Joe Bouton

First, you have to keep it warm. You can stick polystyrene board back to your north wall. Cover hear curtain on your firm at night.
Then, while the temperature goes down, you could heat your greenhouse with hot-water warmming by solar energy , hot air blower, even bath heater.

Oh dear god! You made it yourself? That is amazing. One tip: I see the transmitting of your firm maybe a little low. So you can raise some plants that are shade tolerant.