Food Production

A glasshouse (greenhouse) is a building with glass walls and roof; a polythene tunnel (polytunnel) is a curved frame covered with clear plastic sheeting. Both let growers control the environment around a crop so that photosynthesis, and therefore yield, can be pushed higher than it would be outdoors.

A glasshouse or polytunnel increases yield because it lets the grower manage the factors that limit photosynthesis and growth:

• Higher temperature. The transparent cover traps heat from the Sun (and can be topped up by heaters), so the air inside stays warmer. Warmer conditions speed up the enzyme-controlled reactions of photosynthesis and growth, and they extend the growing season into colder months.
• More light. The clear cover still lets light through, and growers can add artificial lighting to keep photosynthesis going on dull days and after dark.
• Controlled carbon dioxide. The enclosed space can be enriched with extra CO₂ (see section 2).
• Protection. Crops are shielded from frost, wind, heavy rain, and many pests and diseases, so fewer plants are lost.
• Controlled watering. Water and dissolved mineral ions can be supplied steadily so the crop never runs short.

A polytunnel is cheaper to build and cover than a glasshouse, so it is widely used for crops like strawberries and salad leaves, but it gives less precise control than a fully equipped glasshouse.