Your Soil Should Be Warm To Sow Seeds

The woman in front of me as we checked out of a major retailer last night had a tree in her cart. At first I thought it was plastic because the blooming flowers that covered its branches looked too perfect.  Upon closer observation, the “flowering tree” label around its trunk identified it as real, probably a flowering plum. As I walked to the parking lot I observed a man helping her load it into the back seat of her Prius and advising, “Don’t plant it now.” I cringed when I saw the branches with the beautiful flowers hanging out the window.

Unless she drove home at three miles per hour in the 45-degree air, all of the blossoms were left on the roadway before she pulled into her drive. Even if she is patient, stores it in a warm area, and waits until we get through the next few nights with forecast low temperatures of around 22 degrees F, she’ll probably want to plant it soon. If she does everything right from this point on, the tree’s prognosis is not good; it’s too early. I’ve offered advisories before about avoiding the temptation of pretty plants at big box stores until the proper planting time arrives. Often the best lesson is experience, and the loss of an inexpensive tree.

Most plants prefer root temperatures between 50 and 70 degrees F. That’s one reason you have to be careful about planting trees that have broken dormancy and flowering plants from big box stores too soon. Early spring frost is a threat, but cold soil can wither roots, sap energy, and hinder future growth. A weakened plant is much more susceptible to disease and insect infestation.

It’s tempting to start planting as the days warm, but be careful. Locally, we’ve set some new records for high temperatures in the last few weeks and have more warm days this weekend. Just looking at the air temperature it seems like a good time to plant, but what is the soil temperature? That may be more important.

Stick your finger in the soil at various spots in your garden. Do you have to pull it out because the cold is stinging or do you detect warmth? The mass of earth beneath our feet takes longer to warm than the air we breathe. You may assume the soil is warm enough for plants because the sun is shining, but unless you take a few moments to confirm it, you may shock the roots of your new flowering plum or stunt or prevent the growth of your vegetable seedlings.

An inexpensive soil thermometer is a nice garden gadget to own, but any probe will work for temperature confirmation. You need to measure the temperature a few inches below the surface and your finger or a meat thermometer can give the readings you need. Your finger is around 98 degrees F and you can gauge the temperature of things you touch by the relative coolness you feel. A probe thermometer is much more precise however. If your significant other doesn’t like the idea of using kitchen tools in the garden, get a soil thermometer.

Soil temp is 37F after days in the 60s.

Seeds need specific soil temperatures to germinate. Many cool season vegetables can germinate with a soil temperature above 45 degrees F, but will do best when it’s above 60 degrees F. Warm season vegetables may be able to handle 60 F, but would prefer 80 F. Of course, there is a high end; above 85 and 90 degrees F few seeds will germinate, but we usually don’t concern ourselves with that problem in early spring.

Knowing the specific temperature that your seeds need can help ensure germination and growth. Gardener Supply Co. has a nice online chart that shows what soil temperature various seeds need to germinate. As we approach our last frost date, it’s important to be aware that frost is only one factor that affects new plants. If the soil hasn’t warmed up enough, seeds won’t grow.

Naturally, longer days with more sun will warm up the soil. You can accelerate the process if you want an earlier start to your planting. Laying a sheet of black plastic over a planting area will warm it quickly in a short time. Once you remove the plastic, however, the soil will cool if the air remains colder than the soil temperature. Using a mini green house (read my blog on that subject) to warm the soil and help maintain warmer temperatures will add weeks to your growing season.

A small hoophouse covering a raised bed.

How you garden will affect how the soil warms up. Raised beds warm up faster than open soil. Sand and loam warm up faster than clay. Dry soil warms faster than wet. Rich, dark, amended soil will warm faster than average soil. Soil in the sun will warm faster than soil in shade. Soil will warm slower in mulched areas. For the quickest warming, start with dry, amended, unmulched, loamy or sandy soil in a raised bed.

The soil temp near my roses is a few degrees cooler than the raised beds.

You can track the progress of your soil’s temperature by taking a reading every day. Stick your thermometer a few inches deep, wait for the reading to stabilize, and write it down. If the temperature is constant or increases for three days in a row you can use that as a baseline. If the soil is warm enough for the seeds that you want to germinate and if the air temperature will stay above freezing, you can sow. It’s that easy. If the soil or air temperatures are too low, you should wait. Take more readings until you meet the appropriate threshold.

The importance of taking these steps is that they help ensure you’re not planting too early. If you traditionally wait to plant until the sun is high in the sky and the trees are fully leafed out, you probably don’t need to be taking daily soil temperature readings. But if you want to gain every advantage and sow as early as possible, take the time to monitor soil temperature.


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