if ya going to be growning in a polytunnel you need to get a strain that is mold resistant.
hollands hoop,Early Girl and The Church are mold resistant.
this most likely fucked with what you were going to grow.
and you would want to pinch it out to stop it getting to big for the polytunnel.
find out the average height it grows when goes into flowering.
so that you can have it pinched out before it gets to big and flowering at the same time.
Reptile Heating mat i must check this out might be handy for low ryders in the green house.
if ya need any thing else shout always about