I want to specify why I'm going to test this method of reheating in a cold bed. I plant tomatoes in this bed at the end of March. The plants thrive in it, but there is a problem with flowering and pollination. I need temperatures below 11 ° C at night. This time I want to base this bed as a warm hotbed. I will give leaves to the leaves, manure from horses (stabled on wood sawdust), I will add a little urea and pour warm water. I will put a layer of soil on top, in which I will plant tomatoes. I hope that this will allow me to increase the temperature by some 3-5 ° C. I would like to achieve another increase at night by decomposing the organic matter in the bag with air. In the fall, I want to test how the bags in that bed will work. I can also add a little horse manure, or even urea.
One more question: should I use a transparent bag or black?
Vladimír

I use low tunnels for early spring plant protection but, as others have said, it provides frost protection but without heat sinks the overnight temp inside the tunnels will not be much above what's outside the tunnel. I follow about the same procedure as [mention]Salaam[/mention] for regulating the temp in tunnels during the day.

Now that I'm doing corn exclusively in raised beds, I'm going to try something I've been thinking about next spring and that's putting the tunnel on the bed early to heat the soil for that real early Spring Treat corn. Take the plastic off, plant and then put the plastic back on. Once the corn's popped up I can raise the tunnel sides during the day (and night if mild) but still have it in place for a while just in case one of those late frosts comes in.

Compared to me, you have the advantage that you can regulate the temperature several times a day. You have a garden near the house where you live. I take the tram and trolleybus to the garden three-quarters of an hour there and three-quarters of an hour back.
Vladimír