Controlled-environment agriculture and vertical farms
Greenhouses, plant factories and vertical farms: how they work, what they grow well, the energy arithmetic of LED lighting, and why many have struggled.
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Controlled-environment agriculture (CEA) means growing crops in structures where light, temperature, humidity, carbon dioxide, water and nutrients are managed, rather than left to the weather. It runs from simple plastic tunnels to high-tech glasshouses to windowless vertical farms where lettuce grows on stacked shelves under LEDs.
Vertical farms attracted huge investment and headlines in the 2010s and early 2020s, followed by a wave of failures. Understanding what CEA does well, and the physics that limits it, is now part of horticulture, agricultural engineering and food-systems courses, and a frequent topic in essays on the future of farming.
The range of systems
| System | Light | Climate control | Typical crops | Where it is prominent |
|---|---|---|---|---|
| Net houses and shade houses | sun | little; keeps out insects, reduces heat | vegetables, nurseries | tropics and subtropics |
| Plastic tunnels and polyhouses | sun | passive ventilation, some heating | vegetables, strawberries, flowers | worldwide; large areas around the Mediterranean and in East Asia |
| Solar (passive) greenhouses | sun | thick north walls store heat | winter vegetables | northern China and other cold, sunny regions |
| High-tech glasshouses | sun, with supplementary LEDs | heating, cooling, CO2, screens, computer control | tomato, cucumber, pepper, flowers | north-western Europe, North America, East Asia, and arid regions with cooling |
| Plant factories with artificial lighting (vertical farms) | LEDs only | fully enclosed and controlled | leafy greens, herbs, microgreens, seedlings, some berries | cities in East Asia, the Middle East, Europe and North America |
By area, greenhouses and tunnels are by far the largest part of CEA; vertical farms are a small but conspicuous fraction.
How a vertical farm works
- Soilless growing: plants grow in hydroponics, with roots in nutrient solution (nutrient film technique, deep-water culture, ebb and flow), in aeroponics (roots misted), or in inert substrates such as rockwool or coir. The solution's electrical conductivity (EC, a measure of nutrient concentration) and pH are monitored and adjusted.
- LED lighting: tuned to wavelengths plants use most, mostly red and blue, with some far-red and green. Light is measured as photosynthetic photons: the daily light integral (DLI), in moles of photons per square metre per day.
- Climate control: air handling removes the heat from the lights and the water vapour from transpiration, recovering it as condensate to reuse.
- CO2 enrichment, often to two or three times outdoor levels, raising photosynthesis.
- Biosecurity: sealed rooms keep pests out, so most systems use no pesticides.
What it does well
- Very high yields per square metre of floor, from many layers and year-round cropping with short cycles.
- Very low water use, because water is recirculated and transpired water is recovered. A frequently cited comparison found hydroponic greenhouse lettuce using roughly a tenth of the water of field lettuce per kilogram.
- Consistency and reliability: the same quality every day, independent of drought, floods and seasons.
- Location: near consumers in cities, deserts and cold climates, cutting transport and spoilage for perishable greens.
- Clean produce without pesticide residues and with low microbial contamination when managed well.
The energy arithmetic
The sun delivers light free; a vertical farm must buy it. That single fact explains most of the economics.
How much electricity does lighting lettuce take?
Assume lettuce needs a daily light integral of 17 mol of photons per square metre per day, delivered by modern LEDs producing 3 µmol of photons per joule of electricity (good current fixtures are in the range of about 2.5 to 3.5).
- Electricity for light per square metre per day = 17 mol ÷ 3 µmol/J = 17,000,000 µmol ÷ 3 µmol/J ≈ 5.7 MJ ≈ 1.6 kWh per m2 per day.
- Suppose each square metre of a growing layer produces 25 heads of 150 g every 30 days: 25 × 0.15 = 3.75 kg per 30 days, or 0.125 kg per m2 per day.
- Lighting electricity per kilogram = 1.6 ÷ 0.125 ≈ 13 kWh per kg of lettuce.
On top of that comes the electricity to remove nearly all of that energy again as heat through air conditioning, plus pumps and fans. Published estimates of total electricity use in plant factories vary widely, but values of the order of 10 to 20 kWh or more per kilogram of leafy greens are common. At an illustrative electricity price of 0.15 currency units per kWh, 15 kWh is 2.25 currency units per kilogram before labour, rent and capital, which is why electricity prices decide whether a vertical farm survives, and why its climate footprint depends on how clean the electricity is.
