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Soil temperature and why it matters

soil thermometer in the ground
Soil thermometer. Photo: A. Bodkins

Sunshine, increasing temperatures, warm rain showers, and the return of migratory birds are all signs that Spring is getting closer. They are all reasons to be excited about Spring and all the possibilities that the new gardening season will hold.  

It’s always tempting to go out and start sowing seeds at the first glimpse of sunshine, but most seasoned gardeners know that patience is the best policy. It takes several weeks of warm air temperatures and sunshine for the soil temperature to get warm enough to signal the seeds to germinate. Mother nature provides mechanisms to protect seeds from germinating too early (called “seed dormancy”) and there are certain requirements that must be met before sprouting occurs. 

Did you know that every seed has an optimum range of soil temperatures for germination? This factor helps determine which seeds are cool-season versus warm-season. Penn State Extension has a great article regarding Soil Temperature and Seed Germination that you should spend a few minutes reading. 

Gardeners often purposefully stratify seeds to mimic what would occur naturally if the seed was left outside through the changing seasons. Some plants have really complex and specific requirements to meet before they can break dormancy. This principle is why so many people are having such good luck with the popular winter sowing technique, which lets you sow seeds early in the season, and then the seed decides when the conditions are right to grow. 

Winter-sown spinach seeds germinating. Photo: C. Carignan

A few years ago, I purchased a heat mat for seed starting and it has been the best investment that I’ve ever made! The heat mat helps to achieve better seed germination rates by providing optimum soil temperatures, which results in quicker seed germination. It’s a great cheat tool, especially for finicky seeds, like wave petunias, geraniums, and peppers.

I use the heat mat for all seedlings, cool and warm season. Often the cool-season plants sprout in just a day or two. Once they have their true leaves, they are transplanted into cell packs and put in the greenhouse until it is time to plant them in the high tunnel. Some heat mats have a thermostat that you can set to a certain degree, but mine just keeps the mat 15-20 degrees above the air temperature, so managing it can be a little tricky, especially if the sun pops out and heats up the air temperature quickly. You have to be on guard to unplug it in those instances. 

Here is a photo of my setup for the early season. The heat mat is under all of those trays. Several years ago, my family helped me build this mini-greenhouse out of scrap lumber and greenhouse plastic to create a microenvironment inside the larger greenhouse, which we keep unheated for as long as possible to conserve firewood. Once these seedlings get their true leaves they will be transplanted into cell packs and moved into the main greenhouse and more seeds will be planted on the mat. We do this process several times throughout the spring season. Photo: A. Bodkins

Jon Traunfeld, director of the University of Maryland Extension Home and Garden Information Center, shared this awesome site, which shows you the average soil temperature for your region!  

For a more specific reading of soil temperature in your own garden, you can purchase a soil thermometer. I recently purchased a soil probe thermometer and my kids are enjoying comparing the different temperatures from locations in our landscape. Here was the difference between the high tunnel and the raised bed.  

Don’t forget to check out the Vegetable Planting Calendar and be sure you are starting all your seedlings at the correct time. You can check your soil temperature in your own garden and make sure that it matches the ideal range for germinating the crop you want to grow. For example, the planting calendar recommends direct seeding snap beans in May (for Central Maryland, but here in Western Maryland most of the dates need to be moved to the right by at least 2 weeks). We can look at the chart up above and know that beans, a warm-season crop, need soil temperatures close to 70 degrees F to germinate. Happy Spring!

By Ashley Bodkins, Senior Agent Associate and Master Gardener Coordinator, Garrett County, Maryland, University of Maryland Extension. See more posts by Ashley.

This year, the University of Maryland Extension Master Gardener Grow It Eat It Program celebrates the resource that supports all life on earth – soil! Look for soil education programs offered by your local Master Gardener program. State-wide, all Maryland residents are welcome to attend the upcoming Grow It Eat It webinar, Seed Starting and Planning Your Garden for Climate Change, on April 8, 9:00 a.m. – 12:00 p.m.

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