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How to Control Your Indoor Grow Environment – Lighting, Humidity, Ventilation, and Pot Size

The natural environment of a cannabis plant is outside. When starting an indoor grow, we need to think about all the things that a plant has access to outside and do our best to replicate them. Should be easy, right? 

In a previous growing blog post, we looked at the equipment you will need in order to create a well-controlled growing environment for high-grade cannabis. If you haven’t read that, nor thought about the equipment you might need to start growing, you might want to take a look. It will save you doing a whole lot of research yourself, and explains clearly why you might need certain things. 

Now, we are going to give you some tips on how to use the kit to create your own indoor garden.  

Cannabis, like most plants, has a need for light, carbon dioxide, air movement, a medium for its roots to grow into, nutrients, a certain temperature and the correct level of humidity in the air. Cannabis has evolved alongside man for millenia in parts of the world where these conditions are perfect. This is where original landraces made their mark leading to the cannabis varieties we have today. When thinking about setting up your own indoor space you need to to think about keeping control of these variables, as we will discuss below. 

For most people, using a specially made grow tent to control this artificial environment is a very effective method and is how millions of people are growing around the world. They have reflective insides and come in a wide range of sizes to fit a number of spaces – there are even pitched roof tents for lofts and top floor apartments or sheds. One of their advantages is they can be packed down and set up quickly if needed and can be quite stealthy if you need them to be. 


There are two things you really should know about the lighting used for growing cannabis plants. Spectrum (the colour of the light) and photoperiod (how long the light is on). 

Plants need light to photosynthesise (produce energy). They also require specific light spectrums to grow to their best potential: a higher amount of blue/white light is needed when in their vegetative stage to replicate the spring sunlight, and when flowering they require more red in the spectrum to replicate the summer sun – the time of year that cannabis naturally flowers outdoors. 

Fig: Kelvin Scale


There are bulbs that are made for each of these parts of the growing cycle as well as bulbs that have a dual spectrum. In the vegetative stage the plant needs 18 hours of sunlight. But to signal to the plant that it should start flowering it requires a shorter light period emulating the later part of summer. 

Now that this is understood, we can understand why there are a number of different bulbs available for use.

The traditional grow light for cannabis has been the High Pressure Sodium (HPS) and Metal Halide (MH) bulbs. These are similar to street lamps which have been specially designed to put out specific light spectrums. These bulbs come in 250, 400, 600 and 1000 watt bulbs and require a ballast to convert the power to the right voltage. Although they are the most affordable bulbs on the market they are becoming less desirable because newer technology has emerged.

Ceramic Metal Halide (CMH) and Light Emitting Diodes (LED) lights use considerably less power and waste less energy by being cooler when they are running. These have become a prefered choice. If you are looking for affordability and an easy to control environment then the CMH is probably where you want to aim your budget. 

Pic: CMH lighting provides a really natural light spectrum that isn’t as harsh on your eye’s as HPS and you can inspect your plants for health or pest issues with a much more sun-like spectrum. 

Pic: The spectrum of an HPS is much more orange and harder to see the true colour of the leaves, stems and buds for potential issues.

Source: (permission not asked – any picture you have that has this kind of orange lighting though will be adequate to demonstrate)

Because you will need to change the photoperiod (length of time the light is on) manually you will also need a timer to plug your light ballast into. 

When we refer to “vegging” the plants out, what we mean is getting the plants big enough to flower. This is about 4 weeks from seed. But the longer you veg for, the potential for a bigger yield increases. This is the time you would use a bluer/whiter light spectrum around 6400k and have the light on for 18 hours a day. 

When we refer to “flowering” or “bloom”, this is when the plants slow down their production of leaves and focus their energy on producing the buds. This takes a minimum of 8 weeks but anywhere up to 14 for more exotic sativas. During flowering you need a more yellow/red light spectrum around 2-3000k and to have the light on for 12 hours. Don’t forget to change your timer!

Air – Ventilation 

Plants breathe. They inspire carbon dioxide (co2) and expire oxygen (o2). They do this through the stomata on the underside of the leaves which are like little mouths. This gas exchange happens when producing their own energy, which at the same time releases moisture that the plant uses to transport its nutrients. We need to make sure that our indoor environment is able to exchange the air and make sure the moisture level doesn’t rise too high. 

To control this we use an extraction fan that is an adequate size for exchanging the air at the appropriate rate and this can be made even easier by coupling it with a fan speed controller. These can be basic ones that you plug in at the socket and control the speed with a variable resistor, or there are automatic ones that work by use of a sensor. For your basic home grow setup the plug in ones are perfectly adequate. 

