CO₂ enrichment in polyhouses:
what the yield data
actually shows

CO₂ enrichment is one of the oldest and best-supported interventions in protected cultivation. It is also routinely oversold, and the conditions attached to the published gains are usually left out. This is the honest version, with the Indian complication that most international sources do not address.

Start with depletion, not enrichment

The first thing worth understanding is that a closed polyhouse does not sit at ambient CO₂. It sits below it.

Under strong light, a healthy crop canopy consumes CO₂ rapidly. In a sealed or minimally ventilated house, consumption exceeds the rate at which CO₂ leaks in. Measured levels in closed houses commonly fall to 250–350 ppm by mid-morning, well below the 415–450 ppm outside.

The consequence: the crop becomes CO₂-limited long before the day is over. All the light, water and nutrition you are supplying cannot be converted, because the input that limits photosynthesis has run out. You are already losing yield relative to what the house could do — before any enrichment is considered.

This is why measurement should precede investment. A grower who discovers their house runs at 280 ppm at 11 a.m. has found a problem that ventilation alone can partly fix, at effectively zero running cost, by bringing the house back to ambient. Enrichment above ambient is the next step, not the first one.

What the research supports

The physiological mechanism is well established. Most horticultural crops — tomato, cucumber, capsicum, lettuce, most leafy greens, roses and other ornamentals — use the C3 photosynthetic pathway. In C3 plants the enzyme rubisco fixes both CO₂ and oxygen, and the oxygen reaction (photorespiration) wastes energy. Raising CO₂ shifts the balance toward carbon fixation and suppresses photorespiration, so photosynthetic rate rises.

That is real, repeatable plant biochemistry, and it is why commercial greenhouses in the Netherlands, Canada and elsewhere have enriched routinely for decades — they would not spend the money otherwise.

The yield responses reported in the literature vary widely by crop, light level, season and enrichment strategy, which is exactly why a single headline percentage is not a useful thing to quote. We summarise the published figures with their sources on our CO₂ and greenhouse yield research page rather than reducing them to one number here.

The conditions attached

This is the part that gets omitted, and it determines whether enrichment pays for you.

Light must not be limiting. This is the big one. Photosynthesis needs light energy and CO₂ together, and the limiting factor sets the rate. Under low light, adding CO₂ does essentially nothing, because the plant cannot use it. Enrichment pays best in bright conditions — which in India means it is most valuable in the middle of clear days, and least valuable in monsoon overcast or in a house with heavy shade netting.

Water and nutrients must not be limiting. Enrichment increases growth rate, which increases demand for everything else. A crop short of nitrogen or water will not convert extra CO₂ into extra yield, and pushing it can make matters worse.

Temperature must be in range. Enrichment raises the optimum temperature somewhat, but a house that is already too hot has a bigger problem than CO₂.

The crop must be C3. C4 species — maize, sugarcane, sorghum, amaranth — already concentrate CO₂ internally and respond far less. Most protected horticulture is C3, so this rarely bites, but it is worth checking.

Liebig's law of the minimum governs all of this: growth is limited by the scarcest resource, not the total. Adding CO₂ only helps if CO₂ is the resource currently running out. In a closed, brightly lit house, it very often is. In a shaded house in monsoon, it very often is not.

The Indian problem: you have to vent

Here is what makes Indian polyhouse enrichment different from a Dutch glasshouse.

In a cold climate the house is closed for much of the year to retain heat, which is exactly the condition where enrichment works — the CO₂ stays in. In most of India, the polyhouse must be vented for a large part of the day for the opposite reason: to get heat out. And an open vent exhausts enriched CO₂ as fast as you can supply it.

This produces a genuine conflict:

• Vents closed — enrichment is effective and cheap to hold, but the house overheats.
• Vents open — temperature is controlled, but the house sits at ambient CO₂ and enrichment gas is simply blown away.

Growers who buy CO₂ equipment without accounting for this find that most of the gas leaves through the vents, and reasonably conclude that enrichment does not work in India. What does not work is enriching a ventilating house.

What does work

The realistic strategies for an Indian polyhouse, roughly in order of cost:

1. Fix the depletion first. Before adding any CO₂, make sure the house is not sitting below ambient during closed periods. Circulation and modest ventilation bring it back to 400+ ppm. This costs almost nothing and captures the largest and most certain part of the available gain.

2. Enrich during closed periods only. Early morning after sunrise, before the house needs venting, is typically the best window — light is rising, temperature is still moderate, vents are shut, and the crop's demand is high. A controller that enriches only while the vents are closed converts an unaffordable strategy into an affordable one.

3. Enrich to a modest band. 700–900 ppm is often the practical target in a house with any leakage, rather than the 1,000–1,200 ppm used in tightly sealed glasshouses. The marginal yield gain from the top of the band is small and the marginal gas cost is not.

4. Control it, do not schedule it. Vent state, light and crop demand all vary hour to hour and day to day. A schedule cannot track that. A controller measuring actual CO₂ and switching the source accordingly is what makes the economics work.

What this looks like as a control problem

Stated plainly, a polyhouse needs a device that does two opposite things from one measurement:

• Below the lower threshold — switch the CO₂ source on. The house is depleted and the crop is being limited.
• Above the upper threshold — switch the ventilation fan on. Either you have overshot, or heat and humidity need managing anyway.

That is exactly the two-socket configuration the VentPlus greenhouse unit implements: one socket switching a CO₂ generator or solenoid, the other switching a ventilation fan, both driven from the same continuously measured level, with the band set by the grower. Our greenhouse page walks through the logic and the day-cycle in detail.

What to do this season

1. Measure a full clear day. Sunrise to sunset, sensor in the canopy. You are looking for the mid-morning minimum — if it is below 350 ppm, the crop is CO₂-limited and you have found real, recoverable yield.
2. Check it against vent state. Note when the vents open. The window between sunrise and first venting is your enrichment opportunity, and its length determines whether enrichment is worth doing at all.
3. Fix depletion before buying gas. Cheap, certain, and it tells you what the house does at ambient.
4. Then enrich the closed window, to a modest band, under control.
5. Keep the record. Compare CO₂ logs against yield across cycles. That is how you find out what your house actually responds to, rather than what a brochure says it should.