Hotbeds have been used for hundreds of years. Hotbeds are basically the same as a cold frame, but use manure with compost as a heat source in early spring. In France, when transportation was done by horses, not cars, people would take the stall litter which was a mix of manure and straw for heat and make what is called a hotbed. They surmised the bed would get warmer as the manure decomposed and they could start some crops earlier while getting nitrogen in the soil. You can use this method in greenhouses, cold frames or poly-tunnels on top of your raised beds.
The only thing is if you use horse manure, you need to ask whoever you get it from if the hay they fed the horse wasn’t sprayed with an herbicide (like Roundup). Herbicides are weed killers. You don’t want to use any manure with an herbicide in it as it lasts for 4-6 years in the soil and will kill anything you want to grow. Basically it will ruin your bed. Having said that, our hay comes from a safe source. As a thought, you could use bagged cow manure but know that it is hotter than horse manure so it should probably age a little longer-use a soil thermometer to make sure your soil is not too hot.
Looking back in my notes, here’s what I tried in January 2020-I took out all of the soil in my center raised bed in my unheated greenhouse and put the soil on a tarp to put back in one of my other beds. I then put about 14-18″ of hot horse manure and 6 inches of straw in a wheelbarrow and mixed them up. Then put the mixture in the bottom of the bed and packed it down lightly and watered it till it was moist but not soggy. I then added 6″ of homemade compost/soil on top of that leaving a couple of inches of free space at the very top of the bed for plants. Then I watered again. A few days later, I put a compost thermometer in the bed and saw that the temperature had come up substantially deep inside in the manure/straw mix. It will get from 125°-150°F in the beginning but will come down pretty quick by the time you put your transplants in later. I didn’t take the temperature at the top compost layer but the fresh horse manure/straw mix got the compost in the top layer warm, and the 6″ of the aged compost kept it from getting too hot.
If you have a cold frame with a lid or plastic tunnel over a raised bed, you could do the same as in the top photo with spinach in it. I have one cold frame that I use in this way in the spring as well. It isn’t dug into the ground (like in the diagram above-not in my ground-too hard!)) but sits on top of the soil so I put some straw bales around the sides to add some insulation to it.
While the manure was cooking, I started lettuce/spinach seeds inside under lights (with no heat) in late January which took 3-4 weeks to get big enough to transplant in that top layer during which the soil temperature came down more as the manure mixture decomposed. Do not plant in the manure section but only the top compost/soil layer.
Once I planted them, I still had to cover the greens with medium weight row cover at night (and I double it up with layers of row cover on it if the weather was going to be very cold in the day/night (like it is now). It worked!
It was a fun project to learn how to make a hotbed and how ingenious people were in the old days!
giantveggiegardener 