No probs.. I'm in the South of England so weather pretty much same as you I expect god knows the weather has been all over the place the last month.. mini heat wave one week then hail the next.. crazy. 5" to 6" tall should be ok seeing as they will be protected from the elements in the polytunnel. Will you be using coco then as your medium? That's what I used for my grow, you might need a few bags of the stuff if you are growing 15 of them! Yes they love the coco & canna nutes so good option for you. I still use Canna Coco & nutes as they always give me good results that are just as good as a pure hydro grow. Would be great to hear of your progress so keep me posted mate.