How to build a caterpillar tunnel
This will be our second year on the Hurricane Hill property. One of our goals this year was to work on season extension now that some infrastructure has been put into place. We decided that a caterpillar could be an economical solution to our challenges. We farm on leased land and don't want to make expensive investments in the property, especially ones that we can't take with us in the future. We finally experienced nice enough weather to put it up this week even despite the light snow cover.
MATERIALS: 1'' PVC, 20 foot lengths - 14 each
5/8'' Rebar, 30'' sections - 28 each
Large carabiners - 28 each
Small carabiners - 28 each
1'' woven strapping - 50 yards
Lashing straps - 2 each
2'' wood stakes or metal T posts - 2 each
4 mil greenhouse plastic - 24' x 150' - 1 each
#6 (3/16”) poly rope - 1000 ft
Sandbags - 30 each
The cost of our tunnel was just under $500 ($0.62/square foot)
The total length of our tunnel is 100' from tip to tip of the end points. We didn't want the end points to run longer then our 100' beds because it would block our harvest lanes, so our actual planting area is about 80' long by 10' wide. You can make your tunnel as long as you want, just add another hoop every 6'.
The first thing we did was measure out our space and pound in the rebar. The first rebar stakes were pounded in 10' from where you want the tip of the point to be. Then, we continued pounding in stakes every 6' until we got to 10' from were we want the far point to be. We pounded in the rebar until only about 12'' was exposed above ground.
Clip the two carabiners together and slip one over each stake. Make sure they are positioned so the little clip will be on the outside of the tunnel. The style of the larger carabiner fits perfectly around the rebar in the smaller side of the clip.
Once the rebar pieces are pounded in and the carabiners are in place, start to put up the hoops. You definitely need two people to accomplish this step. We suggest receiving the pipe the same day you put it up if the weather is old, PVC can crack and break if it's bent when too cold. Start by slipping the pipe over one side of the rebar while your partner walks steadily towards you, bending it into an arch. Slip the other end over the rebar and push down so it meets the ground.
Next, pound in one stake at each end 10' from the end bows. Pound in the stakes so they are at an angle leaning away from the tunnel. In this picture, we used a metal fencing post which was not strong enough. We would recommend a sturdier T post or a 2'' wooden stake.
The purlin strap is next - we got a 50 yd roll of 1'' woven strapping. Take one of the lashing straps and tie a loop in it. Slip the loop over the end stake and slide it to the bottom. Put the end of the 1'' woven strap through the lashing strap and pull it through so you have some excess to pull tight. Test it to make sure you have the strap inserted in the right direction so when you pull on it from the top, it won't slip.
Working with a partner, toss the strap over then under each bow. Make sure the strap is going the same way around each bow so it runs along the top side of the bows (the picture below shows the wrong way for the second bow and the right way for the first bow-it should go over the bow first before going under. ) As you move down the bows, make sure you are pulling tight to keep the tension and that the straps wrap around the bows right in the center. Walk back through the tunnel before tightening down and adjust the straps left or right and the bows vertically so they are upright and level.
Once you get to the far end of the tunnel, slip the end of the purlin strap through the lashing strap on the far stake and pull tight. Go back to the first stake and tighten the purlin strap down as well.
Walk down both sides of the tunnel and place a sandbag in between every bow and a few extras at each end. Roll out the plastic alongside the bows on the ground making sure there aren't any rocks or sticks that could snag the plastic as you are unrolling.
Working with your partner, hand the plastic up and over the structure. One of us stood on a milk crate inside to help it get over the peak and the other pulled it over and placed sandbags as we went. Be really careful about wind- even a little breeze can throw the whole piece of plastic into the trees!! Better to have at least two helpers, but three would be better. After the plastic is in place, walk back and straighten the plastic, working from one side to the other to take out any wrinkles or folds.
Gather the excess plastic at one end of the tunnel into a point. Wrap a piece of rope around the plastic and tie loosely. Have your partner pull from the loose end (like pulling hair in a pony tail) to make sure the peak is tight and even. Tie the rope tighter to hold it in place, then wrap and tie it around the end stake. We used a few sandbags and a large rock to anchor the peak down.
After the first point is tied down, start on the ropes. Tie the loose end of the roll in a small loop and clip it to the first end bow on one side. With your partner on the opposite side, unroll some excess rope and toss over a loop to the other side. Have your partner clip the loop into the 2nd bow, then tighten the slack on the rope back to the first side. We found it easiest to work from the roll and unroll slack as needed to make loops. This rope will be clipped in to every other bow zig zagging over the top. One side will be clipped into bow 1, 3, 5, 7, 9, 11, 13 and the other side will be 2, 4, 6, 8, 10, 12, 14. Once you get to the end, leave a few extra feet of rope then cut it off the spool, don't tie it to the last clip yet. Starting from the first side you started on, tie the loose end of the rope to the first clip on the opposite side of the bow you started on. Put on the second rope the same way you did the first, tossing ropes over and hooking into every other bow in every remaining empty clip. When you get to the far side leave a few extra feet again and cut it off the spool.
Walk down and remove all the sandbags to the side temporarily. Pull the plastic from the loose end and gather it in the same manner as you did the first point. pull down and away from the structure to make sure it is taught from end to end. Tie it to the stake and pull the plastic tightly to wrap it around. Place a large rock or several sandbags on the point.
Starting from the first side again, work with your parter and remove all the slack from the first rope, working from the side tied on to the first clip all the way to the loose end. Keep the tension taught the whole time as you go with your partner. Once you get to the end, pull the rope tight and and tie it around the last clip. Repeat with the second rope.
Replace the sandbags on the plastic in between each bow and with 3-4 on the angles of the points. Trim any extra plastic from the points and leaving a tail, scrunch it up and tuck it under the point.
To enter/exit the tunnel, the ropes should be able to hold the plastic in place as you scrunch it up from the side.
That's it! It took two of us about 3-3.5 hours including a few short breaks. Our harvest bodies haven't returned yet from winter hibernation so we took it slow! If we had an extra set of hands we could have easily done it in 2 hours.
We plan to seed in here as soon as the soil is dry enough, a week of warm weather is upon us and we hope it will help. We will keep this post updated as to the strength and durability of our caterpillar tunnel, as well as any improvements to the design. We would be happy to share where to purchase any of the supplies we used, just send us an email and we can answer any questions you might have.