Tooele Transcript Bulletin – News in Tooele, Utah
image Inexpensive and readily available soil thermometers will allow you to plant at the best time for germination.

March 24, 2016
The magic of warm soil

While the first day of spring is in the history book for 2016, we are in the throes of meteorological uncertainty. As I write this, the clouds have settled down low across the valley, and large snowflakes have begun descending. While the calendar says its spring, weather patterns are king when it comes to our reality. Besides, this IS spring — one of the times of the year when you can experience fall, winter, and spring-like weather all in a day or two — and sometimes in the same day!

I had to chide one of my Utah Facebook friends the other day. He posted that he had already set out the lawn furniture, as well as his barbeque after enjoying several warm days recently. He posted a photo of his snow-covered patio and asked, “what happened to spring?” I told him that he had fallen for, well, you know… sucker weather (you knew that was coming!). Besides, nothing had “happened” to spring — it was simply doing its thing — like it does every year.

Recently, I made the point that while we gauge comfort and warmth by the ambient air temperature, it’s different for our plant friends. When plants are getting started, either from seed, or being transplanted, soil temperature trumps (no pun on the current political season) air temps every time.

When the correct ratio of soil moisture and warmth comes together, the results for a gardener is, well, magical. Seeds leap to life in just a few days as seedlings emerge, eager to get on with the season. Seedlings that have been transplanted with proper care (transplant at dusk to let the plants acclimate to their new home overnight) don’t seem to have any set back, but simply continue their rapid growth rate.

Germination time is greatly affected by soil temperature. Below a certain temperature, the seed won’t germinate at all. As the soil starts to warm, the seed may germinate, but the time to do so can be quite lengthy. At optimum warmth, the seed germinates in just a few days.

Here are some germination time comparisons for five common veggie crops — spinach, tomato, watermelon, squash, and sweet corn. An important fact to know is that the minimum temperature  to germinate these five AT ALL are (in the same order listed): 32, 50, 68, 68 and 50 degrees (Fahrenheit). It’s interesting to note how long the time to germinate those five when only minimum needed soil temp is attained. For our five friends, the time to sprout is 63, 43, 12, 6, and 22 days respectively at this minimum temperature.

Squash germinates pretty fast, but notice that the soil temp for it to start at all has to be pretty warm — 68 degrees. For the rest of them, the time needed for you to start seeing some greenery is a test in patience.

Look what happens when optimum temperatures are attained though! When the average soil temp is 77 degrees, those same five now germinate in 5, 6, 5, 4, and 4 days. That’s what I’m talking about (sorry, my inner rapper just came out)! If you’d like to see more of this, download a time-to-germinate chart of these and many more crops by visiting I keep a copy of this chart handy, and I recommend you do too.

Of course, what we’ve been talking about is direct seed sowing in the garden plot. You can get further faster by starting seeds indoor under growing lights in a warm place or over heating mats. This will compress germination time. Once your plants have sprouted and have begun to put on true leaves, move them to light only and get them off the heat. Warmed plants will put on a lot of stem growth and become “leggy.”

For all the crops that we will be planting directly in the garden (corn, peas, beans, carrots, etc.), we’ll need to know the soil temperature to do this right and enjoy the most planting success. You can’t do a good job of this by estimating soil temps based on how warm the air feels, or by sticking a finger into the soil. Nor can you simply plant by fixed dates, simply because each year is different climatically. The only reliable way is to use a soil thermometer. Yes, I’m aware this could add to your quirky reputation with your neighbors. Wear your green thumb — and thermometer — proudly.

Don’t use a medical thermometer for this, it’s not the best tool — it’s too fragile in case you encounter hard ground or a rock. Besides, who knows where that thing has been! Rather, inexpensive soil thermometers are readily available at gardening suppliers and online. They are available in dial and digital models, and you can spend anywhere from under $10 up to $60 depending on what you want.

Once you have the unit, place the probe only two to three inches in the soil where you are wanting to sow seeds. Use the handy chart referenced above and make a decision about when you want to plant. One caution. Don’t plant seeds too early before their minimum soil temperatures are attained in your garden space. If you do, you risk having the seed rot from moisture.

I’ve found that it’s safe to plant seeds of about any type when the soil is between 55 and 60 degrees. However, this is an “averaging” approach, meaning some plants could have been sowed earlier, and others will germinate slower, waiting for the soil to warm up.

You know that soil warms up slower and cools down later than surface air. But did you know that’s a good thing? The resistance of the soil to immediately change temperatures depending on whether the sun is shining on it or not, or if it is inclement weather, or simply seasonal change, is highly beneficial.

On hot days, the coolness of the soil keeps the core and root temperature of the plant cooler until the sun is not so harsh or the sun sets. Later in the season, we can plant cool weather crops in the heat of summer to rapidly germinate them, but have them mature as the heat fades off as fall comes on. Not only can plants “harvest” the heat and cool that the ground affords them, but so can we. This is the core principle of geothermal heating and cooling systems for homes and office buildings.

Ground tends to stay temperature constant at depths of four feet and deeper (within reason). Piping loops can be placed horizontally (either in ditches or drilled sideways) or vertically and a water and eco-friendly anti-freeze/conditioner mix is circulated through them. The temperature that is transmitted from the surrounding soil into the liquid is then transferred into the house or building using an exchanger. The benefit is that when it’s hot outside, the soil temp will be lower, so the home’s cooling system only needs to cool the difference in the ground temperature and what’s desired in the home.

During the winter, it works in reverse. While it’s very cold outside, the soil is quite warmer than the air. This heat is conveyed into the building, and the furnace only needs to warm the difference, not the entire range from outside air temperature to desired level in the home. This is all pretty nifty stuff. In fact, my friend Ron Haycock (current president of the county Master Gardeners) has one of these systems in operation at his home in Grantsville. This really works!

So, while the first day of spring has already occurred, it will be several weeks before noticeable soil warming occurs. Even so, the trees, shrubs, and bulb-type plants all know it is coming. Buds are forming, some leaf tips are beginning to show, and the bed of daylilies, iris and tulips are springing to life. So while the feeling of bare feet in warm soil will have to wait for a bit, the anticipation is enjoyable now.

Remember to save the date for the upcoming Spring Garden Expo on Saturday, April 9, from 10 a.m.-2 p.m. This is a speed course on a variety of great gardening topics — all for $5! You’ll learn a lot, have fun doing it, and meet some great people along the way. It’s a winning combination — and just too good to pass up.

Jay Cooper can be contacted at, or you can visit his web channel at for videos on gardening, shop skills, culinary arts and landscaping.

Jay Cooper

Garden Spot Columnist at Tooele Transcript Bulletin
Jay Cooper is a new contributing writer for the Garden Spot column. He replaced Diane Sagers, who retired in November 2013 after writing the column for 27 years. Also known as Dirt Farmer Jay, Cooper and his wife have been residents of Erda since 2001 after moving to Utah from Tucson, AZ. A passionate gardener and avid reader of horticultural topics, for several years he has been a member of Utah State University’s Master Gardeners Program, and served as chapter president in 2013. Cooper says Tooele County has an active and vibrant gardening community, and the Garden Spot column will continue to share a wide range of gardening, landscaping, home skills and rural living themes.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>