Glasshouses, shadehouses and other nurser y structures
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structure cost. To avoid mutual shading between houses, care needs to go into initial positioning. Retractable-roof greenhouse Greenhouses of this type are used where hardening-off is required, as they avoid the labour of moving plants to help them acclimatise to full sun conditions. These houses allow flexible growing conditions – some have two or three covers for plant protection against wind, low temperatures or high light. Table 11.1: Comparison of multi-span and single-span greenhouses Multi-span advantages
Multi-span disadvantages
Single-span advantages
Single-span disadvantages
Less covering material required
Harder to control heat than in single-span
Easier to control heating
Harder to heat
Easier to heat overall area
Easy for pests and diseases to spread
Easier to control pests
Less-efficient use of ground space
Most efficient use of ground space
Provide more specific More structural materials growing conditions for required different types of plants
Greenhouses and shadehouses are now commonly available in kit form for the home gardener, and may be used as a starter greenhouse for backyard nursery growers or for housing displays in retail nurseries. On a full commercial scale, these structures can be readily obtained in a wide variety of sizes and styles, often in multiples of a basic unit (e.g. 6 × 4 m/20 × 13', 10 × 4 m/33 × 13'), or they can be made to order.
Greenhouse construction methods and materials • Shortwall – the transparent material covers the roof and only part of the sides. The lower parts of the walls are covered with non-transparent material such as timber or brick. • Longwall – the transparent material covers all of the walls and roof. • Tunnel – the framework is made from half-circle metal hoops arranged in a row to form a tunnel shape. Transparent material such as PVC film is laid over this framework. • Lean-to – a structure is attached to the side of another building, such as a house, which hence forms one side wall of the greenhouse. Framing materials Metal. Aluminium frames are popular because they resist rust, and are relatively strong and lightweight. Galvanised iron or steel are also used, though over time corrosion can become a problem. Glass goes well with metal framing, but PVC film and other plastics can deteriorate if in direct contact with metal (as metal gets very hot in summer, sections of PVC film touching metal will crack or tear much sooner than parts not in contact). The strength of the various metals allows the use of structural members with smaller cross-sections than timber used for the same task. This results in reduced shading effects.