Unless the greenhouse is in the Arctic/Antarctic or on the moon or Mars
I don't think you'll need heating.
( brb Monkhouse Stampede, Tue 9 May 2023, 17:07, archived)

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)

Seems easier just to get rid of the greenhouse and rely on the earth's own solar powered heating and watering systems.
If anything the glass just gets in the way.
( 2 Can Chunder Word to your mums, I came to prod bums, Tue 9 May 2023, 17:10, archived)

Seems better not to jog with limestone
If anything, your wall just gets in the way.
( mr horrible up yours, dickface, Tue 9 May 2023, 17:36, archived)

The sun is overrated anyway
i.dailymail.co.uk/i/pix/2014/04/17/article-0-1D28F8E000000578-927_634x422.jpg
( Turtle Power & Knuckles, Tue 9 May 2023, 18:48, archived)