Growing quality medical marijuana is not something that comes to one overnight. It is a skill, an art, and it must be learned. There are really only two overall factors in how good the marijuana you’re growing turns out to be: the plants’ genetics, and the internal environment you create for them to grow in. The genetic factor is more or less set when you purchase the seeds or clones from which you’ll be growing, so 99% of whether or not your plants turn out to be top shelf or unmarketable is internal micro-climate you are able to create. The objective is to create essentially the most optimal state of affairs within your grow room possible, for the flowering and flourishing of your cannabis plants.
One critical element of creating the perfect micro-climate is to find the optimal balance between temperature and humidity. Having too much or too little of either can cause serious problems for your plants. Temperatures being too low can cause slower plant growth, and contribute to the development of molds. However, too much heat can also cause slower growth, while also opening up a range of other problems; if you’ve ever visited the south, then you know that with heat and humidity come all sorts of creepy crawly creatures, and if your grow room is too hot and humid, then all sorts of pests will invade, ranging from gnats to spider mites to snails. Other problems that can be caused by overheating your plants are root rot, nutrient burn, and powdery mildew.
Ideal temperatures for your plants vary between the different stages of growth. Remember, since your plants are natural lifeforms, everything you do to create a controlled environment for growing cannabis is trying to mimic their perfect natural environment. That means the earliest part of their life-cycle, the vegetative stage, is hot, like the Summer; ideally 70-85 degrees. Later, the flowering stage is mimicking the onset of Fall, and so ideal temperatures shift to 65-80 degrees, and also, just as is the case in the Fall, there needs to be more of a difference between “night” temperatures (when the lights are off) and “day” temperatures (lights on). Ideally, about ten degrees’ difference.
Humidity and temperature are intimately related, because different temperatures of air can hold different amounts of humidity; the higher the temperature, the more water can be held by the air. The balance between the two is called Relative Humidity (RH), which is the ratio of how much the air is holding, compared to how much it could hold, at the given temperature. So, since you already know your ideal temperatures, you just need to adjust your RH to match them. Since humidity determines nutrient intake, this is extremely important. Again, you have to think of the plants’ natural environment, and how they are made to adapt; if humidity is high, the plants think there’s plenty of water, and they don’t drink as much through their roots. That would be alright, if it wasn’t also their primary source of nutrients. Proper RH is directly related to root growth, when the roots are less developed, humidity should be high, when they are more developed, it should be low. This means that at the earliest stage, where roots are least developed, the RH should be around 75%. During the vegetative stage of rapid growth, it should be closer to 45-65%, and during the flowering stage, 40% or lower.
Also important for your plants to thrive is, of course, the air that they breathe, which in their case is primarily CO2. Although it requires more attention and expertise, the best way to grow marijuana is with CO supplementation. Plants that have a proper CO2/Oxygen balance will grow faster, and produce more buds. CO2 is typically measured in parts per million (ppm), and average CO2 levels in the air are around 500ppm. That’s fine, your plants can certainly grow with that, but if you supplement CO2 and drive the levels up, to as much as 1800 ppm, your plants will actually grow much faster and produce more buds. If you plant to use CO2 supplementation before-hand, you can also use a higher powered light, since plants growing in CO2-rich air can actually handle more light and heat, because photosynthesis is happening more efficiently.
Oxygen is also important, and the best way to get good oxygen to your plants is through their water supply. Oxygenating their water will be the most efficient way to get them the oxygen they need, and this is easily accomplished by increasing flow, as well as aerating the water. Various devices for water aeration, and oxygen measurement, exist in most grow stores.