IT’S COMMON PRACTICE FOR BACKYARD GARDENERS to decide when to transplant vegetable seedlings by using a traditional planting date or the current weather conditions. This may be the most common error made by kitchen gardeners.
It’s certainly an understandable mistake. We pick a day when the air temperature is warm. The calendar says it’s well past the frost free date. But the next few days are cool and rainy and the transplanted seedlings stop growing.
When indoors, they may have been the strongest seedlings … the best of the bunch … but now they’re drooping badly and look miserable. If we’re lucky, we have other replacement seedlings waiting in the house. The replacements are not as strong as the originals, but we make do. (If it sounds like I’m speaking from experience, that’s because I am.)
More often than not, we blame the situation on transplant shock, or the cold rainy weather. But the real mistake was not waiting until the soil had stored enough heat. If we had checked the soil temperature and waited for it to be consistently warm, a few cool, rainy days wouldn’t have been the end of the world.
When the temperature is right
Truth be told, it’s much better to use soil temperature rather than the calendar to determine when to transplant warm weather seedlings. You can still get it wrong, but you have a much better chance of picking the optimum planting time if you check the temperature.
But it’s important to get it right. Transplanting pepper seedlings, for example, before the soil temperature is 70 degrees F could stunt their growth for the entire growing season.
To take proper soil temperature readings, you need to use a soil thermometer on three consecutive mornings. Readings should be taken at a depth of four to six inches for transplants. Using a soil thermometer is easy, and they are not very expensive. All you have to do is push the thermometer in to the specified depth and leave it there for three minutes (depending on the model of thermometer).
Use the following guide for MINIMUM soil temperatures for transplants:
60 F – tomatoes, cucumbers, snap beans
70 F – peppers, watermelons, squash
75 F – cantaloupe, sweet potatoes
Temperature is also an important factor for direct sowing crops. Measure at a depth of one to two inches when you are not transplanting. Here are the MINIMUM soil temperatures for vegetables that are direct sowed:
35 F – lettuce, spinach and other salad greens
50 F – onions
45 F – radishes
50 F – beets, Swiss chard
60 F – beans, snap and dry
70 F – beans, lima
40 F – peas
60 F – corn
If you need a soil thermometer, here are three models from Amazon:
Chaney Soil Thermometer* Price: $5.94 plus shipping.
Luster Leaf Rapitest Soil Thermometer* Price: $7.90 plus shipping.
Luster Leaf Rapitest Stainless Steel Dial Soil Thermometer 1630* Price: $11.52 plus shipping.
* If you use this link, I make a small amount of money from the purchase, which helps me continue to publish Suburban Hobby Farmer.