I would like to use my greenhouse to flower my plants. I realize this means 12 hours dark for them. In winter this is natural, but in summer the Sun is up way too long. So, my thought is to blackout the greenhouse every day. Since it is about 6m x 4m, and about 2.5m high (square, not the tunnel-type), I intend installing a motorized blinds system inside the greenhouse, and thus would like to use a very plyable and light material for the blackout.
Does the material (let’s say as an example I use black plastic at 250 microns) need to be UV treated? Will UV get through untreated plastic even if lights is stopped, and, more important, will the UV influence the “light-cycle” for the plant - ie.: will the plants think it is still daylight, and thus NOT start flowering?