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In 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

To 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.

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