Artificial Spring

One of the main factors that can speed or inhibit germination of seeds is the temperature of the soil. You can sow peas as early as you like, there are even some people who broadcast them over the snow – but if their environment isn’t to their liking, they’ll remain stubbornly dormant until it warms up a little.

Well what good is a warm greenhouse if the soil is still too cold for plants to grow?

One of the main ways I’m warming up the soil in the bed is through compost. Compost is made by microbes digesting organic matter. The more organic matter, the more activity (as long as it’s in the right ratios and moisture level), which generates more and more heat. Compost piles easily reach 140-160*F at their center, and have been traditionally used to slowly heat greenhouses over the winter. There is even a way to heat your water by coiling a hose around and around the core of a carefully constructed compost pile, and pumping cold water through it gets you very toasty water indeed at the other end.

Of course I didn’t want the soil that hot!

But a little warmer than freezing would be better. So I worked all that compost, dry leaves, and all those donated hefty bags of coffee grounds into the soil. I spread it all out nicely so there was no real “core” to concentrate the heat and scorch the plants. It should decompose a bit more slowly over time, dispersing heat more gently and evenly across the bed.

The next day I took the temperature of the disturbed soil right in the middle of the bed – it was up from mid-50s to 62*F, which told me the combination of water and all that compost was probably already beginning to have an impact on microbial soil activity (good news for the earthworms, who love to eat soil bacteria).

Next I changed the bed from flat, angling it a bit more southerly towards the sun. Sloped from back to front, I hope it will catch and sink more sunlight.

Since you should never leave soil naked for a single minute unless your goal is to grow a jungle of weeds, lose moisture, tilth, and nutrients, I immediately mulched it with leaves, even though I knew they would probably act as insulation and keep my soil from warming. But then I remembered that last year I had bought some cornstarch-based biodegradable “plastic”… and it was black.

Perfect! Putting the black sheet in contact with the earth ought to really get the soil warming up. Much like solarizing your soil in summertime with a black sheet to heat it up to kill all your weeds, though the winter sun is not nearly that strong.


Sure enough, 8 hours later the soil was at 72*, a full 10 degrees higher! (And it’s stayed around there all day today too.) That temperature is optimal for planting pretty much anything. Of course I wouldn’t plant my hot weather crops now anyway because of the cold-nights problem, but it should mean that my peas and lettuces will really take off quickly.

More importantly, because earth takes so long to dissipate its heat, this means that even if the air temperature gets cold enough in there to freeze, the plants’ roots ought to be just fine.

At least, they’d better be! Because I spent all day soaking my pea seeds and just planted like 90 of them along the back wall – a full 3-4 weeks before I would normally plant them outside (our date to plant crops like peas is March 15th, but it’s always too wet and cold for that to work for me. April 1 works better.) I’ve never sown peas this early and I’m really excited!


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