It won't let a full spectrum of light in. Also too dark. Check with online garden supply sites for a wide range of options. You can get a roll of clear poly pretty cheap and it won't yellow or break down as fast as the stuff they generally sell at a hardware store.

I'm trying to build one as well. I'm doing the rebar/PVC hoop tunnel thing. Can I use a painting dropcloth -- the cheap clear plastic kind -- for the cover, or will this be too thin/clear? I'm not sure what kind of plastic to use...

Thanks!

Greenhouse grade clear poly is the best. Will give good protection and last for years. It costs a little more, but considering the years of repeated use and healthier plants it is cheaper in the long run.

Here in Wyoming, we absolutely have to use greenhouse film that is UV protected. Ordinary plastic will only last 1 year at best, whereas you'll want your cold frame to last many years without the hassle and expense of replacing the cover.

I believe you also need to be careful about which side is installed outward, unless both sides are UV protected. I use woven poly for everything since we have high winds and hail, and I am told that the material is UV protected on both sides.

Another advantage of woven poly is that it can flex repeatedly and still holds together, and the occasional snag doesn't have to be repaired. You can't rip the stuff, no matter how hard you try.

Clair