Good points Barbara! I must admit I've been thinking the run off as more of an opportunity than a problem. I've already arranged 'stick drains' along the length of the polytunnel, which will drain the water
inside the polytunnel. This is described from
this post on my polytunnel build thread. So far I think it is working well. The soil in the tunnel seems to be staying damper than I would expect otherwise, although I haven't tried digging test pits inside yet.
Many of the plants I'd like to plant, that like it rather damp, tend to be a bit spreading (like mint and marsh woundwort). Being so close to the polytunnel I'd like something that doesn't require digging for harvest too, as it makes it more possible to damage the plastic if I'm working with levers too close to it. Comfrey would probably be an idea, although that has large spreading roots. I had that close to my old polytunnel and it moved in! I think watercress might be a good possibility, as it doesn't actually need water. I wonder about marginal pond plants too....My wasabi looks like it has taken well