A sheet of plastic over a few hoops can change what a farm earns.
Growing under cover protects a crop from cold, heat, rain, wind, hail, insects and birds.
Yields per square metre can be several times those in the open field,
quality is higher and more even, and crops can be grown out of season, when prices are better.
Protected cultivation now covers millions of hectares worldwide.
It runs from the high-tech glasshouses of the Netherlands,
to the plastic greenhouses of Almería in southern Spain, to the solar greenhouses and tunnels of northern China,
and the polyhouses of East Africa and South Asia.
In this video: the main types of structure, how a cover changes light and temperature, ventilation and cooling, humidity and carbon dioxide, water and nutrients, crops and pollination, pests under cover, working safely, and when a greenhouse really pays. The advice is general and global.
Your local extension service comes first.
Start with the range of structures.
A glasshouse has glass on a steel or aluminium frame. It lasts for decades and lets in the most light,
and it suits high-value vegetables and flowers in cold climates.
A polyhouse, or plastic greenhouse, uses UV-stabilised polyethylene film on steel hoops. It costs much less, and it is the most common structure in the world.
The Chinese solar greenhouse has a thick north wall that stores the day's heat, a sloping plastic roof facing south, and insulating blankets rolled down at night. It grows winter vegetables without any heating.
A net house uses insect-proof or shade netting instead of film, to keep out virus-carrying insects and to cut heat and light.
Walk-in tunnels and low tunnels are plastic over hoops, for early vegetables and nurseries.
And mulches and row covers lie directly on the soil or the crop, to warm the soil and protect against frost.
Whatever you build, anchor it well.
Film houses usually fail in storms by uplift, the wind lifting the whole structure, not by collapsing under their own weight.
Every cover changes the crop's climate.
First, it cuts light.
Clean glass lets through about ninety per cent. Plastic film lets through slightly less, and that falls as the film ages and collects dust.
UV-stabilised film typically lasts three to five years. In sunny, hot climates, growers do the opposite and reduce light, with shade nets of thirty to fifty per cent, or whitewash on the roof.
Second, a cover traps heat.
Sunlight passes in, warms the plants and the soil,
and the heat they give off is partly held in by the cover. That is the greenhouse effect.
So in warm climates, overheating is the main problem. In cold climates, the problem is losing heat at night,
and growers use hot-water pipes, hot-air heaters, and thermal screens drawn across at night. Match the structure to your climate.
A costly heated glasshouse is wasted in a mild climate,
and a simple polyhouse overheats in a hot one unless it can be cooled.
Ventilation is the first tool against heat.
Natural ventilation through roof and side vents works well if the vents are large enough, often fifteen to thirty per cent of the floor area.
Roof vents let the hottest air out, and side vents bring cooler air in. If you fit insect nets over the vents, make the vents bigger, because the finer the mesh, the less air gets through.
When vents are not enough, exhaust fans pull air through the house. A useful rule is about one complete change of air every minute in summer. A house thirty metres long, ten wide and four high holds twelve hundred cubic metres of air, so its fans must move about twenty cubic metres every second.
Evaporative cooling goes further.
In a fan-and-pad system, the fans pull outside air through a wet pad,
and the evaporating water cools it. Here is the sum.
On a hot, dry afternoon at thirty-eight degrees, with a wet-bulb temperature of twenty-four and a pad that is eighty per cent efficient, air leaves the pad at about twenty-seven degrees.
On a humid day at thirty-four degrees, with a wet bulb of thirty, you get only about three degrees of cooling.
So pads work best in dry heat.
Air warms again as it crosses the house, so very long houses, over about thirty to forty metres from pad to fan, have hot ends.
Fine fog nozzles are the other option, cooling the whole house more evenly,
but they need very clean water.
Humidity builds up under cover, because the water the plants transpire is held in. High humidity encourages fungal diseases such as grey mould and downy mildew. So growers balance ventilation and heating to keep the air moving and dry the leaves.
In closed, heated greenhouses, there is one more lever: carbon dioxide. On a bright winter morning, a dense crop with the vents shut quickly draws carbon dioxide down below the outdoor level, just when the light is good. Enriching the air to about eight hundred to one thousand parts per million raises photosynthesis of crops such as tomato and cucumber, often increasing yields by fifteen to thirty per cent. But with the vents wide open, the extra gas simply blows away, so enrichment pays only in closed houses.
