CO₂ in the Grow Room

Carbon dioxide (CO₂) enrichment is one of the most misunderstood and most misused tools in indoor cultivation. Done correctly, it can significantly increase photosynthetic efficiency, plant growth rates, and final yield. Done poorly, it adds cost, complexity, and risk with little to no benefit. This guide takes a technical but accessible look at how CO₂ functions in the grow room, why it benefits plant growth, and how to apply it correctly in real-world indoor environments. It’s not a sales pitch; it’s about understanding CO₂ and using it with intent.

Why CO₂ Matters: The Science in Plain Terms
Photosynthesis is the engine that drives plant growth. Using light energy, plants convert carbon dioxide and water into sugars, which fuel everything from root development to flower formation. In ambient air, CO₂ sits at roughly 400–420 parts per million (PPM). For indoor plants under high-intensity lighting, this level is often a limiting factor. When light, nutrients, and environmental conditions are optimised, plants can process far more CO₂ than the atmosphere naturally provides. Increasing CO₂ concentration removes that bottleneck, allowing photosynthesis to proceed faster and more efficiently.

In practical terms, CO₂ enrichment allows plants to do more with the light you’re already providing.

Key Benefits of CO₂ Enrichment
When applied correctly in a sealed indoor environment, elevated CO₂ levels can deliver several compounding benefits:
Increased photosynthesis: Higher CO₂ availability accelerates photosynthesis, particularly under strong LED or HID lighting.
    •    Enhanced growth and yield: Faster photosynthesis translates to larger plants, increased biomass, and improved yields.
    •    Improved nutrient uptake: With increased metabolic activity, plants are able to absorb and utilise nutrients more efficiently.
    •    Greater stress tolerance: CO₂-enriched plants often cope better with heat stress and environmental fluctuations.
    •    Shortened growth cycles: Faster growth can reduce total crop time, increasing annual turnover in controlled environments.

It’s important to note that these benefits only appear when CO₂ is the limiting factor. If light, temperature, or nutrition are suboptimal, added CO₂ will do very little.

The Non-Negotiables: When CO₂ Actually Works
CO₂ enrichment is not a plug-and-play upgrade. It only delivers results when several conditions are met:
  •   High light intensity (typically above 700 PPFD)
  •   A sealed or semi-sealed grow room
  •   Tight control of temperature and humidity
  •   A balanced and responsive nutrient program
If your grow room is actively exhausting air, running low light levels, or struggling with basic environmental control, CO₂ should not be your next step.

Setting Up a Sealed CO₂-Enriched Grow Room
The Grow Space
A CO₂-enriched room must be well sealed to prevent gas loss and outside air exchange. Any gaps, cable ports, or poorly sealed doors will quickly dump expensive CO₂ back into the atmosphere. Weather stripping, lightproof tape, and proper sealing around penetrations are essential. Ventilation ports and fans can still be installed, but in sealed rooms, they are typically used intermittently or for emergency heat management rather than continuous air exchange.
CO₂ Delivery Systems
There are two primary methods of CO₂ delivery:
  •   Compressed CO₂ tanks with regulators
  •   CO₂ burners (gas-powered generators)
Tank-based systems are the most common in small to medium indoor rooms due to their cleanliness, precision, and ease of control. Gas burners are typically reserved for large-scale or commercial environments with appropriate safety systems. Gas distribution lines should be installed at the top of the room, well above canopy height. CO₂ is heavier than air and will naturally fall downward through the plant canopy. Compressed CO₂ solutions such as rapid-release CO₂ systems are often used at lights-on to quickly elevate PPM levels before maintenance dosing begins.
👉 Check Out: Pure CO₂ Blast
Controllers and Monitoring
A CO₂ controller is essential. It monitors PPM levels and activates the CO₂ supply only when needed, preventing waste and dangerous over-enrichment. For accuracy, CO₂ levels should be measured as close to canopy height as possible. CO₂ is heavier than air and can stratify in poorly mixed rooms, meaning readings taken too high or too low may not reflect what the leaves are actually experiencing.
Wall-mounted controller/monitor units are ideal for sealed rooms, as they allow precise setpoints and automated control of solenoids or regulators. Portable or secondary monitors are strongly recommended as a safety backup and for spot-checking levels at multiple points within the room. Consistent air circulation is critical for even CO₂ distribution. Without it, pockets of high or low concentration can form, reducing overall efficiency.
Growers implementing controlled CO₂ enrichment typically rely on wall-mounted CO₂ monitor/controllers for automation, paired with compact standalone CO₂ monitors for safety verification and canopy-level checks
👉 Controller/Monitor: Wall Mounted CO₂ Controller
👉 Safety Monitor: Mini Co₂ Monitor

For growers running more advanced or fully integrated rooms, environmental controllers that manage CO₂ alongside temperature, humidity, and lighting offer a higher level of control and consistency. Systems such as digital CO₂ controllers allow precise PPM targeting, while full-room controllers can coordinate CO₂ delivery with HVAC, dehumidification, and lighting schedules to maintain stable conditions throughout the entire grow cycle. 

👉 Advanced CO₂ Control: TrolMaster CO₂ PPM Digital Controller (Beta-8)
👉 Full Room Integration: TrolMaster Hydro-X Plus Controller
👉 Tent-Scale Control: TrolMaster Tent-X Control System
For safety, a secondary CO₂ monitor is strongly recommended. CO₂ levels above 5,000 PPM are dangerous or fatal to humans and animals.

