Using Soil Temperature to Decide When to Plant
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.
I started my Ailsa Craig onions from seed at the end of February. They’re ready to put into the garden now but my soil temperature was 42 degrees today. A chart I use said that onion plants should be set out when the soil temp reaches 45. So I guess I’ll wait and plant later on. Let’s hope the forecast for warmer weather this week is correct.
One year I was determined to have gladiolas blooming at different times in the summer so I planted some bulbs early, some more 2 weeks later and some more 2 weeks after that. The soil was cold for the first planting and warmed up over the next month. But all the glads bloomed at the same time in the summer.
Hi, gave my daughter a tulip plant.
Can those bulbs be planted when the soil warms up.
Also do you wait for the leaves to die
Of before planting?
Thank you. Waiting for answer.
Gayle — I’ve done a bit of research and it looks like you can plant the bulbs after the danger of frost has passed. (About May 10) Plant them with the leaves and let the leaves dieback naturally. Because those bulbs were forced into bloom early and because critters (deer, rodents) love tulips, there’s no guarantee the tulips will rebloom next year — but they might! Good luck!
Do you have a larger chart to show the minimum planting temperatures for all the other fruit and vegetable crops?
Check out this resource: https://northerngardener.org/wp-content/uploads/2022/02/Germation-Growing-Temperatures.pdf
Thank you Mary. Like the site
Gayle