You can grow greens like lettuce, kale, and spinach all winter in Oklahoma if you build a simple hoop house out of PVC and plastic sheeting. I've spent many hours researching the best way to build a PVC hoop house so that greens can be easily harvested over the winter. This is the best design I've seen by far. This video calls for 2 layers of protection, but we should only need one here in Oklahoma. I plan to finish building mine soon and will follow up with a review of how well it works.