By the Sweat of My Brow...: High Tunnel Greenhouse

We have launched into learning first hand about greenhouses. We have a lot to learn, but we are interested in experiencing an extra month+ on each side of our growing season, and doing more with some intensive gardening within the greenhouse.

Our philosophy at this time is to not add additional heat to a greenhouse, but rather to buffer the lows and shelter the plants from desiccation. We won't be trying to grow tomatoes and peppers (warm loving plants) in the winter, rather we will focus on cold hardy plants with the goal of keeping them going all year long.

Our greenhouse dimensions are 16 x 48 feet and is called a high tunnel because you can walk inside of it. In this post, I will outline the placement considerations, assembly steps and tips we learn along the way. You can DIY (do it yourself) where you can purchase fence poles and do your own rib bending and collect the needed hardware. We decided to start with a kit containing all of the poles, hardware and greenhouse grade plastic included.

Location Considerations

Your placement of the greenhouse is important. You want your site to be level, have good sun exposure, and be conveniently accessible to you, power and water.

We cast about in our minds various locations for the greenhouse, and have come to decide on our oldest garden plot, which is the most level ground we have in garden, and is also closest to our pole barn, power and water supply. There are some tall trees to the west of this location, but perhaps we will trim these down, or they may help shade the greenhouse in the summer when it will likely be too hot anyway.

We have brought in a load of well composted horse manure to amend to the soil and help level it out a bit more. It looks good as a plot and location to start a greenhouse.

Parts

The general parts are straightforward:

Anchor posts of 3 feet long are driven into the ground ~ 2 feet, with a 15" out of the ground. The anchor posts are driven along the perimeter of the greenhouse every 4 feet along the sides.
Hoop poles are set into the anchor posts, held with a connector tube.
Ridge pole attaches with hardware along the top ridge of the greenhouse
Plastic sheeting holder hardware is attached to the frame
End walls are constructed using 2x4s
Plastic sheeting is applied and fastened

Assembly

1) Anchors: After selecting the location for the greenhouse, you need to layout the specific corner posts, and make sure everything is square. We wanted to square the greenhouse with the road in front and the pole barn to the side, so we took some measurements, and set the front two corner posts 16 feet apart that face the road. From these we measured out the back corners, measuring 48 feet along the sides, and 50'7" on the diagonal.

A little geometry review: We want the structure to be square. This means that all of the corners need to be right angles. The Pythagorean formula for a "right angled triangle" is a² + b² = c². This means that for a structure 16 x 48, the hypotenuse would measure 50'7". You measure both sides and both diagonals and make adjustments till every measurement is right. Then you set the back corner posts.

We measured the corner posts from soil line to the top of the post 15". We will use 2x12" side boards along the perimeter and 15" will leave some room above the side board for the attachment of the connector and hoop pole to the anchor post. With the four corners at the proper depth into the ground, we strung a flexible tape measure along the long sides and pounded in the side anchors every 4' and to the depth of the tape line. There is a pounding plug shown in the image which allows you to hammer the anchor in without destroying the mating surface of the pole. Once at the proper depth, you can re-orient the holes in the anchor pipe with a screw driver as shown in the image. We had a small level to help make sure we were placing the anchors straight into the ground, a feat generally accomplished.

2) Hoop poles are mated with the anchor posts. This is a fun step, as the greenhouse quickly takes "shape." There is a connector tube of about 5" long which is just large enough in diameter to slide over and onto the anchor post. There is a matching hole for a bolt to tie the connector to the anchor post. The hoop pole, being the same diameter pipe as the anchor post, also slides down into the connector tube. There is a matching hole for a bolt to tie the hoop pole onto the connector.

3) Ridge pole. The greenhouse kit that we purchased has a ridge pole of tubing that attaches a longitudinal pole with each hoop at the center. This ties the entire structure together as one unit. Larger greenhouses may have several longitudinal poles. The connection hardware is made of two brackets with a pipe holder form. Two brackets are rotated 90 degrees to accept both the ridge pole and the hoop pole. There are 4 bolts to hold the brackets together, and the nuts should be placed downward to not puncture the plastic.

4) Corner bracing. On each corner, there is a specialized pole that attaches to the second connector and the first hoop. These have a twist in them and there are right and left bracing poles (the twist is to the right or to the left.)

5) Construct the end walls. The end walls are secured to the hoops with clamps. These clamps need to be installed before the plastic holding strip is applied to the end hoops.

6) Plastic-holding strips. These are attached along the side boards, and along the hoop pole on both ends. This will tie the plastic tight to the front and back hoop along the curve, and along the base sides. The strips are metal, and are attached with 1" self tapping screws.

7) Attach the plastic. The plastic is drawn over the hoop structure as a double layer. When it is secured, there will be two layers of plastic to contend with. This allows air inflation to add some air space between the two layers as an insulating layer. You want to attach and tension the plastic at a temperature of 65 degrees. If you attach it too cold, it would loosen as it warms up. If you attach the plastic when it is hot outside, then when it gets cold, the plastic may be too tight. When the tension is achieved, Z wire is attached to hold the plastic into place.

Remember to attach the fan motor before completely securing both layers of plastic.

8) Insulate the perimeter. We applied tar paper to the outside of the 2x12, and then piled wood chips against the outside perimeter. We plan to put building styrofoam on the inside of the 2x12 to add just a bit more insulation. These will fit nicely between the anchor posts.

9) Mark out the beds. We have a 16 foot interior, and considered several arrangements for the beds and isles. We concluded with 24" beds on each outside wall, 3 walkways running the length of the greenhouse at 18" wide, and the center beds being 42". This gives us 4 distinct beds, and rather easy access to any bed, even for Sunshine and the boys to plant and harvest from the edges. We will put a work table at the back north of the greenhouse, and will make that area our start house as well.

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