High Tunnel Construction

2010

After reading Elliot Coleman’s book Four Season Harvest I wanted a garden that I could harvest out of every week of the year in my zone 5 climate. To grow enough for my family I needed a high tunnel in my garden. The three pictures above are of my high tunnel and were all taken the same week. I have enjoyed salad greens all winter.

I wanted a high tunnel in my garden but before building one I spent some time thinking about my goals for it. I studied plans for several years but was never comfortable with how they compared to my design criteria. I kept searching and planning until it all came together.

My Design Criteria
I garden in 30 inch beds with a 12 inch walk way between each bed. I want my my high tunnel to cover at least 4 beds. I want the outside edge to be 6 inches past the outside bed which made the tunnel at least 14 feet wide. A fifth bed was desirable so that any interior supports would be in the middle bed instead of down the middle walkway of the tunnel. This made the tunnel requirement at least 17 feet 6 inches wide.
We have a wonderful north facing view from the back side of our home. It includes our garden, a horse pasture and Mount Timpanogos. The dining room, kitchen and my wife’s sewing room windows all face onto this view. It is spectacular. The tunnel must not block the view yet be tall enough to work in.
I want to use poly lock channel and wiggle wire to attach the plastic cover to my high tunnel.
In our high desert climate we get alternating cold and warm periods in the spring and fall. I need to be able to easily ventilate the tunnel when it is warm and seal it up when it is cold. I don’t want to rely on fans to ventilate the tunnel.
I want galvanized steel rather than PVC plastic pipe for the frame. PVC pipe feels like false economy to me. Attaching ends to a steel tunnel is much easier and poly lock channel can not be attached to PVC pipe effectively.
The tunnel must stand up to heavy snow loads. I leave for work early in the morning and during the winter I often don’t get home until after dark. I want my tunnel to be able to handle a lot of wet heavy snow when I am not able to address removing it in a timely fashion.
I want a tunnel that is inexpensive, fast and easy to build.

I studied high tunnel designs all over the internet. When I ran across the Tod Hanley modified caterpillar hoop house design on the Kerr Center for Sustainable Agriculture web site, the design I wanted came into focus. I would use the galvanized steel tube frames and wiggle wire attachment from the Tod Hanley design, the end closure from the Utah State University high tunnel and center supports based on the greenhouse described at The Modern Homestead web site. I would add a center ridge purlin and space my hoops four feet apart instead of six. I ordered a tube bender from Tod Hanley and started accumulating the things I needed to build my high tunnel.

Metal Mart in Lehi Utah stocks galvanized one inch 24 foot long square steel tubing. A friend has a trailer long enough to haul it so I avoided a delivery charge. I ordered greenhouse plastic cut to the length I needed and wiggle wire and channel from FarmTek. FarmTek only stocks 8 foot length of wiggle wire and channel so I needed three lengths of channel for each end. I needed one row of wiggle wire to attach the top plastic and another row to attach the end plastic so I ordered 12 lengths of wiggle wire. In the spring I will add solar operated vents so I ordered the vent openers from FarmTek as well. All the other materials I needed were purchased from my local building supply store.

Construction
Please download and read the Tod Hanley and Utah State plans referenced above. I am not going to provide the details that are given in those plans, I am only going to list the steps I followed and describe the differences in my design.

The first thing to be aware of when you start working with metal tubing is where the weld seam is. You want to avoid drilling through the weld seam so pay attention to where it is. The picture to the right shows the weld seam.

Measure your tubes to see how long they are so you get an accurate middle measurement. Mine were more than an inch longer than 24 feet. The tubes were cut with an abrasive cut off saw so there is a rough edge that should be dressed with a hand grinder. Mark the tubes in the middle (this may not be 12 feet exactly) and two feet from each end. Make a drill start with a center punch at the middle of the tube. I used a 1/4 inch bolt with a Phillips head to attach the center purlin so I drilled a hole slightly larger than the bolt.

With all nine tubes marked and drilled I bent the hoops. You need some high quality help to hold the hoop while you are bending and I had the best. It is easiest to bend from one end to the center and then start from the other end and bend to the center. Start bending at the 2 foot mark leaving the first two feet straight.

Drill attachment holes 1 inch from the ends the poly lock channel and every two feet along its length. Attach this channel to two of the hoops. The channel is 1 1/4 inch wide. Attach it so that it is flush with one tube edge using self drilling screws. The flush edge will be the outside edge of the tunnel. Be sure not to put in the bottom screw on each hoop until the hoop is placed over the rebar.

Lay out the base of your high tunnel by driving 2 foot lengths of 1/2 inch rebar into the ground at a 15 degree angle pointing in. I placed my hoops every 4 feet rather than the 6 foot spacing in the Tod Hanley design.

