Allotments 4 All

I put my new min-max thermometer in my mini plastic greenhouse last Thursday, just out of curiosity.

On Saturday, I checked it. Min was down to -0.5C (not surprising). Max went up to 33C.  :o It was very sunny on Friday and I keep it closed the whole time in the winter.

Will this be detrimental to the plants? Should I open it during the day?

In it there are a new rose bush, autumn-sown sweet peas and various hardy periennial seedlings (most Aquilegias) that I sowed in the autumn.

I put a few sub-tropical plants in one and they turned to mush or mould, I may try a few seeds after February passes, but I'll be keeping the door open during the day. ::)

Not surprising! NO greenhouse is immune.
Nothing likes ups & downs, but they are especially bad for propagating & seedlings. And not much help to toms etc.
Don't know, but wouldn't have thought that your other things would come to much harm.
Common sense dictates here, I think. Do what you think sensible, like ventilate, when you can. Even to add some more insulation at night? But you don't want to be ruled by the thing?  

you can by a gadget from garden centres etc which opens and closes a ventilaion window depending on temperature.