In October we began building the first of 5 caterpillar tunnels on our farm. Our main purpose behind building these tunnels is to grow vegetables on our farm all throughout the winter. However, we also plan on using these tunnels during the rest of the season in order to extend our season for warm weather crops.
We've posted a few pictures of our tunnels on our instagram page and have gotten a lot of questions about various details of the tunnels. Below is a step by step walk through of how we build the tunnels. We're by no means experts, but we are happy to share what has worked for us so far.
Our caterpillar tunnels are 14' wide by 96' long and are designed to withstand high winter winds that we get here in the mountains. So far they've handled 30mph wind. If you live in an area with low winds then you may be able to substitute some of the materials we use with more inexpensive options.
Each of our tunnels cost us around $500. We we able to use our NC tax exemption, a Lowes Pro Account (10% off), found some things on sale and already had a few items laying around on the farm such as t-posts, twine and sandbags. Here's the list of all of the materials for one tunnel:
- 16: 1''x20' schedule 40 PVC from Lowes
- 32: Duckbill Anchors, model 40 from Foresty Supply
- 1: Box of white tomato twine from RainFlo Irrigation
- 64': 1/4'' rebar from Lowes
- 100': 1/2 pvc
- 16: 1/2 mounting brackets for pvc & screws
- 1: sheet of 3mil - 32'x110' overwintering plastic
- 30: Sandbags
- 2: t-posts
The first thing we did was drive our 2' rebar into the ground and set the PVC piping on top of them. We actually bought 10' lengths of rebar and cut them to size. We spaced our hoops 6' apart and 14' wide with about 8'' of rebar above ground. This gives us about a 6' ceiling in the center of the tunnel and covers 3 beds. The width of your tunnel with directly effect the height of it. There's definitely a balance to be struck. We're really happy with our width vs height.
Once your hoops are up then you need to stabilize them. Because we have high winds on our farm we decided to add a PVC purlin to that runs the length of the tunnel. We used 10' pieces of 1/2'' PVC and attached them together with 1/2" couplings (we tried using smaller lengths of scrap pieces we had and I'm not a huge fan of how that turned out). I don't recommend gluing them together - we screwed them together so we can reuse them. The purlin is attached to the main hoops with 1/2'' mounting brackets. You can also use a strong twine wrapped around each hoop and anchored to each end, but we found that this still allowed the hoops to move around too much when the wind was really blowing. If one hoops falls then the whole tunnel has the potential to go down.
It's time to add anchors to the sides and ends of the tunnel. The anchors on the sides will be used to attach twine to in order to strap down the plastic. The anchors on the ends will be what you use to pull the plastic tight in that direction. We purchased duckbill model 40 anchors and put them in at each end of each hoop. These anchors are bulletproof and can are the best option if you are in a windy area.
Some people use wooden stakes in this instance, but they would pull out much too easily in our setting. These duckbill anchors are set in the ground about a foot and provide much needed stability as you will see later on.
Its time for plastic! Make sure all of your anchors are in and your hoops are secure then pull your sheet over the hoops. We were able to easily do this with just two of us on all of our tunnels. Our plastic was a little wider than we needed, so once we had the plastic draped over the hoops we cut the excess off. Once the plastic is draped over and set we attached one end to a t-post and then pulled the plastic tight from the other end and attached that side to a t-post with twine.
Almost done! Now it's time to drape twine over the plastic to strap it down. We did this 'wagon style' by sending the twine diagonally from hoop to hoop and forming a criss-cross pattern. I strongly recommend doing an individual string for each connection. This adds some needed redundancy in case a string breaks at some point and also makes the tunnel easier to adjust later on. Getting this right is one of the most important steps since the twine is mostly what is keeping the plastic on the tunnel.
All that's left is regular maintenance and adjustments to the tunnel. Every week or after every high wind day we'll adjust any hoops that have shifted and check for any broken or loose twine. We've found that the twine stretches over time and periodically needs to have sections replaced or tightened. We also tighten the ends of the tunnel when needed.
Depending on the weather, your tunnel will potentially heat up fairly quick when the sun is shining. This can be resolved fairly easily by rolling up the sides during the day. The twine on the outside of the plastic does a fairly good job of holding up rolled up plastic. We also prop certain areas with Y shaped stakes which work well when we have a little bit of wind blowing but still have the sides up.
Sandbags are another good addition to your tunnel. We load up each end of the tunnel with sandbags and put a sandbag on the plastic between each hoop as well. This gives us some additional security for windy days.
Feel free to contact me with any additional questions.