Last year, at the end of the season, I bought some of the big box store daylilies and asiatic lilies. They were in the bags that don't cost much, especially at the end of the season. Instead of planting them in the ground, I potted them all up in nursery containers.
Knowing they wouldn't survive the winter without some help, I began by making a tunnel out of a piece of fence wire bent on each side. I then taped two pieces of three-foot-wide plastic together with duct tape. A bigger piece of 5-foot-wide plastic would have saved this step, but I already had this on hand. I stapled a piece of 2x2 onto both of the wide sides of the plastic, then just placed that over the tunnel. This makes it very easy to open on warm days.
The easy opening also makes watering a breeze. Just lift one side and flip it over. Don't forget to close it back up at the end of the day!
To seal off the ends of the tunnel, use what you have available. Plywood works well. The picture below was taken in January. I was using two old window frames I had picked up to cover the ends at the time.
By making a little greenhouse, I could be certain that the plants would start growing earlier than if they had been in the ground. This picture was taken in Feburary and you can see that the plants have begun growing and are showing green leaves.
The picture below shows the plants in March. Here you can see how the plastic is just pulled up and over so that it lies on the back. This method has been much easier than the method I used previously, when I fixed the plastic onto the cold frame.
This picture shows a covered cold frame in the back, one of my wire tunnels with plastic over it in the middle, and the wire tunnel in the foreground, covered with a shade cloth (old party tent material salvaged) and used to grow my carrots and lettuce.
I also went a step further and made a cold frame for some of my potted plants. I used two old window frames I had found by the side of the road (the ones I had originally used to seal off the ends of my tunnel) and some old lumber that had been tossed, and I used that to make a free cold frame. I actually had picked up an old band platform a couple of years ago and saved the black carpet-like material. It makes a great cover for cold nights. You can see the black cover in the picture above.
Cold frames don't have to be fancy or expensive, and they can give your growing season a great early start.