Warming Your Soil in the Spring

Warming your soil in the spring can let you plant several weeks earlier each year.

Warming Your Soil in the Spring

I learned this trick for warming your soil in the spring from a very well respected gardening expert in our area. He taught it to us as part of our Master Gardening course. Warming the soil in the spring for early planting is really pretty easy. All it takes is a nice big piece of clear plastic.

Before we talk about this trick for warming your soil in the spring let’s spend a little time learning why it’s important to warm the soil in the spring.

Soil temperature is important for germination

In the early spring we are planting cool season crops like peas, lettuce, radishes, and spinach. All of these cool season veggies will have their optimal germination rate with soil temperatures of between 55 and 65 degrees Fahrenheit. They will germinate at lower temperatures than that, some as low as 40 degrees, but if you really want quick and consistent germination then your soil temps need to be in that 55 to 65 range.

Buy yourself a soil thermometer!

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The only way to really know where your soil temps are is to buy yourself one of these handy soil thermometer. You can find them at many garden centers or you can buy them online.

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To use them, you simply stick the probe in the soil and leave it for a bit. I like to test in several different areas of my garden. It’s interesting to see the variation in soil temps even in my small garden. Some spots really do get more sun!

What does getting the soil warm early do for your garden?

Why do we care about getting the soil warm early? Warming your soil in the spring allows you to get your cool season crops started weeks earlier than you otherwise could. Most importantly the warm soil gets the seed germinating faster than they would otherwise!

Let’s use peas as an example

On a normal year I try to get my peas planted around the 20th of March. That is really the first date when I can reasonably expect the soil to be dried out enough to work and plant. Even before I started warming my soil March 20th was still my target. The difference is that with pre warmed soil my peas germinate right away! Before I started warming my soil I would plant my peas on March 20th but the peas wouldn’t germinate until the 10th of April or later.

With warmed soil my peas will be germinating in 5 to 7 days. Some years that is a whole two weeks earlier. That extra two weeks makes a world of difference in my crop. The peas are ready earlier, they are growing in cooler weather and the whole crop is done and out of the way sooner so that I can plant summer crops in their place in early June.

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The same advantages apply to any cool season crop. In fact if the soil has dried out, you can use this method to get many of the really hardy crops like spinach and kale planted and germinated more than a month sooner than you could otherwise.

Now for the soil warming trick

All you need to take advantage of this soil warming trick is a simple sheet of clear painters plastic. I like to buy a 25 by 5 foot roll of 10 mil plastic. 10 mil is about the thickest plastic sheeting you can find. You want the thick stuff because it will last for several years without needing to be replaced. The thinner plastics may be a bit clearer but they don’t hold up well to wind and sun and will break down very quickly.

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All you do is spread the plastic out on your garden beds and secure it down with some rocks or bricks so it won’t blow away.  Put the plastic out on your beds at least 3 weeks before you intend to plant. You will be amazed by the difference this will make. In my garden the beds that have plastic on them will have a soil temperature of 60 degrees while the uncovered beds will still be in the low 40’s.

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For an added boost once you have planted the bed put the plastic back on the bed.   Leave it there until the plants germinate. Be sure you don’t leave that plastic on for long.  If you leave the plastic on as the temperatures start to rise you risk burning your new seedlings. Once the seeds have all germinated and are up about a 1/2 inch you can remove the plastic and store it away from the sun in a shed or garage. If you leave this plastic outside where it is exposed to the hot sun of summer it will break down quickly.

Remember that clear plastic is the key. Clear plastic lets the suns UV light through to the soil and then holds the heat in. You will not get the same results with black plastic, you need that sunlight to get through to warm the soil.

Warming your soil in the spring

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  1. Dave February 8, 2016 12:17 pm Reply

    Great info here! I’ve used a soil thermometer for years now, and I’ve found I have better results that way than going by the calendar alone.


  2. Jennifer A February 17, 2016 8:24 pm Reply

    I thought about doing this last year, but I wasn’t sure it would work. Thanks for sharing on the Homestead Blog Hop!


  3. NormG December 6, 2016 5:54 pm Reply

    I tried this, and got a city of voles as a result! LOL! Yes, the soil was warmer, but once the plastic was removed, the soil cooled off rapidly until it was in the same temp zone as surrounding soils. Think as if a cup of warm water in a cold lake. My question is, how do you keep it warm…oh, and let’s not talk about the weeds that germinated, too!

    • Mr. Stoney December 6, 2016 9:15 pm Reply

      I can’t help with the voles, sorry! You will note in the post that I mention that I put the plastic on for a couple of weeks before planting. I then take it off, plant the bed and then I put the plastic back on until the seeds I planted (usually peas) have germinated and are up about 1 inch. Then I remove the plastic for good.

      I don’t really have a weed problem, maybe just a few. Things that I have done to reduce weeds in my garden include: 1. NEVER let a weed grow big enough that it sets seeds. 2. Switch to drip irrigation (water only where you want). 3. Stop tilling! Tilling just brings buried seeds closer to the surface and it also destroys the soil structure.

      If weeds are really a huge problem for you, you might consider moving your garden to the “Garden of Eden” method where you cover your garden with a thick layer of wood chips as mulch. I don’t know much about that method, but I know some folks have a lot of success with it.

  4. NormG December 8, 2016 8:37 am Reply

    Thanks! I wonder if you start with the clear for a couple of weeks, plant, and then switch to black plastic instead of clear if that would help soil temp. I read an article indicating such, but it didn’t make sense because on one hand they said it absorbed heat and on the other hand said it kept soil from getting too hot (as it would with clear). I don’t want to cook my soil! By the way, I am just north of Seattle, and we have LONG sunny days, but soil temps stay in 50’s to 60’s…night air temps drop into the 50’s…so I am looking for a way to add heat without burning things up during the day. A greenhouse is not an option for me, I have used tunnels for peppers.

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