well ...isn't it too cold because of the cement and the stone inside of it?
I mean if leaving the walls as it is without cover , the soil will act like cooler/heater throw the winter months.Instead they keep the temp. a little too low.

Just my opinion

Later Edit:

Nice flowers

This is the qumquat in the greenhouse.

nice looking tree , and plenty of fruits

Thanks Danero! This citrus is ideal for tranch culture. It is quite cold hardy, has fast rippening.

We have had -3,5 C ouside this morning. There was +4,1 C inside greenhouse. It is covered with tape only.

it is a small distance between outside temp and inside , isn't too close to the 0C line ?...

I will cover it with styrofoam when the temperature drop below 2 C inside.

and if ...saying only .... is there any risk of dropping below 2C ?
I m so interested 'cause I will build myself one the next year.

Yes, it is. When the temperature outside is -7 C -9 C at night it is possible to get below +2 C inside.

and you are not afraid of freezing ground on citrus ?

I have the digital temperature sensor. It is connected to the my fathers computer. When the temperature is going down below +2 C at night i will cover it immediately. And i will able to see forecast for 7 days.

so you got some heaters then if it drops ? I m thinking of buying some special panels


they have air between the two sides so offers more protection , also the frame must not be from metal or aluminium as it conduct cold throw the panel and inside .I consider wood or PVC frame is the best along with 1 or 2 panel over them. The light level remains the same but the cold stays outside

I'm going to use the PVC-panels in the future. I hope i will buid a new greenhose with large size.
I don't use any heaters if the temperature drops.

larger and larger is the word , as the air volume increases also it is more hardier that the air will freeze

So you are not afraid of ground freezing as I read?

Yes.