If your tent or room is in a hotter part of the house you may want to consider an inlet fan to bring in cooler air from somewhere else. This is usually not necessary but in some cases like loft spaces it really can make the difference from bringing the temperature down from 30 degrees C to 28 C thus ensuring your plants don’t fry during the summer. Some growers who really battle with the heat add in a small air conditioning unit to maintain optimum temperatures.But taking a break during the hotter months is often the safer option if you don’t want to risk losing what you’ve been working on due to the heat, or incur a much higher electricity bill. 

Plants grown in hotter temperatures tend to produce airier less dense buds and can have a higher chance of bud rot and powdery mildew – both health hazards if you smoke or vape them. 

A little trick that growers like to use is having a negative air pressure in their grow space which can help achieve denser buds – do keep your eye out for mould spots though. If you see a dying leaf coming out of a bud this may be a sign. This is easier to tell when growing in a tent because you can see it visually represented by the walls being sucked inwards. All you need to do though is make sure you are removing air quicker than it is entering the space. 

An oscillating fan in the corner of the tent is a great addition to make sure the air really is moving around the tent and you aren’t getting trapped pockets in the canopy where moisture and mould can build up. This doesn’t and shouldn’t be so strong that the plants are getting damaged but it is healthy for the stems of the plants to move and wave around a little as it helps them strengthen in veg for when they grow buds which may become too heavy. 

Remember, your plants will smell, probably more to other people than you. This is firstly because you love the smell of cannabis, and secondly, because being around the smell of something all the time desensitises you to it. Using a carbon filter is going to save you from worry when they start to really hum.


We often take the moisture content in the air around us for granted but plants cannot be so forgiving. RH stands for Relative Humidity and means the amount of moisture in the air in relation to the temperature. In the vegetative stage plants really enjoy RH levels in the 60-70%. When flowering, this drops to 40-50% as higher levels can cause bud rot or mould issues. 

Warm air holds more moisture. Depending on where you live can really impact the baseline humidity levels that you are working with. Some grow spaces won’t have to do much at certain times, whilst others will find they need to keep a close eye on this and employ a few more tactics to keep it on point. 

If your RH is too low? Humidifiers are a small device that you fill with water and it blows out a fine mist of moisture into the grow environment, they usually have a little knob on the front so you can adjust the speed/rate at which it outputs the humidity. If your RH is too high dehumidifiers do a good job, instead of chucking out moisture they actually suck it up and store it. For both of these you can get digital versions that click on and off automatically when they sense the moisture is too high or low, or you can buy a separate device that has a sensor which you plug the humidifier into, so your room stays within the RH sweetspot. You’ll likely have to fill or empty these once a day or every other day unless you get a big enough one that lasts longer. 

Pot Size / Number of Plants

You will want to make sure that your plants have enough room to grow and get the light they need but you also don’t want to waste space as this can also make it harder to control things like humidity. Vegging plants can increase a quarter in size in just a week. When you flip them into flower, this can stretch to 1.5 – 3 times the size, so consider this whilst vegging. 

Growing a smaller number of plants can give you more time to veg a plant out in the space you have before flipping it into flower whereas having 4 smaller pots in the same space will allow you to go into flower sooner. Overcrowding a space can lead to over moisture problems, light not getting to enough of the plant and you could miss bud rot, male plants or hermaphrodites which could seed your crop and leave you with a small amount of usable bud and a lot of seeds. 

A 1m2 tent would be good for growing 1 large plant vegged for 8 weeks, 4 or 5 medium plants vegged for 6 weeks or 9 small plants vegged for 4 weeks. 


It’s often a smart move to have a friend or family that you trust come over and check if they can smell anything outside the house or in the rest of the house as their nose won’t be accustomed to it. 

Check to make sure you have no light leaks going into the tent or you may get hermaphrodites, but also you don’t want light leaks out the tent glaring out of a window and alerting people to your garden.

Check regularly for mould, pests, nutrient deficiencies and male parts on your plants. The sooner you notice something the quicker you can fix it. Remember, when in flower, 1 week is the equivalent of a decade of your life – how long would you wait  to fix a problem with your own health/living environment? 

Cultivation information, and media is given for those of our clients who live in countries where cannabis cultivation is decriminalised or legal, or to those that operate within a licensed model. We encourage all readers to be aware of their local laws and to ensure they do not break them.

This post is also available in: French


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