45-95
Carrots
40
45-85
Corn
50
60-95
Cucumbers
60
60-95
Lettuce
35
40-80
Muskmelons
60
75-95
Okra
60
70-95
Onions
35
50-95
Parsley
40
50-85
Peas
40
40-75
Peppers
60
65-95
Pumpkins
60
70-90
Spinach
35
45-75
Squash
60
70-95
Swiss chard
40
50-85
Tomatoes
50
70-95
Turnips
40
60-105
Watermelons
60
70-95

Calibrate a soil thermometer in ice waterTo test your soil temperature, first calibrate your soil thermometer in a jar of ice water to check its accuracy.  (The thermometer should read 32 degrees Fahrenheit.)  Then stick the thermometer three to four inches into the soil first thing in the morning and read the temperature.  If you don't have a soil thermometer, you can get a rough estimate of your soil temperature by looking at this soil temperature map.

If you're like me and are dying to put spring seeds in the ground as soon as possible, you can make the soil warm up more quickly by raking back your mulch, adding a thin layer of dark compost to the surface of the soil, laying down a sheet of black or clear plastic (although I don't like blocking off air to soil microorganisms), or building a cold frame or quick hoop.  You should also be aware that the temperature of the upper layer of soil in your garden can change quite quickly --- a couple of degrees per day --- if a cold snap or warm spell hits your area, so even if the soil has reached 35, don't plant lettuce if an arctic blast is forecast for tomorrow.

While waiting for the soil to warm up, why not treat your chickens to a homemade chicken waterer that never spills or fills with poop?



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