Polyethylene plastic has many properties that makes it useful as a covering for greenhouses. Its low cost, large sheet size, ease of attachment and good light transmission are some of its properties that have helped many farmers to adopt greenhouse cultivation. Most polyethylene film is manufactured as an extrusion of three layers with different polymers and additives.
Greenhouse covering film has high mechanical properties to withstand maximum stress during installation. The main purpose of the greenhouse is to create an internal environment favorable to plant growth regardless of the external environment and to extend the growing season as well as the quality and the crop yield.
The following summarizes some of the important characteristics of the polyethylene film that you need for your crops.
PROPERTIES OF THE GREENHOUSE COVER FILMS
- High mechanical properties
The film’s mechanical properties depend on the polymers used during the production and the production technique itself.
- Ultra-violet (UV) Stability (film durability)
Film without the UV stabilizer will fail after only a few months (4-6 months depending on the thickness of the film) due to the degradation by the UV rays from the sun. UV stabilizers are required in order to achieve the required durability. The type of UV stabilizer and the rate employed depend on the crop, film’s lifetime desired, area (total radiation), film’s thickness desired, type of greenhouse structure and the agrochemicals used.
Consequently, bees need UV to navigate. Therefore 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.
When sulfur and chlorine are used, a special UV stabilizer should be used.
- Maximum light transmission;
Energy from the sun is transmitted through the greenhouse covering to the plants allowing the photosynthesis process. The total light transmission of the greenhouse covering determines how the photosynthetic active radiation (400-700 nanometers) is used. The intensity of these
wavelengths that is Photosynthetic Active Radiation (PAR) directly influences the growth and the developments of the green plants.
Photosynthetic, active radiation (PAR) light transmission varies with the type of additive in the film. Dust, smog and plastic deterioration can also reduce light transmission. A “rule of thumb” 1% increase in light equals 1% increase in plant growth during the winter or in cloudy weather. Therefore, it is also necessary to wash the film when dusty to enable maximum light transmission for an optimum photosynthesis process.
2. Photo-selective films;
These films absorb or reflect specific wavelengths of light. They can enhance plant growth, suppress insects and diseases and affect flower development. Double layer poly is preferred to single layer if you are growing during the heating season; an inflated double layer is recommended. 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. Single layer is common on high tunnels and nursery overwintering houses.
3. Thermal additives (IR);
This is an additive that traps the air inside radiant heat from escaping.
During the day, the air in the greenhouse heats up, therefore, if you use the normal cover at night when the outside temperature cools down, the greenhouse losses heat.
To prevent the extreme temperature changes, a special additive is incorporated within the film during the manufacturing process. This additives blocks the (IR) radiation and helps to maintain a higher temperature inside the greenhouse, thereby helping you to deal with frost and low temperature damage.
4. Controlled Light diffusion
Direct sunlight, (undiffused light) can cause a “shadow’’ effect and “sunburn’’ on plants. Additives that diffuse lights will cause the direct light to scatter and will prevent the shadow effects as well as the sunburns. The light diffusion increases the photo-synthetic efficiency by providing a more homogeneous light from all directions and is beneficial when cultivating self-shading crops such as cucumber, tomatoes, squash, roses and others. The film is suitable for areas with high radiation.
5. Anti-Fog (AF)/ Anti-Drip (AD)
It is also referred to as Condensate Control (AC). Is a wetting agent that reduces surface tension allowing condensation to flow rather than to form droplets.
Hot air contains more moisture than the cold air. At night, the temperature inside the greenhouse decreases and as a result, the water vapor in the air condenses and forms small droplets.
The effects of fog creates:
Reduction of light transmission which leads to low photosynthesis leading to low yields.
Dripping of water droplets on plants causes fungal diseases.
In order to prevent these situations special additives are incorporated into the films. These additives modify the film’s surface so the water droplets are spread out into a continuous invisible water layer.
The human eye is sensitive to a UV range of 400-700nm, while insects are sensitive to a UV range less than 400 nm; therefore, by blocking the UV range, the insects inside the greenhouses are less active. As a result, less agro chemicals are needed to be used. Furthermore, with red roses, UV blocking prevents the rose petals from blackening.
This is an additive that prevents mists from forming in a greenhouse when the temperature drops and the relative humidity reaches saturation. Mist has a negative impact for both top and bottom. The mist blocks out the sun’s rays for many hours after the sun rises.
The multi-layer construction along with a special polymer results in a smooth film that prevents dust accumulation on the outside. Accumulation of dust outside the film blocks light penetration thus causing shadow effects on the plants. This is important knowledge to ensure maximum light transmission and allows cleaning the film to be much easier.
Better greenhouse cover equals better crop yield and quality. This all comes about because of the ability to control the microclimate in terms of light, temperature, humidity, and the physical protection of the crops from winds, rain, snow etc. Therefore, a good greenhouse covering is a very essential material to be considered while constructing a greenhouse.
By Arthur Shadrack, Agronomist at Farm link Kenya.
We install both wooden and metallic greenhouses and drip irrigation systems. Our prices are pocket friendly. Try us for quality services.
For more information on greenhouse installation, extension services and agronomical support contact us on 0728-082887 or email us on firstname.lastname@example.org. Visit our website www.farmlinkkenya for more agricultural information.