Warming the Soil for Spring Planting

We run a plant business here in New Hampshire, where our growing season is short, so it's important to get seeds in the ground as early as possible. To this end, I tested a few common methods for melting snow and warming the soil. I hope my advice helps you with spring planting, too.

Melting the Snow

First, I put down squares of black plastic, clear plastic, wood ashes, and coffee grounds and tested each for how quickly they melted the snow.

Clear and Black Plastic: On the few sunny and unseasonably warm days we had, Mother Nature did a better job of making the snow disappear than either of the plastics, which actually seemed to protect the snow from melting. I ended up with two snow squares that still haven't melted!

Wood ashes

Wood Ashes: Surprisingly enough, the wood ashes quickly melted right through the snow.

Coffee Grounds: The coffee grounds seemed to insulate the snow from the sun's rays, like the plastic, which resulted in little melting.

Warming the Soil

After the snow was gone, I moved the two squares of plastic to one of the frozen raised beds to gauge their warming effect.

• This time, the clear plastic worked best, thawing the ground to 6 inches and heating the soil from 36° up to 56°F in 4 days! 
• The black plastic only got to 50°F, but still managed to thaw the ground down 6 inches.
• The wood ashes only thawed down 2-1/2 inches in that time and warmed to 48°F.
• The coffee grounds were 1 inch and 40°F.
• My control patch, which had no help with warming other than the sun, was thawed 1-1/2 inch and warmed to 44°F—better than the coffee grounds!

The Conclusion

Use wood ashes to melt the snow and then put down clear plastic to warm the soil. The clear plastic lets in the sunlight and UV rays and holds in the heat, allowing the soil to warm up.

One thing to note is that ashes will raise the pH of the soil (and add potassium), so if you live in an area that has soil with a naturally high pH, you'll want to forego the ashes and wait for Mother Nature to do the melting for you.

In the vegetable garden, we always plan ahead in the fall and put up the pea fence where we will be planting the peas in the spring. This seems to act like a snow fence and prevents the snow from drifting deeply over that bed so it thaws out and warms up first. Good thing, since I am very eager to get those seeds in the ground!

The soil is still pretty cold, it was 45°F on the last sunny day, but dropped back to 40 today since it is cloudy and raw out. Looking back thru my records, March 11 was the earliest I have planted peas in this garden and I had to replant them on April 2 because of poor germination. Probably rushed the season and the seeds rotted. Even though many seed packets say to plant peas as soon as the ground can be worked, the soil temperature really matters. At 50°F, it takes 2 weeks for the pea seedlings to emerge, at 60 degrees only 9 days, and at 77 degrees only 6 days.

Learn More

One old-time trick is to plant peas when the forsythia start to blossom. See more clues of planting by nature's signs.

Now see our video demonstrating how to warm your soil for spring planting.

This article was originally published in 2018 and has been updated.