Growing Marijuana Indoors: Controlling the Climate

To achieve optimal growing results, you will have to keep the climate in your growing space in check. The three most important aspects of climate control are: Humidity, temperature and ventilation.


Humidity Grade

DSC_0121When you are growing marijuana indoors, you need to keep in mind a cannabis plant cultivates best with a high humidity grade during the growing cycle. It’s recommended to keep this level around 80%. By doing this, you will get a faster cultivation of the plant and you can shorten the growing cycle, because the plant will reach the desired height in a shorter time. As the grow progresses we will lower the humidity grade to around 70% when the flowering cycle begins. During the flowering cycle it’s recommended to make use of a lower humidity level. Attempt to keep the humidity level at 65% during the first few weeks of the flowering cycle and lower this to reach 55% at the fourth week. Slowly attempt to lower the humidity level every week during the flowering cycle because when the buds are starting to develop, a too high humidity level can lead to bud rot. During the last weeks of the flowering period we attempt to keep the humidity level around 40-45%. When large buds can be found on the plants during the grow, a high humidity level can lead to mould.

It isn’t easy to maintain a perfect humidity level in your growing room. During the growing period you can easily create a nice humidity level with a humidifier . This will only succeed when your exhaust fan is turned off during the growing period. When the flowering period starts you can turn your exhaust fan back on to make sure fresh co2 rich air will flow into your growing space. During the flowering cycle, it’s best to keep your exhaust fan running because of the fact that they are able to evaporate a lot of water themselves due to their size. The bigger the plants, the more leaves they have and the more water evaporates via them. Larger plants will also consume more co2 and thus it is important to make sure that there is a continues flow of fresh air into your growing space.

DSC05863Humidifiers aren’t made to be on duty in a growing space, so it will be a hard job to maintain a good humidity level during the exhaustion of air. Keeping the humidity level on a perfect level is only possible with a professional humidifier and hygrometer that is powerful enough to maintain a stable humidity level also during exhaustion. However these aren’t very practical for a starting grower because they cost around 500 dollars. The perfect humidity levels are guidelines, so when you don’t achieve to match up with them you still will get good results. It’s a lot easier to maintain a good climate in a large grow room than in a small ones, so the small scale grower will have a little more difficult times. Also, the time of the year and the season in which you are growing will influence the humidity level. During a wet autumn you will quickly get too high humidity levels, during a dry summer too low humidity levels.


Humidity level has a huge influence on cannabis plants. Cannabis plants that have had a high humidity level during the growing cycle are better resistant against bud rot during the last weeks of the flowering period. While cannabis plants that had a too low humidity level during the start will get problems with bud rot faster in the last weeks when the humidity level is too high.


Although there are differences between different varieties of strains, cannabis will develop best between 70 and 80° F. Under 68°F the metabolism of the plant will slow down and at 55°F – 60°F it will stop. Such temperatures may occur overnight but that’s not a problem, since the photosynthesis will take place during the day. A difference of 20°F between the light and darkness period is tolerated.  Bigger differences will cause the plant to stress. Above 80°F the metabolism of the plant will speed up. The plant will use more water, nutrients and CO2. The amount of CO2 in the air has to be raised by then.  To minimize the temperature differences, it can be a good choice to put the lights on during the night and off during the day.

To keep the temperature in your growing space at an ideal value, you will need a fan-controller with thermostat. This way you can choose at what temperature the fans have to run at maximum power.


DSC05856Maintaining a good airflow in your growing space is very important. You need intake holes at the bottom through which fresh air can enter your space and exhaust holes at the top to drain air to outside. It’s important that the intake and exhaust holes are not too close to one another. You want to create a situation in which fresh air will flow along your plants before it reaches the other side and gets exhausted. Cannabis plants need CO2 to grow and flower. If you don’t exhaust the air well enough, no fresh air will enter and also no new CO2. Your plants will consume all the CO2 in the growing space and because of the exhaustion, they will get a new load of CO2.

The biggest problem while making intake and exhaust holes is that you have to make sure that the growing space also stays lightproof. You can put a vent before the holes to block light or you can use a bent pipe. There are many options that will do the trick as long as there is no possibility for light to come in your growing space, because cannabis plants can’t flower without 12 hours of uninterrupted darkness.