Overwintering tomato plants is the aim

( mr horrible up yours, dickface, Tue 9 May 2023, 17:08, archived)

You could paint the (concrete?) floor black
to absorb heat during the day and radiate it at night.

Or you could build a proper passive solar collector heat sink type of thing.

Are you doing soil or aqua? If aqua, fish or no fish?
( brb Monkhouse Stampede, Tue 9 May 2023, 18:01, archived)