Season extension structures resembling low tunnels are a great way to protect overwintering vegetables. I use them to have fresh greens—kale, collards, and chard—on the table through the winter months. They are easy to build with plastic pipe and either clear plastic sheeting or greenhouse plastic. I would love to not have any plastic in my garden, but until I have a better alternative, I make an exception for this.
Plastic pipe easily bends to form the arches that hold up the plastic cover. My garden beds are 4’ wide and I use an 8’ length of plastic pipe (1/2” inside diameter) for each arch, giving me a tunnel with a height of about 30”. Some people use metal electrical conduit for their arches, bending them around a homemade jig. I space the arches about 4’ apart down the length of the bed. Another piece of plastic pipe is put on top, becoming a ridge pole to connect the arches. A screw is used to attach the ridge pole to the top of each arch. It is important to have the ridge pole.
My arches are held in place by either putting them over pieces of rebar extending up from the ground or by inserting them into larger pieces of plastic pipe, also extending up from the ground. Whether rebar or larger plastic pipe is used, the pieces of each are cut to 2’ lengths. Plastic pipe can be cut easy enough and you can buy rebar already cut into 2’ lengths. Look for rebar where cement blocks are sold. One foot of each anchor piece is driven into the ground, leaving 12” sticking up to receive the end of the arch.
Next comes the plastic cover. You can find clear plastic sheeting in a hardware store or big box building supply store (look for it in the paint department). Make sure it is 6 ml thick to withstand the winter weather. This construction plastic has no UV protection, but since you are only using it through the cold months, you can get a couple years use out of it if you store it out of the sun and keep the mice away during the off-season. Greenhouse plastic is good if you can get it since it will last longer. If you are building a structure that will be in the weather all year long, go with greenhouse plastic. A piece 10’ wide is good to go over the 8’ arches covering my 4’ wide beds.
The easiest way to secure the plastic cover to the pipes is with plastic clips, called garden clips or snap clamps, that are sold for this purpose. Johnny’s sells them and they are available at other garden and greenhouse supply sources. You can make some from plastic pipe, but if you need to take them on and off, the ones you buy are easier to work with. Okay, I know it is December already and if you had greens to protect, most likely you have already put up a structure like this if you intended to. I’m really writing this post to talk about the covers. You can build a low tunnel from these directions, but if you stop here you will have problems when the wind picks up or when it comes to harvesting from your tunnel through the winter.
The plastic covers on my low tunnels stick out 12” on the sides. Some gardeners put sand bags, rocks, or pieces of wood on that extra to hold the cover down. On a calm day, it might seem to do the job, but the wind will easily whip the plastic out from under these things. Besides, if you have 18” wide paths like I do, there is no extra room for sandbags, rocks, or pieces of wood. You will be tripping over these long after the covers were removed in the spring, unless you are diligent in taking them up. It would take putting many clips across each arch to secure your plastic cover enough to hold it through high winds. Even if you were willing to work with that many clips, you need to be able to access the plants inside through the season and it isn’t practical to be messing with so many fasteners each time.
My solution is to put a cord across from one arch to the next, alternating sides. You need the ridge pole to hold the cord up. I usually use 1/8” nylon cord found in hardware/building supply stores, but have used old clothesline if that was available. If you already have a low tunnel and have experienced problems with wind, you can add this feature and alleviate problems the rest of the winter. It involves putting a screw eye near the base of each pipe the cord attaches to. I use a drill to make a pilot hole for the screw eye.
Years ago when I first did this I thought I needed to build a wood box and use pipe clamps to hold the arches, screwing the screw eye into the wood beside the pipe. Later I discovered that it is fine putting the screw eye directly into the plastic pipe. Of course, there is more material to screw into if there are two layers of pipe (the anchor pipe and the arch pipe), but it also works well if the arch is put over rebar. I have not put a screw eye into a metal pipe, but I imagine it would work well, also. If anyone has done that, I welcome your comments. Using a bungee cord between the screw eye on one end arch and the cord helps to apply tension to the cord.
There are so many great things about securing the cover this way. Most importantly, it doesn’t come off in the wind. Another advantage is that all those things you put in the path to hold the plastic down are not necessary anymore. And the harvest—it is so easy! You can lift the plastic at any point along the sides to harvest and it is held in place under the cord. You will still use clips, but only on the end arches. The cover can be cut to come a few inches over the end arches and be secured with the clips. A separate piece of plastic sheeting can be cut to fit the ends. In mild weather it can be left off. When it is needed, it can be secured with the same clip that holds the tunnel plastic, holding two pieces at once. There are so many ventilation advantages with a separate end piece. Once the weather gets severe enough for me to put on the end pieces, I will fold the top edge down for ventilation on the warmer winter days.
I got the idea for using a cord over the plastic cover from Eliot Coleman in his book Four Season Harvest. He used wire arches with a loop bent into it to anchor the cord. Arches from plastic or metal pipe with a ridge pole can withstand more severe weather than the wire arches he described. If you have been having trouble with the plastic covers on your row tunnels and haven’t used a cord to secure them, take the time on a mild day to go out to your garden and make the upgrade. You will be happy you did.