Protected cultivation and greenhouses
Greenhouses, polyhouses, net houses and tunnels, covering materials, heating, ventilation and fan-pad cooling, crops grown, economics, and a cooling sum.
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Growing crops under a cover, whether a glass greenhouse, a plastic polyhouse, an insect-proof net house or a simple low tunnel, protects them from cold, heat, rain, wind, hail, insects and birds, and lets growers control more of the crop's environment. Yields per square metre can be several times those in the open field, quality is higher and more consistent, and crops can be grown out of season. Protected cultivation now covers millions of hectares worldwide, from the high-tech glasshouses of the Netherlands to the plastic greenhouses of Almería in Spain and the vast solar greenhouses and tunnels of northern China.
Structures, climate control and crop management under cover are standard exam topics in horticulture and agricultural engineering.
Types of structures
| Structure | Description | Typical use |
|---|---|---|
| Glasshouse | glass on a steel or aluminium frame; long life, high light transmission | high-value vegetables and flowers in cold climates |
| Polyhouse (plastic greenhouse) | UV-stabilised polyethylene film, often 150–200 µm thick, on steel hoops | vegetables and flowers worldwide; lower cost |
| Chinese solar greenhouse | thick north wall that stores heat, sloping plastic south roof, insulating blankets at night | winter vegetables in northern China without heating |
| Net house or shade house | insect-proof or shade netting | keeping out virus vectors; reducing heat and light |
| Walk-in and low tunnels | plastic over low hoops | early season vegetables, nurseries |
| Mulches and row covers | plastic film or fleece directly over soil or plants | raising soil temperature, frost protection |
The greenhouse environment
Light
Covers reduce light; clean glass transmits around 90 per cent, and plastic films slightly less, declining with age and dust. Diffuse-light films and glass spread light deeper into the canopy. In sunny, hot climates, shade nets (often 30 to 50 per cent shade) or whitewash reduce excess light and heat.
Temperature
Short-wave sunlight passes through the cover and is absorbed by plants and soil, which emit long-wave heat radiation that glass and many films partly trap: the greenhouse effect. Overheating is therefore the main problem in warm climates, and heat loss at night in cold ones.
- Heating: hot-water pipes, hot-air heaters, combined heat and power plants, and heat storage; thermal screens at night.
- Natural ventilation through roof and side vents, which works well if vents are large enough (often 15 to 30 per cent of floor area).
- Forced ventilation with exhaust fans.
- Evaporative cooling: fan-and-pad systems, or high-pressure fogging, cool the air by evaporating water.
Humidity and CO2
High humidity encourages fungal diseases such as grey mould and downy mildew, so ventilation, heating and dehumidification are balanced. In closed, heated greenhouses, CO2 enrichment to about 800 to 1,000 ppm raises photosynthesis of C3 crops such as tomato and cucumber, often increasing yields by 15 to 30 per cent.
Water and nutrients
Most greenhouse crops are grown with drip fertigation, in soil or in soilless media such as rockwool, cocopeat or perlite, with nutrient solutions adjusted by electrical conductivity (EC) and pH.
Crops grown under protection
- Vegetables: tomato, cucumber, sweet pepper (capsicum), lettuce and other leafy greens, strawberry, melon, eggplant.
- Flowers: rose, gerbera, carnation, chrysanthemum, orchids, and cut foliage.
- Nurseries: seedlings and grafted plants, raised in protected nurseries for strong, uniform, disease-free transplants.
Crops are often trained vertically: greenhouse tomatoes and cucumbers on high wires can grow more than 10 m of stem in a long season, yielding over 60 kg of tomatoes per square metre in the most advanced Dutch glasshouses, compared with typical open-field yields of a few kilograms per square metre.
Pest and disease management
Covers help, but greenhouses are warm and humid, so pests such as whiteflies, thrips, aphids and spider mites, and diseases such as powdery mildew and grey mould, can multiply quickly. Insect-proof nets on vents and doors, double doors, hygiene, resistant varieties and biological control (for example Encarsia formosa against whitefly and predatory mites against spider mites) are standard; biological control is used on most of the greenhouse vegetable area in northern Europe.
Economics
Protected cultivation needs high investment and skilled management. It pays when:
- crops are high-value and markets are reliable;
- the structure suits the climate (a costly heated glasshouse is wasted in a mild climate, and a simple polyhouse overheats in a hot one without cooling);
- growers are trained in fertigation, climate control and pest management.
Many developing-country programmes subsidise polyhouses; failures often come from poor design for the local climate, lack of training, or weak market links.
How much can a pad cool the air?
The air temperature leaving a wet pad is:
Tout = Tdry − η × (Tdry − Twet)
where Tdry is the outside dry-bulb temperature, Twet the wet-bulb temperature and η the pad's efficiency (often 0.75 to 0.85).
On a hot, dry afternoon, Tdry = 38 °C and Twet = 24 °C, with η = 0.80:
- Tout = 38 − 0.80 × (38 − 24) = 38 − 11.2 = 26.8 °C.
On a humid day, with Tdry = 34 °C and Twet = 30 °C: Tout = 34 − 0.80 × 4 = 30.8 °C, only about 3 degrees of cooling. Air warms again as it crosses the house, so long greenhouses (over about 30 to 40 m from pad to fan) have hot ends.
Fan capacity. A greenhouse 30 m long, 10 m wide and 4 m high has a volume of 1,200 m3. To change the air about once a minute in summer, the fans must move about 1,200 m3 per minute, or 20 m3 per second.
Key points
- Structures range from glasshouses and polyhouses to Chinese solar greenhouses, net houses, tunnels and mulches.
- Covers trap heat (greenhouse effect); cooling is the main challenge in warm climates and heating in cold ones.
- Ventilation, shade nets, fan-and-pad and fog systems cool; pipes, heaters and screens heat; CO2 enrichment boosts C3 crops.
- Main crops: tomato, cucumber, capsicum, leafy greens, strawberry, roses, gerbera, carnation and nursery seedlings.
- Pad outlet temperature = Tdry − η (Tdry − Twet); evaporative cooling works best in dry air.
- Insect-proof nets, hygiene and biological control manage pests.
What the exams ask
- School-leaving and entrance papers: types of protected structures, advantages, and crops grown.
- Degree and research-fellowship entrances: covering materials, greenhouse effect, ventilation and cooling systems, CO2 enrichment, fertigation, and pest management under cover.
- Agricultural engineering and professional examinations: greenhouse design, heating and cooling calculations, economics, and choosing structures for different climates.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is a polyhouse worthwhile in a hot climate?
Only with adequate ventilation and cooling, and usually with shade nets. Many growers in hot regions get better results from naturally ventilated net houses that keep out insects without trapping heat.
How long does greenhouse plastic last?
UV-stabilised films typically last three to five years before light transmission and strength decline. Glass lasts decades.
Do greenhouse crops need pollinators?
Tomatoes in closed greenhouses are usually pollinated by commercial bumble bee colonies or by mechanical vibration. Parthenocarpic cucumber varieties need no pollination.
Sources
- Nelson, P. V. Greenhouse Operation and Management, 7th edition. Pearson, 2012.
- Heuvelink, E. (ed.) Tomatoes, 2nd edition. CABI, 2018.
- Castilla, N. Greenhouse Technology and Management, 2nd edition. CABI, 2013.
- FAO. Good Agricultural Practices for Greenhouse Vegetable Crops: Principles for Mediterranean Climate Areas. FAO Plant Production and Protection Paper 217, 2013.
- ASABE. Heating, Ventilating and Cooling Greenhouses (ANSI/ASAE EP406). American Society of Agricultural and Biological Engineers.
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