Warm soil jump starts spring

Posted by Carol O'Meara, Horticulture Entomologist, CSU Extension Boulder County

For gardeners, seeing the first, green shoots of spring is like hearing the NASCAR announcement “Gentlemen, start your engines.”  Our engines rumble, our minds become sharply focused.  We act like rookies on the line by digging a bit too early, our eagerness to get started getting the better of us.

If you’re itching to plant, get a jump on the season by warming your soil.  With a few simple tricks, you can get your spring salad off to a quick start.  Before you start, a note of caution:  make sure you’re not working wet soil.  Turning it can damage the tilth of soggy ground. 

One of the simplest ways to warm your soil is covering the ground with plastic sheets. Use 6 mil or thicker, UV resistant clear or black plastic and lay it over the soil, weighing down all edges with rocks or soil to prevent winds from whipping it up, up, and away to Kansas.  Alternately, you can anchor it down with wire U-shaped pins.

Check the soil after ten days to see if it’s warmer; for germination of cool season vegetables the minimum temperature needed is 40-degrees F.  Typically, it takes two to three weeks for it to rise, depending on the soil type. Sandy or manufactured "planters mix" soils warm faster than wet, heavy clay. 

If you’d like to speed the process, combine the plastic cover with an insulating layer.  Using only clear plastic, lay a sheet on the ground, anchoring it as described above.  Then drape a second layer of clear plastic slightly above the first, using bricks or other objects to make a small space between the two layers.  Anchor the second layer securely, by tucking its edges under the bricks or by weighing them down on the ground.

To plant, fold back the plastic drape and remove the plastic sheet covering the soil, cleaning, drying, and folding it away for use another time. Plant seeds of lettuce, radish, kale, collards, cabbage, broccoli, spinach, peas, onion and carrots and then replace the plastic drape over the spacers, creating an impromptu cold frame, anchoring the cover securely so it remains to keep the seeds snugly warm in the bed.  There is no need for a plastic sheet on the soil once seeds are planted.

Patience is required when warming the soil as seed take longer to germinate at minimum soil temperatures than they would later in the spring.  Leaving the plastic on until the temperatures have risen higher than 40 degrees won’t take much more time and you’ll be rewarded with better germination if you wait.   Monitor soil moisture and add water as needed.

Watch the weather and your plants closely; once the weather warms, the plastic tenting will trap heat and can reach temperatures hot enough to sizzle your plants.  Open the cover on sunny days, partially folding back the cover and clipping the flap to prevent it from whipping in the breeze and tearing.  Be sure to close the cover in the late afternoon to retain heat.

When the weather has warmed, remove the cover gradually over a week to harden off the seedlings.  Provide wind protection to keep the worst of spring away from them by making a low wall from straw bales or plastic.