There’s a lot of mythology about techniques to warm up soil, including discussions about organic versus sustainable gardening, so you can start growing your tomatoes early.
Here’s the research.
- If your plants are under stress from other environmental activities (lack of food, lack of water, too cold nights) then warm soil is going to help a bit.
- If your plants are not under stress, the work to warm up the soil is irrelevant.
Bottom line: You can do it if you’re looking for an early planting date and your plants will be under stress. But if you’re not trying to push your season by planting early, it may not be of much use other than making the gardener feel better.
Warming Up The Soil May Not Really Help. Here’s Why
Classic methods of heating up the garden soil usually involve using clear or black plastic. But when you think about it, this may be an ineffective method.
- Yes, it heats up the soil faster.
- Yes, it allows your tomato transplants to be planted into warmer soil.
- But, it doesn’t allow you to plant earlier.
You see, if you try to plant earlier because your soil is warm, you’re still going to run into cold night air temperatures.
And it is often these cool night temperatures that the early fruit trusses get blossom end rot.
Please see the section on Blossom End Rot for more details.
You could use them to increase the soil temperature so it’s high when the night air temperature rises enough to warrant planting. That will assist the plant.
Organic versus Sustainable
That leads me to organic versus sustainable. Using plastic to warm up the soil is neither organic nor sustainable. Some folks might make a case for organic because the material isn’t left in the garden and only used mechanically, but they can’t pretend using an petroleum based product for getting a jump on the growing season is sustainable.
If you must use clear plastic to warm up your garden soil, then know it’s extremely effective at doing this.
But do not leave it on the soil after you plant the tomatoes. It will produce too much heat and damage (or kill) the roots.
Black plastic is no more sustainable than clear plastic and while it does an adequate job of heating up the soil, it’s not as effective as clear. It can however be left on the soil all season long in northern areas. In more southerly gardens, the heat may be too high.
My preference is for an organic mulch. It is clearly organic and sustainable. And as it degrades, it feeds the garden rather than sterilizing the soil.
Unlike plastic, it also supports a diverse, active and healthy population of beneficial microorganisms in the soil.
I pull the mulch back from the growing areas of tender plants as early in the spring as I can. The sun acts to heat up the soil.
I try to leave the mulch pulled back for a few weeks after planting but as soon as I start seeing weed seed germinating, I pull it back up around the plants (after killing the weeds)
Note that in a perfect world, new mulch would be put on in the fall so it wouldn’t disturb young plants. In my world, I often get the mulch in July so I can either put it on immediately or have it sit around and rot. I carefully put it around plants then.
Frost fabric is relatively quick and easy to lay down. You could build a framework for the sheets but it’s just as easy to pin them to the ground.
If you’re going to heat up the soil to get an early crop planted then you do want to use frost fabric to protect the young transplants! In other words, if you heat up the soil, use frost fabric to protect from cool air temperatures.
I did get asked once about laying pipes under the ground and hooking them up to a hot water solar heater. 🙂 I suspect that would work nicely to give you heated soil (see note above re cold air) but it’s a lot of work and cost for very little gain. Now, if this were in a greenhouse it would be completely different.