People have come up with many tricks to coax plants to grow. One of the most important tricks was the creation of the greenhouse. Greenhouses use glass to trap heat and let light in, creating a mini climate that is warmer than the outside. Today, greenhouses have sophisticated systems of vents and heaters to control temperature and the movement of air.
Most greenhouses are out of reach to the normal person. Glass greenhouses are quite expensive and difficult to move after installation. The wrong site placement can make a greenhouse worse than useless.
Fortunately, there is an easy way to make a greenhouse structure that can provide all of the benefits of a standard greenhouse at a fraction of the price. Plus, they can be made to fit large spaces, rows, or even an individual garden bed. These greenhouses have a number of different names. In the US, they’re normally called hoop houses or hoop greenhouses. In the UK, they’re called polytunnels, and that’s what we’ll call them in this article.
What is a polytunnel?
Polytunnels are tunnels made out of a plastic called polyethylene. Polyethylene works similarly to glass in a standard greenhouse. It lets an adequate amount of light through to nourish plants and it also retains heat in the walls of the structure. This heat warms the air and soil surrounding the plant.
Polytunnels are a proven technology. They are used in many specialty growing operations where plants are particularly temperature sensitive. Floriculturalists and nurseries are two big users of polytunnels due to their low cost. In the UK, polytunnels are used heavily in the berry industry.
They do more than just temperature control. By using doors and other equipment, a gardener can control humidity, airflow over the plants, and protect plants from inclement weather conditions. They can also prevent birds from attacking seedlings. Some are even big enough to allow harvesting machinery to operate inside them.
If you’ve been keeping up with our blog, you may remember our articles on extending the season. A polytunnel is an excellent upgrade to the technologies in that article, and they are not that difficult to build with adequate instruction and some carpentry experience. While there is some initial outlay in cost, it is far less expensive than a standard greenhouse.
How to build a polytunnel
The easiest way to build a polytunnel is to buy a kit. There are several manufacturers out there. Some of the best are, not surprisingly, in the UK, where polytunnels are extremely popular due to the colder weather of that climate. One of the largest vendors is First Tunnels. I highly recommend going through their site at http://www.polytunnels.co.uk/. Even if you’re in the US and can’t order one from them, they have a very extensive video library on the uses of polytunnels and construction videos that could give you inspiration when you’re building your own.
In the US, you can take a look at manufacturers such as Poly-Tex in Minnesota (in particular their Kool House cold frame high tunnel [http://www.poly-tex.com/products/kool-house-cold-frame-high-tunnel-greenhouse.html]) or look at the EZ-Build & Gro Cold Frames [https://www.farmtek.com/farm/supplies/prod1;ft_high_tunnels_cold_frames;pg105158f.html] offered by FarmTek, starting at $235 complete with a 6 mil 4 year greenhouse film.
We’re going to assume that you’re not going to build one from a kit, but rather build it yourself. In this article, you’ll be learning how to build a 4’ wide, 8’ long, and 2’-3’ high tunnel. While this won’t be tall enough to walk in, you’ll learn all the skills necessary to build a larger one. Also, it’s much cheaper to build a smaller house than a larger one. The instructions are from the book “Greenhouse Plans: How to Build a Portable, PVC Hoop House” by Taylor Birch.
What you’ll need to buy for a 4’ x 8’ x 2’ PVC polytunnel:
- Three 8 ft. 2x12s
- Six pieces of 3/4” PVC piping
- Ten 1” EMT conduit straps, two hole, metal
- One pack of 8” cable ties
- One roll of 6 mil UV resistant polyethylene sheeting that is 8’ on one end (available at most DIY stores)
- One pound of #10 3.5” exterior screws
- One pound of #10 1.5” exterior screws
- Three 1x2x8 furring strips
- One extra piece of 3/4” PVC piping for clamps
- Drill with a #10 bit and a 1/8” bit
- Carpenter’s Level
Selecting a site
Your polytunnel will need to be put on a site that gets plenty of sun and isn’t too sloped. Ideally, you’ll want the tunnel in an east-west configuration to get maximum sun exposure. Don’t place it under trees, and remember that trees grow over time too.
Also make sure that there is enough room around your site location that you can move equipment to and from the polytunnel. With this smaller one that’s not as big of a concern, but if you build a large permanent one then it can become a real problem. Use rocks or other objects to lay out a 4’x8’ rectangle around where you want to plant to check the size.
Cutting the pieces
Cut one of the 2x12s in half so you have four pieces for the wood foundation. Next, the PVC pipe needs to be cut. Part of the structure of the hoops comes from inserting the ends of the bent pipe into the soil. It needs to be able to go down a minimum of 3.5” for stability.
