Starting your own garden veggies from seed can save money, and open a much wider and more exciting vista of possibilities. One way to give your baby plants a head start for spring is with a Hotbed.
First, some vocabulary:
Cold Frame: A simple low-slung structure to enclose garden beds and/or flats of seedlings under a glass or plastic top. Like a tiny unheated greenhouse.
Hotbed: A cold frame with a source of heat to warm the soil and/or flats of seedlings from below. Keeps things warmer than a cold frame. Can be heated with an electric heating cable in sand, or by decomposing organic matter - a kind of 'slow burn' composting method tailored to produce gentle heat over a long period.
In the 1800s before modern shipping, lettuce was profitably produced at scale in the winter months in France and New England using hotbeds heated with horse manure. Wealthy population centers supported large operations that produced 40 heads of lettuce per 6'x3' bed.
The "hot" material -- usually spent fresh horse stable bedding -- can be packed into a pit (most common in the old production scale model) or packed into a raised frame (see above).
Build a hotbed
There are, of course, many resources on the web with different anecdotes and plans for hotbeds. In this age it's easy to click around and see a variety of pictures and opinions, so here I'll lay out some important fundamentals to keep in mind.
- Use fresh manure. Ideally, you can find a source of freshly removed muck from a horse stable. "aged manure" will not do. Likewise pure manure will not do. The mix of manure to carbon (sawdust, hay, or straw bedding etc) is pretty forgiving, but with too much nitrogen (as in the case of pure manure) there will likely be too much ammonia produce, it can overwhelm the layer of finished compost or soil and scorch the seedlings.
- Compact your "hot mix" as much as possible and water it thoroughly! Stomp and spray the mix periodically as you add. Unlike regular thermal composting, we aim to exclude oxygen from the manure/bedding, so smash out that air and fill all the pore space up with water. This makes for a slower longer thermal period. Use at least 2 feet of hot mix, more is fine and will probably make heat for longer.
- Don't forget a 6" layer of finished compost or rich soil above the "hot mix" this is critical because it hosts organisms that will consume the ammonia gas generated by the "slow burn" of the manure/bedding. It can be planted directly as in the case of 1800s lettuce producers, but it is still necessary if you're growing seedlings in flats.
- On cold nights throw an extra layer of insulation over the transparent lid. When the sun is not shining, this will really improve heat holding. A blanket, tarp, or even a second layer of plastic makes a huge difference because of the air gap formed between it and the lid.
- Lay a few boards or stakes to prop up flats so that seedling roots won't grow through the bottoms into the soil/compost top layer. Alternatively, I've scooted the flats a few inches every two days or so to disturb those roots. I feel like this makes a bushier root system inside the flat.
- Orient the face of the lid towards the south (if you're residing in the northern hemisphere)
- Square bales of straw or hay make great insulation, they can line the outside of your hot mix:
- Finally, here's a pro tip that I haven't seen on too many other sites: This thermometer is fantastic. It has two probes and saves the low and high daily temperature over a ten day period so you can know exactly how much heat your hotbed holds relative to the outside overnight low.
Timing is important, a horse manure powered hotbed will make heat for about two months. And it'll take about 4 days to heat up after it's made. This is good timing for most veggie seedlings as they will need about 2 months of coddling before going into the ground.
One final word: a hotbed can overheat in sunny weather, especially on warm spring days. It's critical to crack the lid on sunny mornings.
Cheers, and if you do try this at home, let us know how it goes!