A small polytunnel can be quite useful throughout the year for many different purposes. It can extend your growing season and allow you to grow more tender crops than in the open ground. In the early spring, it can be used for germinating and growing bedding and vegetable plants earlier than would otherwise be possible. In the summer it can be used for growing more tender vegetables such as Tomatoes and Peppers or for root cuttings. It can be used in the autumn to start winter vegetable plants. During the winter, it can protect patio plants and other hardy but containerised woody plants.
Commercial polytunnels can be expensive to buy but a home-made polytunnel can be built easily and inexpensively in several hours using simple hand tools and materials available at most building supply or D.I.Y stores.
These instructions and plans for building your own polytunnel can easily be adapted to suit your owns needs as to size. Measuring 12 feet by 14 feet (168 square feet), it is small enough to be unobtrusive but large enough to meets the needs of the serious grower. The completed structure is shown above.
Choose a level, well-drained plot for the polytunnel. If it will be used primarily for plant propagation in the summer, place it in partial shade to minimise heat build-up. If a partially shaded site is not available, you can use shade cloth or a white plastic cover to control the amount of sunlight reaching the interior.
If the polytunnel will be used for starting transplants or growing plants to maturity, it will need maximum exposure to the sun. It should also be located where air drainage is good; avoid low areas surrounded by woods or buildings.
Constructing the Polytunnel
16 3/4" PVC pipe, 10 feet long
6 3/4" PVC crosses
(Please Note: Since I wrote this article in 1998, I have since found that the PVC cross sections are no longer in production. I therefore suggest 20mm electrical cross inspection boxes.)
2 3/4" PVC tees
32 3/4" Plastic or copper clips
2 2"x6"x14' treated pine boards
2 2"x6"x12' treated pine boards
4 2"x4"x7' treated pine boards
4 2"x6"x6' treated pine boards
4 4"x4"x2' treated pine boards
2 2"x4"x3' treated pine boards
2 1"x4"x12' treated pine boards (to be cut up for door parts)
1 Set of door hinges
1 Sheet of plastic, 24'x20'
1 Can of PVC cleaner
1 Can of PVC cement
1 1/2"x10' galvanised electrical metallic tubing
Plus Miscellaneous nails, screws, and staples
The polytunnel consists of a wooden frame to which are attached hoops of PVC water pipe. The hoops support the clear plastic film covering.
All timber for construction should be treated with preservative. This can either be purchased as ready treated (ask for Tanalised Timber) or treated separately. Avoid wood preserved with creosote as this can be detrimental to the plants grown in the polytunnel. Tanalised timber is pressure treated with the preservative and therefore will last much longer than untreated or lightly treated wood. Except for the covering, the structure should last 10 to 12 years.
Unless the site is very flat, you will need to level the ground prior to construction. The purpose of the 4-inch-by-4-inch post at each corner is to anchor the polytunnel and prevent it from moving in high winds. It also provides a convenient way of jointing the boards together.
The PVC pipe hoops are attached to the inside of the foundation boards with clips. These are sold to secure plastic and copper water pipe in place and are usually copper or plastic. The plastic ones are probably better as they will not rot but the copper ones are probably stronger but will have to be replaced every few years. It is best to attach the clamps loosely to the side boards with wood screws before you secure the side boards to the end boards and corner blocks. They may be tightened after the PVC pipe has been slipped into place.
The 12-foot width of the polytunnel was chosen so that each half of the rib section of pipe is 10 feet long. PVC pipe may be easily cut with a hand saw, although a neater cut can be made with a pipe-cutting tool specially designed for that purpose. The joints at the cross sections are cemented using a special adhesive that is sold specifically for this purpose and can be bought from the supplier of the pipe. Care must be taken during assembly because the cement used to fasten PVC is fast-acting and permanent and care should be taken when using the adhesive. Always follow the directions on the tin.
It is best to lay all parts out on a flat surface for assembly. For the ribs to be 2 feet apart, the midrib pipe must be cut into pieces about 22 1/2 inches long. After all joints have been cemented and allowed to harden for a few minutes, position the entire assembly over the foundation boards, bend the ribs, and secure them in place with the clips.
Assembly is best done by two or more people to prevent over stressing the cemented joints. Ten feet of galvanised metal tube positioned inside the midrib will give added strength to the structure.
After all the PVC ribs have been secured, the end frames and door may be constructed. Although there is a door on only one end, both frames are essentially the same. The two diagonals on each end give strength to the structure and make it easy to secure the cover. The top of the end frame may be fastened to the end ribs with either a wood screw or a small metal strip bent into a U shape. If wood screws are used, the PVC pipe must be pre-drilled to prevent splitting. (2005 Note: It has been pointed out to me by a couple of people making this polytunnel that they have required additional diagonal supports on the ends of the polytunnel. This is to stop the end pieces of tubing from distorting when the polythene is tightened. They have fitted the additional supports from the base of the door to the center of the end hoops on each side. Personally, I did not find this a problem so perhaps they were over tightening the polythene!)
Select the type of cover material according the intended use of the polytunnel. Clear plastic polytunnel film is generally best if the tunnel will be used for growing plants. If it will be used exclusively for propagating or overwintering plants, consider using milky or white copolymer film. White copolymer film reduces the amount of heat and light within the house and therefore limits the fluctuations in these factors, keeping conditions more nearly constant during propagation or over the winter. However, shading clear plastic with a 30 to 50 percent polypropylene shade netting or polytunnel whitewash available from horticultural supply companies will produce nearly the same effect.
The cover may be attached with small tacks, but staples are much faster and are more secure. Allow the cover to extend several inches past the bottom of the foundation boards and cover the extra material with packed soil to prevent the entrance of rainwater.
It is essential that the house be as tight as possible so that it retain both heat and humidity. Problems with overheating can usually be solved by simply opening the door. If the polytunnel is placed in a site where electricity is made available a fan can help in ventilation. Supplemental heat can be provided if needed during very cold weather or at night by one or more heat lamps or a small electric heater. Any electrical devices used in the damp environment of a polytunnel should be connected only to a circuit equipped with an approved circuit breaker. Other source of heat can be used, but even very small gas or oil heaters will usually overheat a polytunnel this small and may not burn well if not properly ventilated.
The polytunnel will satisfactorily support up to 4 inches of dry snow. If the snow is deeper or wet, one or more temporary supports should be provided along the midrib to prevent the structure from collapsing. If a larger polytunnel is required, the length may be increased. Do not increase the width of the house without increasing the size of the ribs. Doing so will seriously reduce the ability of the house to withstand snow and wind loads.
3 April, 2005
© copyright 1999, P. A. Owen