Frequently Asked Questions – Using Film Plastic

1. Should I use a single or double layer of plastic for my high tunnel?
2. What type of plastic is used to cover a high tunnel?
3. How do I repair small tears in plastic covering?
4. Is it possible to use 4-year film plastic for longer than 4 years?
5. Why does film plastic sometimes deteriorate where it contacts the high tunnel structure?

1. Should I use a single or double layer of plastic for my high tunnel?
A double layer of plastic will significantly increase your average daily temperature. This may be of particular importance if you plan to grow or harvest vegetables during late fall and winter. An inflation fan will be needed to blow air into the area between the two layers.

2. What type of plastic is used to cover a high tunnel?
Four mil, 4-year, greenhouse-grade polyethylene is suitable plastic for covering a high tunnel.

3. How do I repair small tears in plastic covering?
A special tape is available for repairing ripped polyethylene. It can be purchased in various widths from greenhouse suppliers such as Hummert’s and FarmTek. For most effective patching, delay repairs until the sun in shining so that the plastic around the tear and the tape are warm. Thoroughly clean the area around the hole with glass cleaner. Patch with tape on both the inside and outside of the tear, if possible. Alternatively, a patch may be cut from a section of extra polyethylene. Place the patch over the tear and tape the edges with poly repair tape, followed by taping over the entire patch. These methods result in strong patches that will hold air pressure.

4. Is it possible to use 4-year film plastic for longer than 4 years?
Greenhouse location, ultra-violet light and temperature exposure, greenhouse structure design, film plastic installation practices, single-verses double-layer use, and maintenance of the covering material are all factors that influence the longevity of film plastic. Greenhouse polyethylene producers include enough ultra-violet inhibitors to protect the film plastic from degradation for 4-years of northern exposure. While polyethylene manufacturers all know of instances where growers report that their film plastic outlasted the warranted 4-year life, additional longevity is possible but not guaranteed.

5. Why does film plastic sometimes deteriorate where it contacts the high tunnel structure?
Film plastic is often weakened when in contact with greenhouse structural components, especially PVC framing materials. The background on this starts with the green tint greenhouse films produced from 1960 through the early 1980s, which used a nickel-based ultra-violet (UV) stabilizer that was not affected by free chlorine, fluorinated hydrocarbons, and the volatile plastizers found in PVC. By 1983, long-life films were UV-protected not by the nickel-based UV-stabilizer, but by clear UV-inhibitors called ‘hindered amine light stabilizers’ (HALS). These films had high clarity and did not darken with age. One of the drawbacks with the HALS UV-stabilizers is that as the film is exposed to free chlorine, fluorinated hydrocarbons, selected pesticides, sulfur, and volatile plastizers found in PVC pipe, the stabilizer complex becomes inactivated. When this happens, the life of the greenhouse film is shortened, especially where it contacts the PVC. For this reason, all polyethylene manufacturers do not advocate direct contact of long-life polyethylene with PVC pipe. Sisal baler twine, which is often used for plant support and may be tied to the purlins for this function, can also cause breakdown of polyethylene. The petroleum-based preservative will, over time, degrade the polyethylene.

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