As a person who actually calculated the last frost dates (90 percentile) for Wilmington from the available data you would think I would know better. The above graph is the temperatures near my house for the past several days. Blue is below freezing, the dips to 25F were especially concerning to me and my poor plants. I knew ahead of time that temperatures would dip below freezing, but I now had lettuce, Brussels sprouts and strawberries newly planted and I wasn't sure they would be able to handle it.
I just started with just bottles with the tops or bottoms cut off to cover the plants I could and a black plastic bag for the row of lettuce.
Bottomless seltzer bottles just fit over the Brussels sprouts.
Topless jugs, a bottomless milk jug and a bucket serve to cover the new strawberry plants.
For the lettuce plants I improvised a cold frame from a plastic bag, duct tape and six dry wire hangers saved from the dry cleaning.
Linus helped me lay out a piece of black plastic a little longer than the row of lettuce. I just used a heavy duty 55 gallon garbage bag that I slit up both ends to make a double long piece of plastic.
Leftover hangers from the dry cleaning will provide a high enough support in the middle to be taller than these very short plants. I started by bending the tops of the hangers flat so that I could duct tape them to the plastic in the middle. I made sure that I had enough plastic on either side so that when I invert it and cover the plants, I can put some wood on the plastic to seal the end and keep it from blowing away.
I used four hangers, two on each end that will also be the sides and two dividing the middle space to provide support along the length. After I taped in the four hangers, I took two more hangers, unwound them and bent then into straight pieces and then duct taped them to the middle of the plastic to provide support to the spine of the cold frame.
Starting at the hanger on the end, I then taped the bottom of the hanger to the plastic to form the side of the cold frame, be sure that you have enough plastic at the end to cover the side and have a flap that you can put a piece of wood on to keep the cold frame in place when it is completed. Finally the frame is becoming more three-dimensional. Do the same to the hanger at the other end.
Next tape the sides of the wire hanger to the plastic for the two hangers in the middle. This will now make the cold frame into an upside down diagonal structure. Do this with both hangers.
I made sure that the hangers were taped well and that the straightened spine hanger wire was also taped well. Above is the completed cold frame, before it was inverted onto the plants.
The cold frame in place. I did use two sticks in the ground between the end hangers and the middle ones in order to give the spine extra support. Gently make sure the sticks line up with the duct taped spine so they don't poke through. The plastic overlapped at the edges gives a spot to put some wood down to seal the frame against the ground to keep some heat in and freezing temperatures out or at least moderated. They also keep it from blowing away.
Here is an angled shot to show the three dimensional structure a little more prominently.
I would estimate that I spent almost nothing but my less than an hour of time on this, since the ingredients were left over from other projects or free with other things.
So far my plants have survived the 4 or so days of deep freeze so the frame and the bottles seem to be working. I was finally able to begin the retirement of the device since last night's temperatures stayed above freezing, and what we have for the next few days is rain not freezing or frost.
For a detailed plan of the above project with measurements and a list of materials ... you will need to forget that you aren't watching This Old House or the New Yankee Workshop.