Polyethylene plastic has many properties that make it useful as a covering for greenhouses. Its low cost, large sheet size, ease of attachment and good light transmission are properties that have helped to expand its use so that today it is the most common glazing.
Since the early 1960's when polyethylene film was first used to cover wood frame greenhouses, many improvements have been made. Early films lacked durability and had to be replaced annually. They didn't stand up to the abrasion from the structure and the weather. They also had a short life due to deterioration from the ultra violet rays of the sun.
Most polyethylene film is manufactured as a coextrusion of three layers with different polymers and additives. Each of them contributes to the quality of the film and enhances its performance. The following summarizes some of the characteristics that you need for your crops.
Life – the life of polyethylene films is limited due to degradation processes induced by sunlight and heat. Co-poly is a low-cost material that is good for one season. It is a good choice for seasonal greenhouses, overwintering structures and high tunnels. Avoid construction grade material that has less strength. Greenhouse grade poly is warranted for 4 years or more and costs about double that of co-poly. It contains an ultra-violet (UV) stabilizer that reduces degradation. If additional strength is needed, such as windy in locations, a woven poly or nylon scrim-reinforced material should be considered.
Thickness – one-year co-poly film is available in 3, 4 and 6 mil thickness. Three or four mil film is common for one year use on narrow tunnels and overwintering houses. Greenhouse grade material, only available in 6 mil thickness, is best for multi-year application.
Condensate control (AC) – also referred to as anti-drip is a wetting agent that reduces surface tension allowing condensation to flow rather than form droplets. This can be sprayed on the film or incorporated in the center layer and usually lasts a couple of years. Condensation droplets reduce light transmission and can lead to disease problems when they drip onto plants. An anti-fogging additive may be included to prevent early morning and late afternoon fog formation in the greenhouse.
Reduced nighttime heat loss (IR) – this is an additive that traps the inside radiant heat from escaping. In heated greenhouses, the savings have been measured to total from 10 – 20% depending on whether the sky is cloudy or clear. In double layer poly installations, the IR film is always placed as the inner layer to retain nighttime heat. Research has shown that IR film can increase color and/or compactness and accelerated crop development. This is most likely due to increased nighttime plant tissue temperature. Costing only a couple of cents more per square foot, the payback is only a few weeks for a greenhouse heated all winter.
Reduced daytime heat gain – in areas with strong sunlight, blocking part of the infrared spectrum can lower inside temperature up to 10ºF. Selective pigments can be added to the outside layer in copolymer film to reflect or absorb the near infrared radiation which is useless for plant growth. Research has shown that the higher the outside temperature, the larger the temperature difference achieved by use of these films. The advantages include lower cooling costs, greater worker comfort, lower irrigation needs, reduced plant stress and improved fruit taste.
Ultra-violet (UV) – bees need UV to navigate. If you are using bees to pollinate plants in the greenhouse, purchasing a film that allows some of the UV part of the light energy spectrum to pass through may be important. Otherwise, UV blocking film will reduce whiteflies, thrips, aphids and other insects. It can also control some fungal diseases.
Controlled diffusion – light diffusion is another property that has recently been added by manufacturers. This increases the amount of diffused light that reaches the plants, reducing scorching and increasing light to lower leaves. It is especially important with tall crops such as tomatoes, cucumbers and peppers. Research has shown that diffused light also reduces fungus spore development and insect propagation.
Light transmission – photosynthetically active radiation (PAR) light transmission varies with the type of additive in the film. Typical values are UV stabilized film – 88 - 91%, IR-AC film – 82 - 87%, IR-AC with diffusion – 77 - 88%. Dust, smog and plastic deterioration can also reduce light transmission. A "rule of thumb" is one percent increase in light equals one percent increase in plant growth during the winter or in cloudy weather. Some growers replace the plastic every year just to get a few percent higher light levels when growing plants during the short days of winter. Some manufacturers make a film with anti-static properties that repels dust, dirt and smog.
Photoselective films – these absorb or reflect specific wavelengths of light. They can enhance plant growth, suppress insects and diseases and affect flower development. Red films such as Dupont IR and Smartlite Red film reduce PAR light and create a shading effect. They have also been shown to improve rose yield and quality.
Single or double layer poly –if you are growing during the heating season, an inflated double layer is desirable. It reduces heat loss at night by about 40%. It also reduces the stress at the attachments and the rippling of the plastic on a windy day. Air inflation at ¼" water static pressure is best. A slightly higher pressure should be used in windy or snowy weather. Connecting the blower to use outside air will reduce condensation between the two layers. Single layer is common on high tunnels and nursery overwintering houses.
Plastic failure – early failure of poly can be attributed to attachment stress, abrasion on rough surfaces and sharp edges or heat build up in the area of rafters, purlins and extrusions. Contact with chemicals from pesticides or pressure treated lumber can also affect the life of the plastic. Poly may also be subject to cuts from blowing ice especially if there are multiple greenhouses adjacent to each other. A scrim reinforce poly may be desirable in these situations.
The high quality and long durability make today's copolymer plastic a good choice for greenhouse glazing. Make your selection from the many options that are available to enhance plant growth.
John W. Bartok, Jr., Extension Professor Emeritus & Agricultural Engineer, Department of Natural Resources and the Environment, University of Connecticut, Storrs CT - 2013