CO2 Enrichment: When It Is Worth It and How to Do It Safely

Every grower measures temperature. Most measure humidity. Almost nobody measures the one input a plant gets through fastest of all: carbon dioxide. Plants are built out of carbon, and every gram of it came out of the air around the leaves. In a closed tent under a strong light, a healthy canopy can strip that air of usable CO2 in about an hour, and from then on the light you are paying for is doing very little. This guide covers what CO2 does, how to tell whether your room is short of it, when enrichment pays and when it does not, what the bags, bottles and controllers actually do, and the safety rules that come with putting a gas into a sealed room.

Why CO2 is the input nobody measures

Photosynthesis is simple to state: light, water and carbon dioxide go in, sugar and oxygen come out. The sugar is what the plant is built from. Roughly half the dry weight of any plant is carbon, and none of it came from the pot — it was pulled out of the air through the pores on the underside of the leaves.

A plant grows at the pace set by whichever input is scarcest. Give it more light than it has CO2 to use and the extra is wasted, some of it as heat stress. The room can look perfect on every meter you own and still run at half speed because the one thing you are not measuring has run out. Outdoors a breeze replaces what a leaf takes; in a tent with the zips shut and the extractor turned down, it happens every day, invisibly.

The numbers: 420, 200 and 1,200

CO2 is measured in parts per million. A thousand ppm is a tenth of one percent, which sounds like nothing and is the whole game. Six figures are worth knowing.

A room is not at “ambient” because the door was open this morning. Its air is whatever the plants, the extractor and the people in the house have made it, and in a working tent it can be a long way below outdoor air by mid-morning.

Measure before you spend anything

This is the most useful thing in the guide and it costs less than any of the kit below. Before you buy anything, put a CO2 reading in the tent at canopy height, run a normal lights-on period, and watch the number. There are three answers.

1. It sits around 400 ppm all day. Your extraction is keeping up with the plants. You are not short of CO2, but you are capped at outdoor levels, and the only way up is to add some — which also means changing how you ventilate.
2. It falls well below 350 ppm during lights-on. Your plants are starving themselves and the extractor is not keeping up. This is the cheapest problem in horticulture: turn the extraction up, add an intake, open a passive vent. The cheapest CO2 in the world is the 420 ppm outside your window. Our fan sizing guide covers how much air exchange a tent needs.
3. It swings — high when the extractor rests, low when it runs. Common on a thermostatic fan controller. This room gains most from a controlled top-up, because it already holds air for part of the cycle.

Any NDIR CO2 monitor will do for this test, including the household air-quality type. If you already own one of the controllers below with a CO2 sensor, it shows the live figure on its display or in its app.

When enrichment pays — and when it does not

Enrichment is expensive to do properly and pointless to do badly, so be clear-eyed about the three conditions under which it makes any difference.

1. Light has to be the thing you have plenty of

CO2 only helps when CO2 is the bottleneck. Under modest light the plant is already using every photon it gets and has no spare energy to turn extra CO2 into sugar. Roughly, below about 500 µmol/m²/s at the canopy, enriched and unenriched plants grow at the same speed. From about 700–800 µmol upwards the room runs out of CO2 before it runs out of light, and enrichment starts to pay; a room at 1,000–1,200 ppm can then use 1,200–1,500 µmol that would stress the same plants at ambient CO2.

Put numbers on your own room. A 200 W LED over a 1.2 m tent, at a typical efficacy of about 2.7 µmol per joule, delivers roughly 375 µmol/m²/s averaged across the canopy. That tent is light-limited and CO2 will do almost nothing for it; the money belongs in the light. The same tent under 600 W is in a different position entirely. Our light height and PPFD guide explains how to work out what your canopy actually receives.

2. The room has to hold the gas

An extractor running continuously replaces the whole volume of a tent every minute or two. Add CO2 to that tent and it goes straight up the duct. This is why a bag or generator hung in a normally ventilated tent achieves so little: not that it makes no CO2, but that the extractor removes it as fast as it arrives. Enrichment only works in a room that is sealed, or one where the extractor runs in short bursts and the CO2 is re-dosed afterwards. That is a bigger change to how you run a tent than the CO2 itself.

3. Everything else has to be right already

CO2 is the last 15–20%, not the first. If temperature drifts, humidity is uncontrolled, the feed is out of balance or the pH is wrong, the plant is limited by those things and adding CO2 will not show. Fix the environment, fix the feed, fill the light, then look at the air.

