A greenhouse acts as a controlled microclimate, allowing gardeners to extend their growing season, cultivate delicate plants, and start seedlings earlier in the year. The structure captures solar energy, converting light into heat to create an environment warmer than the outside air. Choosing the best greenhouse requires careful consideration of individual needs, the local climate, and the available budget. This guide analyzes structural options, material comparisons, sizing factors, and necessary components to help you make an informed decision about your investment.
Understanding Greenhouse Structural Styles
Greenhouses are categorized into two primary structural styles: freestanding and lean-to. Freestanding greenhouses are independent structures that can be placed anywhere with optimal sun exposure, offering maximum flexibility in location and design. These structures are typically larger and receive light from all four sides and the roof, resulting in a uniform growing environment throughout the day. The drawback to a detached model is the higher energy cost required to heat the exposed surface area and the need for a dedicated foundation.
Lean-to or wall-mounted greenhouses utilize an existing building wall for one side, which offers significant energy efficiency benefits. The shared wall retains and radiates heat back into the greenhouse, reducing heating costs, and provides easy access to utilities from the main building. However, this style is limited in size and constrained by the sun exposure of the existing wall, often making it unsuitable for gardeners requiring large growing spaces. For temporary, low-cost options, hoophouses or cold frames, constructed with curved pipe and plastic film, serve as useful season extenders but lack the durability and climate control of permanent structures.
Evaluating Frame and Glazing Materials
The longevity and performance of a greenhouse depend on the materials used for its frame and glazing. Aluminum framing is a popular choice due to its lightweight nature, resistance to rust, and minimal maintenance requirements. Since aluminum is thermally conductive, it can draw heat away from the interior unless a thermal break is incorporated into the design. Wood frames, such as cedar or redwood, offer a traditional aesthetic and provide good natural insulation. However, wood demands higher maintenance, requiring regular sealing or painting to prevent rot and decay in the humid environment.
For budget-conscious or temporary projects, PVC (polyvinyl chloride) plastic is the most economical and lightweight framing option. Its lower durability and susceptibility to UV degradation make it unsuitable for large, permanent installations intended to last for decades.
The glazing material affects light transmission, insulation, and durability. Glass provides the highest light transmission, typically around 90%, and is aesthetically pleasing. However, it is fragile, offers poor insulation, and is the heaviest option, requiring a robust frame.
Polycarbonate panels are a preferred modern option, especially twin-wall or multi-wall varieties. These feature internal air pockets that trap heat, offering superior insulation compared to single-pane glass. Polycarbonate is virtually unbreakable, resists impact, and diffuses light to prevent plant scorching, though its light transmission is slightly lower than glass (generally 80–85%). Polyethylene film is the least expensive glazing, offering moderate light transmission, but it has the shortest lifespan, often requiring replacement every three to five years due to UV degradation.
Determining Size and Optimal Placement
Sizing a greenhouse should account for current gardening needs plus an allowance for future expansion, as purchasing a structure that is too small is a common mistake. A small hobby greenhouse for starting seedlings might be 6×8 feet, while a serious grower cultivating a variety of vegetables may require 8×12 feet or larger.
Sizing Considerations
Calculating the necessary footprint involves factoring in the space required per plant (e.g., 1.5 to 2 square feet for a tomato plant), in addition to dedicated areas for pathways and benches. Pathways should be at least 24 inches wide to allow for comfortable movement and access to all plants.
Placement Factors
Optimal placement requires maximum solar exposure, meaning the structure’s longest side should ideally face south or southeast to capture at least six hours of direct sunlight daily. The site must have excellent drainage to prevent water accumulation and should be protected from harsh winds, which increase heating costs and can damage the structure. Local climate factors, such as anticipated snow and wind loads, influence the necessary structural strength, making a reinforced frame or a Gothic arch design a better choice in regions prone to heavy snowfall. Proximity to water and electricity sources is also a practical consideration that simplifies daily operation.
Essential Operational Components
A bare structure requires operational components to manage temperature and humidity effectively. Ventilation is necessary to expel hot, stagnant air and exchange moist internal air with drier outside air, which helps prevent fungal diseases that thrive in high humidity.
Natural ventilation is achieved through strategically placed roof and side vents, utilizing the stack effect where warm air rises and exits through the upper vents, drawing in cooler air from below. For larger greenhouses or hotter climates, mechanical ventilation using exhaust fans is necessary to ensure a specific air exchange rate. This is often paired with evaporative cooling systems, like a fan-and-pad setup, to lower the temperature in dry conditions.
A heating system may be required to maintain temperatures, with options ranging from electric heaters for small spaces to gas-powered heaters for year-round warmth. Internal staging, such as shelving and benches, organizes the space efficiently, maximizing the available growing area. A dedicated watering system, like drip irrigation, ensures plants receive consistent moisture and reduces the labor involved in daily maintenance.