Building a Caterpillar Tunnel

This is the second instalment in how we build and use tunnels on our farm. The first part presented our first couple tunnel iterations. Today, let’s look at the latest design we’ve tried: a caterpillar tunnel.

We first read about caterpillar tunnels three years ago in the manual High Tunnels: Using low-cost technology to increase yields, improve quality, and extend the season. You can download it for free here. We were immediately attracted to the caterpillar design because it seemed even more affordable to build then our already inexpensive tunnels, and also promised to be easier and more convenient to move. Our only concern was whether the structure would be sturdy enough. This year we decided to build a caterpillar tunnel and  find out whether it was as great as we thought.

We decided to keep the same 14′ width as previous tunnels but build this tunnel the full length of our 300′ field beds – 3 times the length of our previous tunnel design! If this is the first tunnel you build I would caution against building one this big.

Building a caterpillar tunnel is as simple as

• make (or purchase) arches
• install the arches
• put on the plastic

MAKING THE ARCHES

We bent our tunnel arches ourselves inspired by Paul and Sandy Arnold’s directions. The first arches we’d made a couple of years ago were 14′ wide at the base from 21′ long steel pipe. This time we decided to keep the 14′ width but make a taller tunnel and used 24′ pipe. We bent the pipe a round a  13′-diameter half-circle jig on a hay wagon. We left 2.5′ unbent on each end of the 24′ pipe so it can later slide over rebar ground stakes.

One person holds the pipe in place.

Another walks the pipe around the jig.

This is elegantly illustrated in our first ever Going to Seed YouTube video:

We then use a pipe bender to add a couple features to the arches.

A gothic peak so the structure can better shed snow.

And a straight part on each side to slide over the rebar ground stake. (This corresponds to the length that wasn’t bent around the jig.)

We do a couple of test runs with the pipe bender to get the angles right. This is done with one person working the bender and another measuring the width of the arch until it reaches 14′ – this measurement is taken just above the straight part unbent by the jig. We measure the displacement of the hydraulic tube on the bender that corresponds to desired angles. After a few tries, we can quickly replicate the arch shape.

Though precision is important, these arches have a bit of play and close enough is usually good enough.

PUTTING THE ARCHES UP

The arches slip over 24″-30″ pieces of rebar placed in the ground. We aim to have 12″ above ground.

We set up two parallel rows of stakes 14′ apart though it is more important that the rows follow the growing beds than be perfectly straight. Inrow spacing varies in function of the season:

• 4′ spacing for a snow bearing tunnel
• 8′ spacing for summer use (some folks supposedly use 10′ spacing!)

The arch slides over the stake.

We then set up a rope as a ridge, wrapping it around the peak of each arch. On both ends of the tunnel, the rope is anchored to a t-stake sunk in the ground.

PUTTING ON THE PLASTIC

On a caterpillar tunnel, the plastic is held in place by 1/4″ rope.

The diagonal stakes holding the ropes are 33″ long (12″ above ground). They are placed between arches. Prior to placing the plastic, we tied the ropes to the stakes on one side of the tunnel.

We unrolled the plastic beside the tunnel.

Pulled it over the arches.

And anchored it to the T-stakes at each end.

Then, we tossed the ropes over and tied them to the opposite stake and voila!!!

Ideally, the plastic on a caterpillar tunnel can be pushed up and held in place with the tension between the ropes and arches.  None of the caterpillar tunnel owners I’ve spoken to is quite satisfied with this solution. Each grower has different strategies from clips to y-shaped branches on the ground. Emily and Reid thought up a solution for Tourne-Sol farm …

A hook made from a bent piece of wire (the same wire we use as arches for row cover) attached to the arch with a hose clamp. The hook can be rotated into the tunnel when not in use.

And with the tunnel in place, we’d revolutionised tunnel technology on our farm!!!

Or so we thought …

LATER THAT WEEK

What can our heroes do to overcome the wild forces of nature that wreak havoc upon their simple livelihoods???

Find out next on Going to Seed …

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