Make the ropes that will hold the plastic in place over the hoops and attach them to the rebar posts. Place a large washer on the rebar to protect the rope from the bottom of the hoop. Ropes holding on the plastic rather than attaching the plastic to side baseboards gives this type of tunnel it’s name of caterpillar tunnel.

Place a redwood 2x4 over the top of the end pair of rebar posts. This will be the base of your tunnel end. Finding 18 - 20 foot long redwood 2x4’s is difficult so it may be necessary to splice two boards together. Be sure to arrange the board lengths so that the splice is not in the door.

Place your hoops over the rebar posts and install the bottom channel attachment screws in the end hoops.

Make your center purlin by drilling holes every 4 feet along a length of tubing. Make the center of first end hole a little less than 1/2 inch from the end so that the end of the tube does not extend beyond the hoop and rub on the plastic. Attach the purlin using 3 inch bolts on the ends and on the even numbered hoops. Use 4 inch bolts on the odd numbered hoops. The extra bolt length will provide an attachment point for the vertical supports during the winter. My tunnel is 32 feet so I needed to splice in an additional length of tube for my center purlin. Measure the length of tube required and cut to length, drill the end hole and measure every 4 feet for the additional holes. I spliced my purlin tubes using 1 inch aluminum angle attached with the same self drilling tech screws I used to attach the the channel to the end hoops.

To build the end walls lay out the door frame and two additional uprights on the end bottom board. Drive a steel T post along the bottom board at each of these 4 locations. Place a board at the bottom mark and mark where the hoop intersects the board. Use a chop box to cut the board at the angle the hoop intersects it. Attach the board at the bottom with an angle bracket and at the top with a metal strap that attaches to both the hoop and the board. Run a metal strap around the T post and the vertical board. Be sure to attach both ends of the door frame strap on the outside of the door frame.

Build the doors to fit inside the door frame and vents in the top of the door. Attach the doors to the end wall.

Before attaching the plastic check where each section of poly lock channel butts up against its adjoining one. If there is a rough edge file it down smooth so it does not rub a hole in the plastic. Check your purlin screw tops to make sure you did not make a rough spot when tightening the nut.

Attaching the plastic is not a single person job. Roll out the plastic along side the high tunnel. If you purchased a plastic that reflects infrared back into the tunnel make sure that side is facing inside the tunnel. It will be marked with printing on the plastic that says “This side towards the soil”. Work the plastic over the hoops and square it. Attach one end by working the wiggle wire into the channel. After the roof plastic is attached trim the ends. Roll up the excess plastic on the sides and throw the ropes over the top of the tunnel at each hoop and tie them off to complete the installation of the roof plastic. Then lay out the end plastic, pull it tight, attach it in the same channel with another wiggle wire. Trim the end plastic after installing. Install lath to hold the plastic to the end wall framing and cut the plastic along the outside and top of the door. There is no need to cut along the hinge side of the door.

The tunnel is now completed. Notice the inner low tunnel made of floating row cover.

In the spring of 2011 I wanted to add a cable that I could hang tomato roll hangers to grow single stem indeterminate tomatoes. I changed the end bolts out to 3 1/2 inch bolts and then added two links of chain to each purline bolt. I used these pieces of chain as cable supports to hold a 1/4 cable. I connected one end to a turn buckle so that I could tighten the cable. This worked very well and I would do it again.

Lessons Learned:
The perline bolts should not be phillips head bolts. A phillips head develops a point which wears a hole through the plastic.
It is very difficult to grow cool season and warm season crops in the same tunnel. In the spring the cool season crops need to have a cool vented tunnel and the warm season crops need a warmer unvented one. In the late summer it is too warm in the tunnel to get the cool season crops to germinate assuming the garden plan opened up space to plant.
Harvesting 1 to 1.5 feet of a 30 inch wide bed of spinach will fill a salad spinner. The spinach will be ready to harvest again in 30 days most of the winter. From mid December to mid January growth occurs but it is a little slower. This is a good planning guide to help determine how much spinach to plant.

High Tunnel Construction

Stats

Size 18 X 32 Feet

location Lindon Utah Zone 5a

Cover 6 mil Greenhouse Plastic

Cost About $600

Tube Weld Seam

Drill the Center Purlin Hole

Start Bending at the 2 Foot Mark

Bend from one direction then the other

Drill holes in the poly lock channel

Use Tech Screws to attach the channel

Hoops on the rebar purlin going on

Splice on Center Purlin

Attaching Poly with Wiggle Wire

End Wall Construction Detail

Tunnel Completed in Time for Winter

Completed End Wall