The sealed-room problem

Here is the catch that makes CO2 more than a purchase. The moment you stop the extractor to keep the gas in, you have also stopped removing heat and humidity. A 600 W light in a closed 1.2 m tent pushes the temperature past 30°C in well under an hour, and the plants keep transpiring into air that has nowhere to go. Sealing a room means taking over two jobs the extractor used to do for free. Growers handle it three ways.

Most home growers end up semi-sealed, and it works well provided the room is not fighting heat all day. The CO2 controller and the fan controller need to know about each other, and several units below do this natively: the Pro-Leaf has a mode that runs an extractor to pull levels down, and the AC Infinity AI+ can trigger exhaust fans from the CO2 reading. Vent for two minutes every quarter-hour and you lose four rooms-full of gas an hour; every hour, one. Our guides to keeping a tent cool and controlling humidity are the groundwork.

Bags, bottles and burners: how the methods compare

Bags and generators: honest expectations

The passive options are cheap and there is nothing to plumb in, so be realistic about them. A mycelium bag takes four to six weeks to reach full output and then gives a slow, steady trickle. With the extractor running, that trickle leaves as fast as it arrives; in a tent kept mostly closed it lifts the level meaningfully, but you cannot set a target and you cannot turn it off. The pumped generators are a step up because the pump is on your light timer, so the output goes in while the plants can use it; the manufacturer’s figures for the unit we stock are a lift from around 400 to around 800 ppm over 2.4 m², with the culture needing 22°C or more to stay active.

Neither will take a room to 1,200 ppm and hold it there. What they will do is stop a closed tent being drawn down to 200 ppm, which for a light-limited grower may be all the benefit that was available anyway.

Bottled CO2: the method that actually holds a number

A cylinder of compressed CO2 is the only home-scale method that can take a room to a target and keep it there. The parts are a cylinder, a regulator with an electric solenoid valve, and a controller with a CO2 sensor. The controller reads the room, opens the valve when the level drops below the set point, and closes it when the target is reached. That is the whole system.

We sell the regulator and the controllers; the cylinder comes from a gas supplier. The food-grade cylinders pubs use for drinks dispense are what most growers run — 6.35 kg is the common size — on exchange from pub-gas, fire-safety and welding suppliers. Check the regulator’s thread matches the cylinder valve when you order; adaptors exist for a reason.

Burners

Propane and natural-gas generators are how large rooms and glasshouses do it: burning gas makes CO2 in quantity, and the heat is wanted in a big cold space. In a tent the heat is the last thing you need, and a burner brings the combustion risks of a gas appliance into a small enclosed room. The Pro-Leaf controller will run an LPG generator as well as a bottle, but for anything smaller than a room you can walk around in, bottled CO2 is the sensible choice.

Controllers and sensors: how the dosing actually works

Every serious CO2 system is built around the same sensor technology, so it is worth knowing what you are buying.

NDIR means non-dispersive infrared: the sensor shines an infrared beam through a small chamber and measures how much the CO2 absorbs. It is accurate to around ±50 ppm in most grow-room units and does not wear out like a chemical cell, but it needs occasional calibration. “Self-calibrating” usually means the sensor assumes the lowest reading it sees each day is outdoor air; in a room enriched all day that assumption fails and the sensor drifts. The fix is a manual calibration in fresh air every few months, as the manufacturers below recommend.

The photocell is the other feature to look for. Plants do not take up CO2 in the dark — they release a little — so gas dosed at night is wasted. A controller with a built-in light sensor refuses to dose until the lights are on. Every controller below has one.

Deadband and dosing logic decide how tightly the level is held. A deadband of 100 ppm around a 1,200 ppm target opens the valve at 1,100 and closes it at 1,200; too narrow and the valve chatters, too wide and the plants see a sawtooth. Better controllers dose in short pulses and learn how fast the room responds, which also protects against the classic bottled-CO2 failure: a solenoid held open so long it has frozen and stuck.

