Hi, new here so go easy

i very nearly ordered a polytunnel the other day but read that they are not suitable for overwintering plants such as palms, bannanas etc that i currently put in a small greenhouse. I understand they are slightly less efficient at heat retaining but by how much.

I wanted a large 20' x 10' (around £300) and the equivalent greenhouse would cost me a fortune. Do you think a ploy tunnel would be ok? I plan on adding some heat and can always line with bubble wrap etc but then i do that in the small greenhouse. Im in southampton by the way

cheers