A plastic sheeting greenhouse provides a cost-effective solution for extending the growing season and creating a protected environment for plants. This structure utilizes flexible polyethylene film stretched over a rigid frame to capture solar energy. Building a functional greenhouse requires careful planning regarding materials, structural integrity, and environmental control. This guide outlines the practical steps for constructing a structure that optimizes plant health and withstands local weather conditions.
Material Selection and Site Preparation
Selecting the proper covering material is the first step, with polyethylene film being the industry standard due to its durability and cost-effectiveness. For structures intended to last multiple seasons, a thickness of at least 6 mil is recommended for increased tear resistance and longevity. The plastic must also incorporate a UV-resistant additive to prevent premature degradation from solar radiation.
Frame materials include wood, metal conduit, or PVC piping, chosen by balancing cost against structural strength. Metal conduit offers superior wind resistance and durability for larger structures, while PVC is lightweight and easy to assemble for smaller projects. Site selection requires a location that receives maximum sun exposure, ideally six or more hours of direct light daily, and possesses good drainage. The site must be cleared and leveled, as an uneven base compromises structural integrity and makes securing the film difficult.
Constructing the Structural Frame
The structural frame acts as the skeleton, bearing the load of the plastic, snow, and wind forces, requiring precise measurement and assembly. When using metal conduit or PVC, the material is bent into arch shapes to create a resilient, wind-shedding structure, connected using specialized fittings or brackets. Increasing the diameter and wall thickness of the frame material improves its load-bearing capacity and resistance to environmental stresses.
Anchoring the frame securely to the ground prevents the structure from lifting in high winds. For a robust foundation, the frame can be bolted to concrete footings or a wood platform, or secured using heavy-duty earth anchors. For less permanent structures, steel reinforcing bars (rebar) can be angled into the ground and attached to the frame with clamps or wire. Diagonal cross bracing should be added to the corners and along the sides to prevent lateral movement and add stiffness against gusting winds.
Applying and Securing the Plastic Sheeting
Applying the plastic sheeting requires stretching the material tautly over the frame to achieve “drum tension.” This tautness minimizes flapping, which causes tears and premature failure under wind stress. The process involves unrolling the plastic over the peak, temporarily clamping it on one side, and then pulling it tight before securing it on the opposing side.
The most effective fastening method without creating weak points is the use of a wiggle wire and channel system. This involves screwing an aluminum or galvanized steel channel to the frame and inserting a PVC-coated spring wire to press the plastic firmly into the groove. This technique distributes pressure evenly, providing a continuous grip resistant to wind damage and allowing for easy film replacement. For a wooden frame, wood battens or specialized batten tape can be used by sandwiching the plastic between the frame and a thin strip of material before screwing it down, though this pierces the film.
Essential Ventilation and Access
Ventilation is necessary to regulate the internal environment, controlling both temperature and humidity, which directly impacts plant health and disease prevention. Without proper airflow, the temperature inside the greenhouse can quickly rise 10 to 20 degrees Fahrenheit above the ambient outdoor temperature due to solar heat gain. Excess humidity creates conditions favorable for the development of fungal diseases such as mold and mildew.
Passive ventilation systems rely on thermal buoyancy, where warm air rises and escapes through high vents. This requires openings near the base for cooler replacement air to enter, creating a convective flow that exchanges the entire volume of air.
A simple access door can be constructed by creating a rectangular frame within one end wall and covering it with a separate piece of plastic sheeting. The door sheeting is secured around the perimeter, allowing it to be latched shut or opened completely for air exchange. Roll-up sides, created by attaching the bottom edge of the plastic to a pipe that can be manually rolled upward, offer an efficient way to achieve cross-ventilation and temperature moderation.