The structure of a greenhouse creates a controlled microclimate, but the choice of covering material determines its effectiveness. This plastic skin must manage temperature, humidity, and light to optimize plant growth. Selecting the correct plastic is a fundamental step that directly impacts long-term operational costs and crop success. The ideal material must balance light availability with thermal insulation and resistance to environmental degradation. Understanding the specific properties of available materials allows growers to make an informed decision tailored to their unique needs.
Primary Material Options
The field of greenhouse coverings is dominated by two main categories: flexible films and rigid panels, each made from distinct polymer compounds. Polyethylene (PE) film is the most widely used choice, primarily due to its low cost and ease of installation on hoop structures. This flexible plastic sheeting is typically UV-treated to prevent rapid breakdown from sunlight, making it a highly affordable and versatile option for many growers.
Polycarbonate (PC) panels represent the leading rigid covering material, often employed in more permanent and commercial-grade structures. Polycarbonate is exceptionally strong, offering high impact resistance that can withstand severe weather conditions like hail and heavy snow loads. It is frequently manufactured in a twin-wall or multi-wall corrugated structure, which creates air pockets that significantly enhance its insulating properties.
Polyvinyl Chloride (PVC) was historically common for its excellent heat retention capabilities. However, modern PVC films are less frequently used now, as they are prone to degradation from high temperatures and can chemically interact with some sulfur-based chemicals. More recently, advanced films like Ethylene-Vinyl Acetate (EVA) and Polyolefin (PO) have emerged as premium alternatives to PE. These offer superior flexibility, light diffusion, and enhanced thermal performance, though at a higher initial cost.
Essential Performance Characteristics
The scientific performance of a greenhouse plastic is defined by how it interacts with solar radiation and internal heat. Light transmission refers to the total amount of light energy that passes through the material, which is necessary for photosynthesis. However, maximum transmission is not always ideal, as intense, direct sunlight can cause plant scorching or create uneven growing conditions with “hot spots.”
Light diffusion is a significant factor, describing how the material scatters incoming light across the entire canopy. Diffused light penetrates deeper into the plant structure, reaching lower leaves and promoting more uniform growth. This increases crop yields by reducing shadows and distributing energy more evenly. Quality greenhouse plastics are engineered to balance high overall transmission (often 88% to 90%) with a substantial diffusion rate (sometimes exceeding 20%).
Thermal retention, or insulation, dictates how well the plastic traps heat inside the structure. This property is especially important in colder climates and is enhanced by multi-layer materials, such as the air-pocket design of twin-wall polycarbonate. UV stabilization involves incorporating chemical additives like Hindered Amine Light Stabilizers (HALS) during manufacturing. These stabilizers prevent the plastic polymer from breaking down and becoming brittle or yellowed under ultraviolet radiation, which significantly extends the material’s lifespan.
Practical Durability and Cost Considerations
Translating material composition into practical use requires understanding metrics like thickness and expected longevity. For flexible films, thickness is measured in “mil,” where one mil equals one-thousandth of an inch. Standard greenhouse films typically range from 4 mil, suitable for short-term use, to 6 mil, which is the most common thickness for multi-year applications. A thicker film, such as 6 mil polyethylene, provides greater tear resistance and mechanical strength.
The material’s composition and thickness directly correlate with its lifespan and cost profile. A standard, UV-treated 6 mil PE film is typically warranted for four years, making it the most affordable option with the lowest initial investment. Rigid polycarbonate panels have a much higher upfront cost but can offer a lifespan of 10 to 15 years or more. Choosing a more durable material provides better long-term value by minimizing replacement frequency and reducing maintenance and labor costs.
Matching Plastic to Climate and Crop Needs
The final selection of greenhouse plastic must synthesize material properties with the grower’s specific environmental and agricultural goals. In cold climates, the priority shifts to maximum heat retention and insulation to minimize heating costs. Options like twin-wall polycarbonate or a double-layered PE film system with an air blower are preferable, as they create an insulating air gap to maintain stable internal temperatures.
For growers in hot, high-sun regions, the focus moves toward light management to prevent plant stress and heat buildup. Highly diffused PE film is an excellent choice, as it scatters intense light to prevent scorching and reduce hot spots across the canopy. Temporary structures or those built on a limited budget are best served by a UV-stabilized 4-year PE film, which offers a good balance of durability and low material cost.
For permanent, commercial operations growing high-value crops, the investment in durable materials is justified by the need for long-term stability and optimal performance. In these scenarios, high-quality, long-warranty polycarbonate sheets or premium polyolefin films are recommended. This is due to their superior strength, longevity, and consistent light diffusion properties. The choice ultimately depends on balancing the required lifespan and thermal performance with the initial financial outlay.