When to Stop CO2 During Flowering: The Art of Right Timing

Carbon dioxide (CO2) is one of the raw materials plants use to make sugars. Indoor growers sometimes add CO2 to a sealed grow room to speed up growth.

However…

More is not always better. Too much CO2 can damage plants, and high levels are a real health risk for people in the room.

This guide covers what CO2 does, safe levels, and when many growers stop adding it during flowering. Home growing is legal only for adults in some states; cannabis remains illegal under federal law.

The Role of CO2 in Growing Marijuana

Using CO2 in the grow room can help, but only when light, temperature, water and nutrients are also adequate. This is true for most green plants, not just cannabis.

Enhancing Photosynthesis

Plants use CO2 in a process called photosynthesis. They use light, water and CO2 to make sugars, turning light energy into chemical energy.

According to Ontario's agriculture ministry, raising CO2 to about 1,000 ppm can increase photosynthesis by roughly 50% over normal outdoor levels in most greenhouse crops.

Boosting Growth and Yield

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Faster photosynthesis means more sugars for growth. In a well-lit, sealed room, that can mean bigger plants and heavier harvests.

The benefit shrinks at low light. If your lights are weak, extra CO2 adds little.

Read Also: Best Cannabis Bud Boosters

What CO2 Does Not Do

CO2 is not a cure-all. There is little evidence that it makes plants more resistant to disease.

Claims that CO2 changes the amount or type of terpenes your plants produce are not well established for cannabis. Genetics, light and harvest timing matter more for aroma.

When to Stop CO2 Supplementation

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As plants move toward the end of flowering, buds stop adding much new growth and focus on ripening. This applies to every photoperiod plant, from the most potent marijuana seed’s flowering stage to milder strains: extra CO2 adds less at this point.

Many indoor growers stop supplementing CO2 during the last one to two weeks of flowering.

This is a grower practice, not a rule backed by research. The main reason is cost: plants grow less at the end, so extra CO2 does little.

What Happens if You Continue CO2 Supplementation

Keeping CO2 running through harvest at a normal target level is unlikely to ruin a crop. It mostly wastes gas or fuel.

The real problems come from levels that are too high:

• Plant damage: Ontario's greenhouse guidance warns that higher than recommended CO2 levels can damage leaves on some crops.
• Burner byproducts: Fuel-burning CO2 generators can release ethylene and carbon monoxide when combustion is incomplete, and ethylene can harm plants.
• Human health: High CO2 is dangerous for people, as the next section explains.

Best Practices for CO2 Supplementation in a Grow Room

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CO2 helps only when the rest of the environment is right. Below are the basics.

Keep People Safe

The OSHA permissible exposure limit for carbon dioxide is 5,000 ppm averaged over an 8-hour workday. Ontario's guidance notes that 5,000 ppm can cause dizziness or lack of coordination.

Never enter a CO2-enriched room without checking the level first. Ventilate before you work inside.

If you use a burner, use only equipment rated for indoor use and install a carbon monoxide alarm. Colorado's health department advises using only CO2 generators that are safe for indoor use.

Monitoring and Measurement

Use a reliable CO2 monitor to track the level in your grow room.

For most crops, Ontario's greenhouse guidance puts the saturation point at about 1,000 to 1,300 ppm under ideal conditions. Many cannabis growers aim for a similar range during flowering, with strong lighting.

Installing a CO2 controller in your grow room can automate this. It switches CO2 on and off to hold your target level.

Tailoring to Growth Stages

Young seedlings and clones need less. Ontario's guidance suggests lower levels, around 800 to 1,000 ppm, for raising seedlings of other crops.

Supplementation pays off most in late vegetative growth and early to mid flowering, when plants grow fastest.

Timing for Maximum Effect

Add CO2 only while the lights are on. Plants do not photosynthesize in the dark, so CO2 at night is wasted.

Air Circulation and Ventilation

Fans help spread CO2 evenly around the plants. Without air movement, the air right next to the leaves can run low.

Enrichment works best in a sealed room. If exhaust fans run constantly, the added CO2 leaves the room. Time CO2 for periods when the exhaust is off, and ventilate fully before anyone works inside.

Conclusion

Photo from Grow Weed Easy

Knowing when to stop CO2 during flowering matters less than keeping levels safe. For indoor marijuana growers using sealed rooms, a target around 1,000 to 1,300 ppm with strong light is a common approach.

Many growers stop adding CO2 in the last week or two of flowering, mainly to save cost. Always protect the people in the room: monitor CO2, use indoor-rated equipment, and ventilate.

Frequently Asked Questions

What is the recommended CO2 level during the flowering stage of marijuana?

Many growers aim for about 1,000 to 1,300 ppm with strong lighting. That matches the saturation range Ontario's greenhouse guidance gives for most crops.

Does CO2 help during the plant’s vegetative stage?

Yes. Vegetative plants can use extra CO2 when light is strong. Seedlings need less.

Is too much CO2 bad?

Yes. Very high levels can damage plants and are dangerous for people. OSHA's exposure limit is 5,000 ppm over an 8-hour workday, and 5,000 ppm can cause dizziness.

Sources

• Supplemental carbon dioxide in greenhouses, Ontario Ministry of Agriculture, Food and Agribusiness
• Carbon Dioxide, Occupational Safety and Health Administration
• Home grow laws, Colorado Department of Public Health and Environment