Most are really only meant to protect a few degrees. 15 is COLD!!! I use a floating row cover but it is lighter and it does protect, BUT, we got down to 27 for an extended period and it zapped most of my plants, except the Pac Choi! Previously I used the floating row cover AND plastic when I knew it was going to get below 32, then remove the plastic after the threat has passed. I don't know why I didn't do that this year. My floating row cover stays on all season to protect from insects and wandering guineas.
