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Ventilation in the cannabis grow space

Why ventilate?

Nearly all living things depend on air of good enough quality. For people, that means enough oxygen is in the air and that the air is not too polluted. For plants, and so too for cannabis, good air means sufficient carbon dioxide (CO₂) and little to zero pollution. Furthermore, relative humidity and temperature play large roles in plant growth.

Outdoor air contains about 0.03 to 0.04% CO₂. The quantity of carbon dioxide is also measured in parts per million (ppm), when 0.03% translates as 300ppm. There are differences in CO₂-requirements between different plants, but for cannabis specifically, raising the CO₂ level to about 1400 ppm can help to accelerate plant growth. Beware however, for with this the law of diminishing returns applies, so creating raised CO₂ levels has its limits.

Raising it to about 1400 ppm (0.14%) generally delivers good results: plants will grow faster. Above 1400 ppm, the effect of adding CO₂ tails off quickly. Not only for humans but also for plants, a high concentration of CO₂ is poisonous - for most plants a CO₂ concentration of 1800 ppm or more is fatal.

A simple method of guaranteeing the level of carbon dioxide is to ventilate the space. There needs to be enough ventilation that the plants have a constant supply of fresh air. If they’re being grown out in the open, then this is not something we have to worry about, there’s fresh air aplenty. If we switch to growing indoors, we need another way of providing this fresh air that contains both oxygen and CO₂ for our plants, such as carbon burners and ventilation.

A second and equally important reason for ventilation is the removal of heat. When the temperature gets too high, the growth of the plants is hindered. This applies not just to the temperature in the grow space, but also for the temperature of the plant itself. When the temperature of the plant is too high (we humans call it a fever), then the sap stream in the plant is reduced, which in turn leads to growth retardation.

There is no standard solution for air refreshment. The need for fresh air is to a large extent dependent on the volume of the grow space in cubic metres. In principle every 2-3 minutes the complete volume of the grow space must be refreshed.

Manual ventilation

With manual ventilation you can cover a space where a couple of plants are stood, where they have access to fresh air for a few hours a day. In order for this room to have the fresh air, all you need to do is leave a door or window open for a period of time. If you’re not certain you can always do the odour test: if you walk into the room and it smells stale, then you can be sure it is time to crack a window or two and refresh the air manually.

Electrical ventilation

This method of refreshing the air involves the use of an electrically powered gizmo ranging from the simple bathroom-style fan, up to industrial ventilators depending on the size of your space and number of plants.

Methods of electrical ventilation:

  • A PC fan is suitable for small grow spaces, though if your lamps total more than 250 Watt you will need a different solution.
  • A ventilation hood or extractor fan is suitable for large grow spaces.
  • Tube ventilation is also suitable for large grow spaces.

Calculating ventilation needs

Suppose you have a space that’s 2.00 metres long, 1.00 wide and 3.00 high. You need the following calculation: L x B x H x 100 = 2.00 x 1.00 x 3.00 = 6.00 x 100 = 600 m3

Assuming a grow space that is 3 metres long, 2 metres wide and 2 metres high (= 12 m3), that means a ventilation capacity of 30 x 12 = 360 m3 per hour is needed.

That is the volume of air that you have to move with your ventilation and by doing so ensure that your plants have fresh air available to them at all times.



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