Inside a greenhouse, there is no rain, so every drop comes from the irrigation system.
Most greenhouse crops are grown with drip fertigation,
which means fertiliser dissolved in drip irrigation water. They grow either in soil, or in soilless media such as rockwool, cocopeat or perlite.
Two numbers run the system.
Electrical conductivity, or EC, measures how much fertiliser salt is in the solution.
Most crops grow at roughly one and a half to three units, lower for lettuce and higher for tomato.
The pH is held roughly between five and a half and six and a half, so that phosphorus, iron and manganese stay available.
Growers on substrates usually let about twenty to thirty per cent of the water drain out, to flush salts away.
Check your water before you build.
Water quality is often the deciding factor for soilless growing,
and for evaporative cooling too.
The main vegetables under cover are tomato,
cucumber, sweet pepper, lettuce and other leafy greens, strawberry, melon and eggplant.
The main flowers are rose, gerbera, carnation and chrysanthemum. And protected nurseries raise strong, uniform, disease-free seedlings and grafted plants for the open field.
Greenhouse tomatoes and cucumbers are trained up on high wires,
and can grow more than ten metres of stem in a long season.
The most advanced Dutch glasshouses harvest over sixty kilograms of tomatoes per square metre, against a few kilograms in a typical open field.
Do not forget pollination.
In closed greenhouses, tomatoes are usually pollinated by colonies of bumble bees, or by vibrating the flowers.
Parthenocarpic cucumber varieties set fruit with no pollination at all.
A cover keeps many pests out, but a greenhouse is warm and humid, so the pests that do get in multiply quickly. The usual ones are whiteflies, thrips, aphids and spider mites,
and diseases such as powdery mildew and grey mould. Keep them out first.
Fit insect-proof nets on vents and doors, for example forty or fifty mesh against whitefly, and use double doors so one is always shut. Repair torn nets at once, keep the house clean, and grow resistant varieties.
Then let natural enemies work.
The tiny wasp Encarsia formosa parasitises whitefly, turning the young scales black.
Predatory mites such as Phytoseiulus persimilis eat spider mites. In northern Europe, biological control is used on most of the greenhouse vegetable area. If you do need to spray, use only products registered for that crop under cover, at the label dose, and respect the re-entry and pre-harvest intervals.
Sulphur and chlorine compounds also attack the stabiliser in greenhouse film and shorten its life.
Now, safety.
A greenhouse is a closed space, and that changes the risks.
Heaters and carbon dioxide burners that burn gas or oil must be vented,
and must burn clean fuel completely.
Incomplete burning makes carbon monoxide, which harms people, and ethylene, which harms crops. In many countries, the workplace limit for carbon dioxide itself is five thousand parts per million, averaged over eight hours.
Read the label before you mix or spray anything.
Wear what it asks for, and gloves matter most.
After spraying, keep people out until the re-entry interval has passed.
Store pesticides locked, in their original containers, away from food, feed and children.
And in summer, work in the cooler hours and drink plenty of water. The air inside a closed house in the afternoon can be far hotter than outside.
Protected cultivation needs high investment and skilled management. It pays when the crop is high-value and the market is reliable, when the structure suits the climate, and when the grower is trained in fertigation, climate control and pest management.
Many programmes subsidise polyhouses, but failures often come from poor design for the local climate, lack of training, or weak links to buyers.
In a hot climate, a polyhouse works only with enough ventilation and cooling,
and usually shade nets.
Many growers in hot regions get better results from a naturally ventilated net house, which keeps insects out without trapping heat.
And windowless vertical farms?
They must buy all their light as electricity.
Laisha's worked example finds about thirteen kilowatt-hours of lighting for every kilogram of lettuce.
That is why greenhouses, which use free sunlight, remain by far the largest and most energy-efficient form of protected growing.
To recap.
Choose the structure for your climate: glass, film, net house, tunnel or row cover, and anchor it against wind.
Covers cut light and trap heat, so ventilate, shade and cool in warm climates, and heat and screen in cold ones.
Fan-and-pad cooling works best in dry heat.
Control humidity against grey mould, and enrich carbon dioxide only in closed houses. Run fertigation by EC and pH.
Plan pollination.
Keep pests out with nets and double doors, and use biological control.
And build only when you have the market, the training and the water.
For grow guides, the pest and disease encyclopedia,
and more explainers, visit laisha.ai. Thank you for watching.
