When to Add CO2 to a Grow Tent
The most expensive CO2 mistakes are timing mistakes, not equipment mistakes. Perfectly good tanks get run over seedlings that can't use the gas, through late flower where pushing harder hurts, and in dim tents where no amount of CO2 changes anything. The question 'when should I add CO2' has a precise answer, and it is two thresholds deep: one about light, one about the plant's stage.
Threshold one: light, because CO2 amplifies light
The first gate is light intensity. Photosynthesis needs photons and carbon in proportion, so extra CO2 changes nothing while light is the limiting factor — the plant simply cannot process more carbon than its incoming light allows. Practically, this means enrichment belongs in setups with substantial PPFD at the canopy: the recent average light your plants receive determines the enrichment range that makes sense, and the range rises with the light. The corollary is just as useful: a tent running modest light levels gets no enrichment target at all, and the correct action there is making sure ventilation keeps the level near ambient during the light period. If you have not measured your canopy's light, measure it before you buy gas — it is the input everything else depends on.
Threshold two: growth stage, and the two stages to skip
Even under ideal light, two stages should never be enriched. Seedlings are too small to consume meaningful CO2 — enrichment there is gas poured into an empty room. And late flower is the stage where pushing the plant harder works against you: the plant is finishing, not building, and forcing extra growth pressure at that point is counterproductive — which is why enrichment is deliberately not recommended during late flower or the seedling stage regardless of how strong the light is. The stages where enrichment makes sense sit between them: a vegetative plant building structure under strong light is the textbook case, and mid-flower under adequate light can benefit. The stage cap is not a style preference; it reflects what the plant is physiologically able to do with the extra carbon.
Timing within the day and the cycle
Within the light period, CO2 only matters when the lights are on — photosynthesis stops in the dark, so injecting CO2 during the night cycle is pure waste. The enrichment window is the lights-on schedule, and consistent coverage through it beats an intermittent blast: plants respond to a steady level, and a tent whose ppm spikes and collapses with a timer or fan cycle never holds a target at all. Across the cycle, start enrichment when the plant has enough canopy under enough light to consume the gas — usually once vegetation is established and light levels are at their working maximum — and stop it as late flower approaches. Between those bookends, the weekly logged reading against your target is what tells you whether the setup is actually holding the plan.
The cheap alternative: never fall below ambient
There is a version of 'when to add CO2' that most growers should answer first: when is my tent losing CO2 it already has. A sealed or semi-sealed tent under strong light can pull the ppm below ambient within hours of lights-on, because dense canopies consume CO2 faster than a closed room replenishes it. In that situation, the payoff is not supercharging photosynthesis above ambient — it is restoring the level your plants assumed they had. Venting fresh air or injecting just enough CO2 to hold ambient through the light period costs a fraction of full enrichment and fixes a real, measurable deficit. Check your logged readings first: if they never dip meaningfully below ambient, you have no CO2 problem, and if they do, you may not need enrichment at all — just less starvation.
How GrowScope helps
GrowScope encodes both thresholds directly: the CO2 target range is calculated from your plant's recent average PPFD — so a light-limited setup gets no enrichment target at all — and is capped by the current growth stage, with enrichment deliberately not recommended during seedling or late flower. Weekly logged ppm readings against that target show whether your tent is holding the plan or sliding below it.
Key takeaway
Add CO2 only when two thresholds clear — canopy light strong enough that photosynthesis isn't light-limited, and a growth stage between seedling and late flower — and check first whether your tent is even falling below ambient, because preventing CO2 starvation is cheaper than chasing enrichment.