Standalone or part of a system

You can buy a CO2 controller as a single box, or as a module in an environmental controller that already runs your fans, heaters and humidity. If you have no controller yet the system route is usually better value, because the CO2 module inherits the fan logic a sealed cycle depends on. The cheapest way in the shop to actually read ppm is the AC Infinity AI+ with its CO2 and light sensor, under £190 as a bundle, which also runs your fans against the reading. The TrolMaster Hydro-X takes a base station, the MBS-S8 sensor and the DSC-2 switching station to do the same job in a more expandable form; the Dimlux Maxi Controller does it with the dual-beam sensor professional Dutch rooms have used for years.

Setting it up: placement and mixing

CO2 is about one and a half times as dense as air. Left alone in a still room it sinks and pools at floor level, which is bad for two reasons: the leaves are not at floor level, and neither should your face be when you crouch to check the pots. Everything about placement follows from that.

1. Release it above the canopy. Run the outlet tubing along the top of the tent, or hang a bag or generator above the plants, so the gas falls through the leaves on its way down. A length of drip-irrigation line with a few holes pricked in it makes a perfectly good distribution rail.
2. Mix it. A circulation fan is not optional in an enriched room. Without one the gas stratifies, the sensor reads a different number from the one the leaves see, and the controller is confidently wrong. An oscillating clip fan or two, stirring the whole volume, is enough in a tent.
3. Sensor at canopy height, away from the outlet. Level with the top of the plants, out of the direct stream from the release point, out of the fan blast, and not where you stand to work.
4. Close the room while dosing. Extractor off, passive intake flaps shut. If the extractor is on a controller, that controller and the CO2 controller need to agree: vent for heat when necessary, then re-dose.
5. Watch temperature and humidity. A sealed tent climbs fast on both. A thermo-hygrometer at canopy height, ideally one that logs, tells you how long you can hold the room closed before you must vent — the number your whole gas bill hangs on.

What it actually costs to run

The gas is cheaper than most people expect, and the reason also tells you where the money really goes.

Take a 1.2 x 1.2 x 2 m tent, about 2.9 cubic metres. Raising it from 420 to 1,200 ppm is a rise of 780 ppm, or 0.078% of the volume: about 2.25 litres of CO2 gas. A kilogram of CO2 expands to roughly 540 litres at room temperature, so one fill uses around four grams, and on paper a 6.35 kg cylinder holds well over a thousand fills. The plants are not a heavy draw either: a canopy photosynthesising hard fixes around two to five grams of CO2 per square metre per hour, so that tent’s plants take perhaps 40–80 grams over a twelve-hour day. In a perfectly sealed tent a cylinder would last a whole crop.

What actually empties the cylinder is venting. Every time the extractor runs to dump heat it takes the room-full of enriched air with it, and the controller has to buy it back. Vent ten times an hour and you throw away ten fills an hour, roughly ten times what the plants use. That is why a cylinder lasts a season in a well-controlled room and a fortnight in a hot, leaky one, and why the real cost of CO2 is almost always the cooling that lets you keep the room shut. Refill prices vary by supplier; the cylinder deposit is often the larger first-time cost. Our ducting guide covers the passive leaks — duct ports, zips, poorly sealed intakes — that bleed a tent between vent cycles.

Safety: the bit that is not optional

CO2 is not toxic the way carbon monoxide is, but it is an asphyxiant, colourless, odourless and heavier than air, and in a small sealed room with a cylinder holding thousands of litres those facts deserve respect. The UK workplace limits are 5,000 ppm averaged over eight hours and 15,000 ppm over fifteen minutes; a few thousand ppm brings headaches and drowsiness, higher brings breathlessness, dizziness and eventually unconsciousness. A room at 1,200 ppm is nowhere near those figures. A room where a solenoid has stuck open on a full cylinder can be. These rules keep it boring.

• Fit an independent CO2 alarm — a second, separate monitor that sounds if the controller or its valve fails. If the room adjoins a bedroom, put one there too.
• Never sleep in or next to an enriched room. Tents in bedrooms and cupboards off bedrooms are the case to think about: gas leaking out at floor level into a room where someone is asleep is the realistic worst case.
• Use a controller with a time cut-off. Good units close the valve after a set dosing time regardless of the reading, so a failed sensor cannot empty a cylinder into the room. Pulse dosing also stops the solenoid freezing open.
• Secure the cylinder upright, chained or strapped, and out of the direct heat of the lamp; cylinder pressure rises with temperature.
• Leak-test every joint with soapy water when you first connect the regulator and after every cylinder change. A hiss you cannot hear over the fans is a cylinder that empties in a day.
• Air the room before working in it. If the monitor shows more than 1,500–2,000 ppm, run the extractor for a couple of minutes first. A glance at a plant is fine; an hour of maintenance at 1,500 ppm is not.
• If you use a burner, add a carbon monoxide alarm. CO, not CO2, is what kills people with gas appliances, and no CO2 controller can see it.