Target CO₂ Levels and How to Match Them to Light
Most indoor CO₂ enrichment programs operate between 1,000 and 1,500 PPM during active photosynthesis.
A useful rule of thumb is to match CO₂ levels to light intensity:
  •   If canopy PPFD is below 1,000, CO₂ PPM should not exceed light levels
      →   700 PPFD ≈ 700 PPM CO₂
      →   900 PPFD ≈ 900 PPM CO₂
  •   If canopy PPFD exceeds 1,000, CO₂ can be run between 1,000 and 1,500 PPM
This relationship exists because CO₂, light, and temperature all interact to drive photosynthesis. Increasing one factor without the others simply shifts the bottleneck. Running high CO₂ levels under insufficient light does not increase growth and can stress plants. During the final weeks of flowering, many growers reduce CO₂ levels to around 700 PPM as metabolic demand decreases. Maintaining stable CO₂ levels becomes increasingly difficult without automated control as light intensity, temperature, and plant demand change throughout the day - particularly in sealed or high-PPFD environments.

Timing: When CO₂ Should Be On (and Off)
CO₂ should only be supplied when plants are photosynthesising.
  •   Activate CO₂ during the light cycle
  •   Disable CO₂ during dark periods
If your controller does not include a light sensor, ensure CO₂ is manually disabled at lights-off. Supplying CO₂ in darkness wastes gas and provides no benefit.

CO₂ enrichment is also most effective once plants are established and light intensity is high. In early vegetative stages, plants are rarely CO₂-limited, and enrichment provides minimal benefit. The largest gains are typically seen from late vegetative growth through flowering, when canopy density increases and PPFD levels are highest.

Temperature Management in CO₂-Enriched Rooms
CO₂ enrichment shifts a plant’s optimal temperature range. With elevated CO₂ (700–1,500 PPM) and LED lighting, plants can thrive at temperatures 1–2°C warmer than standard targets for each growth stage. This is because increased CO₂ concentration reduces photorespiration and alters stomatal behaviour. With more CO₂ available, stomata do not need to remain open as widely, reducing transpiration and allowing plants to tolerate higher leaf temperatures without stress.

This directly ties into Vapour Pressure Deficit (VPD). In CO₂-enriched environments, plants can operate efficiently at slightly higher VPD values than they would under ambient CO₂, provided temperature, humidity, and airflow remain stable. This flexibility is one of the reasons sealed, CO₂-enriched rooms can outperform vented rooms under high light. For example:
  •   If the optimal flowering temperature is normally 28°C
      →   CO₂-enriched flowering can be run at 29–30°C
  •   Maintaining these conditions typically requires:
      →   Appropriately sized air conditioning (split systems preferred)
      →   Dehumidification matched to plant size and transpiration rates
      →   Strong internal air circulation to maintain consistent VPD across the canopy

Nutrients and EC Adjustments
As CO₂ drives faster growth, nutrient demand increases. When enriching above 1,000 PPM, base nutrient EC can often be increased by 5–15%, provided plants are healthy and environmental conditions are stable.
CO₂ will not fix nutritional imbalances. Overfeeding, underfeeding, or pH lockout will still limit performance, often more quickly in accelerated growth conditions.

CO₂ Systems vs CO₂ Bags
CO₂ Generating Systems
CO₂ systems offer precision, automation, and scalability.
They are best suited to:
  •   Sealed rooms
  •   High-light environments
  •   Growers seeking repeatable, dialled-in results
Advantages include consistent CO₂ levels and stage-specific control, balanced against higher upfront cost and ongoing refills or fuel.
Compressed CO₂ systems paired with controllers allow growers to maintain stable enrichment without relying on biological variables. When paired with integrated controllers, these systems can also synchronise CO₂ delivery with lighting and climate control, ensuring enrichment only occurs when plants can actively use it.
CO₂ Bags
CO₂ bags use biological processes to release CO₂ gradually into the grow space.
They are:
  •   Simple to use
  •   Cost-effective
  •   Well-suited to small tents or supplemental enrichment
CO₂ bags are among the most popular entry-level options for growers who want to trial enrichment without committing to a fully sealed room or a controller-based system.
They are available in different formats depending on region, including mushroom-based and non-mushroom CO₂ bags, as well as longer-life passive CO₂ generators designed for continuous low-level supplementation
👉 NZ: CO₂ Growbag
👉 AU (Non-Mushroom): Pure CO₂ Bag
👉 AU: ExHale 365
However, they offer limited control, and output depends on environmental conditions. In spaces with active extraction, CO₂ bags are most effective when extraction is run intermittently, allowing CO₂ to accumulate.
Placing bags low in the room and using fans to draw CO₂ upward through the canopy improves effectiveness.

Safety First
CO₂ is colourless, odourless, and dangerous at high concentrations.
Always:
  •   Use a controller and monitor
  •   Avoid entering sealed rooms during active enrichment
  •   Ensure emergency ventilation options are available
Levels above 5,000 PPM are hazardous to humans and animals.

Is CO₂ Right for Your Grow?
CO₂ enrichment is a force multiplier, not a shortcut. If your lighting, environment, and nutrition are already dialled in, CO₂ can unlock another level of performance. If they’re not, focus there first. Used correctly, CO₂ allows plants to fully capitalise on modern high-intensity lighting and controlled indoor environments, turning potential into measurable results.

From advanced advice to top-quality CO₂ products, we’re here to help you get the most from your grow. Come see us and take your setup further.