Start by cutting one of the PVC pipes to 104”. That will leave plenty of extra pipe. Bend the pipe into a four foot wide hoop and plant the ends into the ground to get an estimate of the height. With that much pipe, you can get anywhere from a 2’ to a 3’ height. Cut off pieces until you can get at least 3.5” into the ground and still have the height you want. Then cut four of the other 10’ PVC pipes to fit. One will be left over for the spine. Save the cut parts.
Assembling the frame
The best way to maximize square footage inside the foundation is to sandwich the shorter end pieces between the longer side pieces. Align the pieces and then drill at least three pilot holes from the outside board into the end pieces with the 1/8” bit in each corner. Take off the long side boards. Screw the 3.5” screws into the 8 ft boards until they show ¼” through the other side. Line up the points with the pilot holes you drilled into the end boards and drive them in.
Next, the frame must be leveled. Lay your frame down on the ground and see how level it is. If there is a high spot, lift the frame up and rest it on something before digging it away. Put the dirt on any low spot. Repeat until the frame is level. Don’t worry too much about being exact. After levelling, use a soft mallet to square up the entire frame.
Some people prefer to sink the wood foundation 3” into the ground for extra stability, but this can be a problem when it is time to move the polytunnel. If you choose to do this, sink it first and then square.
Attaching conduit straps
The ends of the hoops will go through the conduit straps and into the ground for extra stability. Measure 3.5” from the outside of the foundation into the frame along the inside of the long board. Make a mark. That’s where the center of the first strap will go. There are a total of five hoops total, so measure 22 1/4” from one center to the next down the length of the board to divide the space up evenly between the hoops. The center of the last strap should end up being around 3.5” from the outside of the other end. Repeat on the other side, then use the 1.5” exterior screws to secure the straps.
Inserting hoops and spine
Now it’s time to place your hoops. Put the end of one hoop through the conduit strap and into the ground to the depth you like. Bend the hoop over and put the opposite end into the other strap. Make sure the height is okay, then place the other hoops in. The idea is to make the tops of the hoops as level as possible.
Once the hoops are level, loosely attach a zip tie around the top of each of the hoops with the tag end pointing down. This is important to prevent tearing the cover later. Insert the spine through the loose ties. This is easier said than done, so you may want a helper. Once the spine is inside of the ties, tighten them up and clip off the excess. Put another tie around the hoop and spine in the opposite way so the ties make an X, and tighten the same way. Remember to watch for the tags! Once the spine is attached, saw off the excess pipe from the ends.
To attach the sheeting to the frame, you will need to make clips. That’s what all the extra PVC is for. Make a variety of lengths from 3”-6” of the extra pipe, then cut out a 3/8” gap down the side of the pipe. This doesn’t need to be too exact. The wider the gap the easier it will be to put the clamp on. This is best done by clamping the piece in a vise and using a hack saw.
The cover and door flaps
The cover is made up of one main body cover and two door flaps. To make sure we have enough excess material for attaching the cover to the frame, cut one piece of polyethylene 8’x12’ and two pieces 5’x5’. The best way to cut the material is to use an open pair of scissors and to go slowly to keep the line straight.
After the plastic sheeting is cut, drape the body panel over the frame and put an equal amount of sheeting on either side. There will be extra. Now put a long clamp in each of the corners. Due to the offset of the hoop from the edge of the frame there should be plenty of plastic to clamp. Pull the PVC clamp apart and snap it around the hoop. Be careful of your fingers! An optional, but helpful, step is to take two pieces of garden hose and split them to help spread the clamps without pinching your fingers when putting them on. Snap the clamp around then pull the hose pieces out.
Next we’ll attach one of the long ends of the plastic permanently to the frame. Wrap the end of the plastic cover at least twice around the furring strip. Make the wraps straight and tight. Duct tape can help. Once wrapped, screw the strip to the base frame every 12”-18”. The other side can be weighed down with blocks of wood or bricks so that the cover can be pulled aside for watering, weeding, or emergency cooling.
For the door flaps, cut the second furring strip in half and attach the bottom of the door panels in the same way to the base on both sides. Then lift the flaps up and clamp them and the body panel to the end hoops. Use larger clamps on the straight sections and shorter ones on the curved part.
When you’re done, your polytunnel is ready for use. Here are some things to watch for:
- You may need to ventilate on very hot days, but don’t forget to close it back up again at night.
- If small holes develop, cover them with clear packing tape.
- Most polytunnels last about 3-5 years before the film needs replacing.
If you would like more information about how to operate a PVC polytunnel, I highly recommend checking out the First Tunnels website for another way of constructing more permanent polytunnels with extra features like rainwater collection.
This is a great weekend project for anyone that needs a greenhouse and has access to a decent DIY store. If you’d prefer a larger professionally-made polytunnel there are plenty of companies out there that sell kits. Try building a polytunnel when it starts to warm up and let us know how it worked out for you this season. You’ll be growing all year before you know it.