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!
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.
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.
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.
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.
For an added boost once you have planted the bed put the plastic back on the bed and 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.
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