Growing Marijuana in Rockwool

Almost all living beings are dependent on light of satisfactory quality. For humans, that means that sufficient oxygen must be present in the air, and that the air is not too polluted. For plants, and thus also for cannabis, it means good air quality, enough carbon dioxide (CO2), and not to much pollution. Relative humidity (RH), and temperature also play a large role in the growth of plants.

Influencing air quality

The amount of CO2 in the open air is appoximately 0.03 to 0,04%. The amount of carbon dioxide is also expressed in parts per million; ppm. 0.03% is equal to 300 ppm. There are differences in the CO2 needs among plants. By raising the CO2 content, growth can be accelerated. The law of diminished returns still holds true, however. Raising the CO2 level has limits, but at approximately 1400 ppm (0.14%), good results (a faster growth) are generally achieved. Above 1400 ppm, the effect of a higher percentage of CO2 decreases. A high concentration of CO2 is poisonous even for plants. A CO2 concentration of 1800 ppm or more is deadly for most plants. A simple method for guaranteeing the supply of carbon dioxide is to ventilate the room. Sufficient ventilation must be provided, so the plants keep getting enough fresh CO2. A second and just as important reason for ventilation, is to dispose of excess heat. If the temperature gets too high, (see Section 5.4), growth is stunted. This counts not only for the temperature in the grow room, but also for the temperature in the plant itself. When the plant’s temperature is too high (humans get a fever), there is less sap flow, causing growth disturbances. There is no standard solution for refreshing the air. The need for fresh air is, for a large part, dependent on the size of the grow room in cubic meters. In principal, the total air content of the room must be exchanged every 2-3 minutes. Using for example a grow room 3 meters long, 2 meters wide, and 2 meters high (12m3), this means that the ventilator capacity must amount to 30 x 12 = 360 m3 per hour. A standard bathroom ventilator can only handle up to 100 m3 per hour Many growers ventilate their rooms with table fans. The point is the control of the temperature as well as the circulation of the air with sufficient carbon dioxide. Table fans are primarily intended to keep people comfortable on a hot summer day. They are much less suited to run continually for heat removal, and for CO2-content maintenance. Table fans have a tendency to melt with intensive use. You can imagine the consequences: not only the danger of fire, but also massive plant death . . . There are, of course, plenty of fans on the market which will take care of proper ventilation. These have been specifically designed to be able to run continually. The CO2 content in the grow room can also be heightened by adding CO2 from a tank. If the system is set with a timer clock, the desired amount of CO2 can be regularly released. Work with care, because you don’t know how much CO2 is in the room at any given moment. An overdose can easily occur To prevent this, it’s sensible to ventilate the area well before each CO2 ‘injection’. The most professional option is to use a CO2 controller. This apparatus continually measures the CO2 content in the room. When the programmed minimum value is reached, CO2 is automatically added. If the programmed maximum is exceeded, the controller turns on the ventilating system. If CO2 is added to the room via a tank, or a controller, cultivation can take place at a higher temperature. (More about this aspect in Section 5.4.) Ultimately, attention must be given to the relationship between ventilation, and the relative air humidity. The humidity of the air is dependent, among other things, on the amount of air moved through the room. Changing the air draws more moisture out of the plants, because the stomata release more moisture. If the relative humidity of the air drops too low, the stomata close, delaying the growth process.

Relative humidity (RH)

The relative humidity of the air influences the functioning of the stomata, among other things. Cannabis flourishes the best with an RH of 60-70%. At higher RH percentages, the stomata have problems getting rid of excess water. At a lower RH, the stoma keep releasing water until the plant dries out. At that moment, the stomata close. Then, the intake of CO2 stagnates, and plant growth is impaired. The relative air humidity is also influenced by the temperature in the growing space. In the chart below, you can see the number of grams of water which can be absorbed in a 25 m3 room (for example: 3 x 3 meters, and 2.5 meters high). Absorption in grams of water (degrees C) 0 degrees 120 10 degrees 240 20 degrees 460 25 degrees 630 30 degrees 840 35 degrees 1120 40 degrees 1460 It may be concluded from this chart, that with every rise of 10 degrees in temperature, the air humidity doubles. Ventilation influences the relative humidity. Ventilating a space makes the RH fall. In some cases it’s necessary to install a humidifier in the grow room. The best results can be achieved by using a discharge fan with a variable speed control. This way, you can easily regulate the quantity of air to be removed. When the plants are in the dark, the temperature is lower (the lamps don’t give off any heat). So, you would expect the relative humidity to fall (less moisture can be absorbed by the air). But this is not the case; RH increases in the dark. The plants breathe out water in darkness. Therefore, sufficient ventilation must be provided. Too high a humidity level provides considerable risks for the health of the plants. Generally, pests and diseases (see Chapter 8) have a better chance with a high humidity level. Too low an RH is also risky; the plants can easily dry out. Prevention is better than cure . . . Finally, it should be stated that young seedlings and clones generally perform better at a humidity level of 65-70%. Their root systems are not yet developed well enough to take in water fast enough. A higher humidity insures that the young plants will be protected from drying out.


The high-pressure gas lamps we use for cultivation cause a considerable amount of heat in our closed-off grow space. This heat can be damaging to the plants. In the first place, we have to make sure the plants are not too close to the lamps. A distance of approximately 40 centimeters (for 400 Watt lamps), or 50 centimeters (for 600 Watt lamps) is good. The lamps also warm the air in the room. This heat must be discharged via the ventilation system. Cannabis seems to grow best at a temperature of 25 to 26 degrees Celsius. This temperature must not be allowed to rise any higher in grow rooms where no CO2 enrichment takes place. When working with bottled CO2, or even a CO2 controller, the temperature can be a little higher; 27 to 29 degrees. When working at higher temperatures, the RH must be closely monitored. Every 10 degree rise in temperature means that the absorption capacity of the air nearly doubles (see Section 5.3). In the dark period, the temperature may drop a little, but not too much. If the temperature is too low during the dark period, molds have a better chance A temperature of approximately 20 degrees Celsius is ideal for darkness. In order to maintain an optimal temperature, you need a discharge ventilator. The discharge ventilator has a double function: refreshing the air, and drawing off the heat. As described earlier, the capacity has to be great enough to replenish the air content of the grow room at least thirty times every hour. Accordingly, when working at higher temperatures (by adding CO2), the plant needs more water and more feeding. Remember the law of minimums. We can raise the CO2 supply, but if we don’t give extra water and extra fertilizer, plant growth adapts itself to the aspect of poor care.

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