Seven mistakes worth avoiding

1. Enriching a tent the extractor is emptying. The most common mistake by a distance. If the fan runs continuously, you are paying to enrich the garden.
2. Buying CO2 before buying light. Under a small light the plant cannot use it. Fill the light first.
3. Dosing at night. Nothing happens except the cylinder emptying. Use a photocell, or a timer that follows the lights.
4. Sensor in the wrong place. Next to the outlet or on the floor it reads high and under-doses; in the fan stream it reads low and over-doses.
5. Trusting “self-calibrating” forever. Automatic calibration assumes the room sees fresh air daily. An enriched room does not.
6. Sealing the room and forgetting the humidity. A closed tent of transpiring plants hits 80% RH quickly, and 80% at 28°C invites grey mould. The humidity plan comes before the CO2 plan.
7. No independent alarm. The controller cannot warn you about its own failure.

Quick answers

What CO2 level should a grow room be?

Outdoor air is around 420 ppm and a well-ventilated room sits there. For enrichment, 800–1,200 ppm is the working band, with 1,000–1,200 ppm the usual target in a sealed room under strong light. Above about 1,500 ppm the gains flatten, and sustained levels well above 2,000 ppm do harm. The number most growers never check is the low one: a closed tent with a healthy canopy can fall to around 200 ppm within an hour of lights-on.

Does CO2 enrichment work in a grow tent with an extractor fan?

Not while the extractor is running. It replaces the whole volume of a tent every minute or two, so any CO2 you add leaves almost immediately. Enrichment only works in a room that is sealed, or one where the extractor runs in short bursts on a temperature or humidity controller and the CO2 is re-dosed afterwards. That change to how you ventilate is a bigger job than the CO2 itself.

Do CO2 bags actually work?

They produce CO2, and in a tent kept mostly closed they lift the level meaningfully above what the plants would otherwise have. What they cannot do is reach a target and hold it, because there is no sensor and no control: expect them to stop a closed tent being drawn down to 200 ppm, not to take it to 1,200. In a normally ventilated tent most of the output leaves with the exhaust.

Should CO2 run at night?

No. Plants only take up CO2 while they are photosynthesising, which needs light; in the dark they release a little themselves. Gas dosed at night is simply wasted, and it is the most common reason a cylinder empties faster than expected. Every controller worth buying has a photocell that blocks dosing when the lights are off; a pumped generator goes on the same timer as the light.

Where should the CO2 sensor go?

At canopy height, level with the top of the plants, because that is where the reading matters. Keep it out of the stream from the release tube and the blast of a circulation fan; CO2 pools low, so a sensor on the floor reads high and the controller under-doses. And not where you stand to work: your breath is around 40,000 ppm and throws the reading off for minutes.

Is CO2 dangerous in a grow room?

At enrichment levels, no — 1,200 ppm is a fraction of the 5,000 ppm UK eight-hour workplace limit. The danger is a failure: a solenoid stuck open on a full cylinder can take a small sealed room to hazardous levels, and CO2 is colourless, odourless and heavier than air. Fit an independent CO2 alarm, never sleep in or beside an enriched room, use a controller with a time cut-off, secure the cylinder upright, leak-test every joint, and air the room before working in it. With a gas burner, add a carbon monoxide alarm too.

How much difference does CO2 make?

It depends entirely on whether CO2 was the thing limiting your plants. Commercial glasshouse growers would not pay for it if the effect were small; in a room with plenty of light, good climate control and a canopy running short of CO2, the gain in growth rate is substantial. In a light-limited room — a small LED over a tent — the honest answer is close to nothing. Measure your room and work out your light level before spending a penny.

Can I use CO2 enrichment for tomatoes, chillies and salad crops?

That is where the technique comes from. Commercial glasshouses have enriched tomato, cucumber, pepper, lettuce and cut-flower crops for decades, and in the Netherlands industrial waste CO2 is piped directly to growers for it. Every figure in this guide applies to any plant with green leaves; fruiting crops under strong supplementary light are the textbook case.

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