On April 21st, we hosted a work-party to help erect our two caterpillar tunnels. Much like a greenhouse, these structures will provide a sheltered, warmer growing environment for various crops, but there are a few important differences between these two types of structures. Caterpillar tunnels are a) less expensive, b) simpler and faster to construct, and c) easily moved from one location to another in the field.
There is no one way to build these structures. The design for our tunnels was inspired by Ted Blomgren from Windflower Farm in NY. We also sought inspiration from the construction of the caterpillar tunnels at La Ferme Cooperative Tourne-Sol, which are well documented in a number of blog posts by Daniel Brisebois.
After some preliminary ground prep – Bryan had disced and rototilled – we squared the site of each tunnel.
We pounded in 2ft tamarack ground-stakes every 4ft along the two lengths of each tunnel area. The dimensions of one finished tunnel are 12ft by 132ft.
Next, we fitted 20ft PVC pipes over the wooden ground-stakes, creating arches.
Then we inserted Duckbill ground-anchors 4 inches away from the structure at the base of every arch, on both sides of the tunnel. The purpose of the ground anchors is to fasten the ropes that tie down the plastic on the structure. Two different sizes were used: the larger ones secure the ends of the ropes, and the smaller ones act like belt-loops with ropes travelling through them freely. At the ends of each tunnel, where we wanted extra strength, we alternated between larger and smaller ground-anchors. For the majority of the tunnel’s length, however, larger anchors were placed only at every fourth arch.
Then, with the help of our entire work-party (7 of us in total) we simultaneously unrolled the plastic and pulled it over the arches working end-to-end. We were careful to drape the plastic symmetrically, distributing plastic excesses equally.
Next, starting at one end, we bunched and tied the remaining plastic together, and anchored it to a metal T-stake that had been driven into the ground at a 45 degree angle pointing away from the structure.
Then from the other end, we pulled the plastic along the structure as tight as we could, and then repeated the anchoring process at that end.
Then came the ropes. Starting at one end of the tunnel, we tied ropes to the large anchors on each side, and then criss-crossed them across the structure until they reached the next large anchors.
For additional wind-bracing at the tunnel ends, we secured a rope about waist-high on both sides of the third arch from the end, looped it around the remaining two arches, and tied it to the T-post.
Entry into the tunnels from the sides is also working as hoped: by pushing/scrunching up the plastic, the tension between the ropes and arches hold it up. We think the criss-crossing set-up of the ropes, combined with having the anchors at the base of the arches are making this effective.
It’s been five days. The tunnels are still standing, and they’ve had to deal with some pretty windy conditions!
We’d like to thank Carrot Cache for their grant to purchase the materials to put up these caterpillar tunnels so we can extend our growing season! They are amazing supporters of sustainable food production! They also sponsored a great webinar on season-extension structures: http://www.agriwebinar.com/Preview.aspx?id=c108943b-633b-461a-8950-389648ff4740
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