This is also why vertical farms grow leafy greens and herbs: they have a high harvest index (almost the whole plant is sold), short cycles, high value per kilogram and low light needs. Staple crops such as wheat, rice or potatoes would need far more light per unit of food and value, making them uneconomic indoors except for research and seed production.
Why many ventures struggled
From about 2022, as energy prices rose and investment dried up, several high-profile vertical-farming companies in Europe and North America closed, filed for bankruptcy protection or shrank. Common problems:
- High capital costs for buildings, lights and climate systems.
- Energy costs far above those of greenhouses using sunlight.
- Labour for seeding, transplanting, harvesting and packing, only partly automated.
- Narrow product range competing with cheap field and greenhouse greens.
- Scaling faster than the technology and markets justified.
Operations that have done better often sell premium or very fresh products to dedicated buyers, produce seedlings and propagation material (where controlled conditions add great value), operate where field growing is very difficult (arid or very cold places, dense cities with high import costs), or combine indoor production with renewable energy and waste heat.
Greenhouses: the bigger story
High-tech greenhouses use sunlight, so their energy use per kilogram is far lower than vertical farms, though heated glasshouses in cold climates still use a lot of fuel. Their tomato and cucumber yields per square metre are many times those of open fields, and integrated pest management with biological control is standard in them. Large areas of simple plastic greenhouses supply winter vegetables across Mediterranean, East Asian and other regions, with their own challenges of plastic waste, water and labour conditions. For most of the world, the growth of CEA will continue to be in greenhouses and tunnels rather than in windowless vertical farms.
Key points
- CEA ranges from net houses and tunnels to high-tech glasshouses and fully enclosed plant factories (vertical farms).
- Vertical farms use soilless culture, LED lighting, climate control and CO2 enrichment; they need no pesticides in most cases.
- Strengths: very high yield per floor area, very low water use, year-round consistency, location near consumers.
- Weakness: all light must be bought as electricity; lighting alone can take around 13 kWh per kg of lettuce under typical assumptions.
- Leafy greens, herbs, microgreens and seedlings suit vertical farms; staples do not.
- Greenhouses, which use sunlight, are by far the largest and most energy-efficient form of CEA.
What the exams ask
- Horticulture and protected cultivation courses: types of greenhouses and their climate control, hydroponic systems, nutrient solution management, and crops suited to protected cultivation.
- Agricultural engineering examinations: greenhouse heating and cooling loads, lighting calculations (DLI, photon efficacy), and energy use per unit of produce.
- Food-systems and policy essays: the promise and limits of vertical farming, urban agriculture, water saving, and energy and climate trade-offs.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is food from vertical farms more sustainable?
It saves water, land and pesticides, and transport for perishable greens. But its electricity use is high, so its carbon footprint can be larger than field or greenhouse production unless the electricity is low-carbon.
Can vertical farms feed cities?
They can supply a share of leafy greens and herbs. Calories and protein for cities will continue to come overwhelmingly from fields, because staple crops need too much light and space to grow economically indoors.
Is hydroponic food less nutritious?
Not inherently. Nutrient content depends on the variety, the nutrient solution, light and harvest stage. Well-managed hydroponic produce is comparable to field-grown produce, and light can even be adjusted to raise some compounds.
Sources
- Kozai, T., Niu, G. and Takagaki, M. (eds) Plant Factory: An Indoor Vertical Farming System for Efficient Quality Food Production, 2nd edition. Academic Press, 2020.
- Graamans, L., Baeza, E., van den Dobbelsteen, A., Tsafaras, I. and Stanghellini, C. "Plant factories versus greenhouses: comparison of resource use efficiency." Agricultural Systems 160, 31–43 (2018).
- Barbosa, G. L. et al. "Comparison of land, water, and energy requirements of lettuce grown using hydroponic vs. conventional agricultural methods." International Journal of Environmental Research and Public Health 12, 6879–6891 (2015).
- Kusuma, P., Pattison, P. M. and Bugbee, B. "From physics to fixtures to food: current and potential LED efficacy." Horticulture Research 7, 56 (2020).
- FAO. Good Agricultural Practices for Greenhouse Vegetable Production in the South East European Countries. FAO Plant Production and Protection Paper 230, 2017.
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