We were hot. And, now we’re not. What’s a gardener to do in spring in our changeable northern climate when deciding when to plant vegetables. One option is to use soil temperature as your guide.
There are several guides for when to plant, including using frost-free dates or using nature signs (phenology). Soil temperature is another way to decide when to plant because the soil temperature is one of the key factors in seed germination.
Cool But Not Too Cool
Some seeds will germinate at fairly low temperatures—so-called cool season crops. For instance, both lettuce seeds and pea seeds can germinate when the soil is in the 30s. However, it takes them longer to germinate, so a seed that might sprout in a week when the soil is in the 60s could take three weeks (or longer) if it’s in the 40s. Leaving them in the soil that long gives plenty of time for rot or other problems to set in. So planting early is not always the best approach.
Warm-season crops, such as peppers or tomatoes, need much higher soil temperatures in order to germinate. Tomatoes, for example, like the soil to be about 75 degrees, which is why many home gardeners use a heating mat when they are starting seeds indoors.
Too Hot to Germinate
Many northern gardeners use row covers of plastic or cloth to warm up the soil in spring. This is a great way to stretch the season. However, soil can be too warm for seeds to germinate. For instance, lettuce won’t germinate if the soil is above 85 degrees. For spinach, 75 is too hot to germinate.
Taking the Temperature
How do you take a soil’s temperature? Get a thermometer! A meat thermometer will do the trick, but you can buy thermometers specifically for soil for about $8 at your local garden center. To take the temperature, insert the probe end at about the level where the seed or the roots will be, if you are planting transplants. Give it a few minutes to register the temperature. If it’s too cold for your crop, wait a week or so to let it warm up.