To achieve a successful harvest, it’s important to understand both how a marijuana plant works and what it needs. All sorts of chemical and physical processes take place inside a marijuana plant to let it grow and flower. To do this, a plant needs things such as light, water, nutrition and carbon dioxide.
In this guide you will learn
In this course, I will precisely explain how the metabolism of a plant works and what the plant needs to grow optimally and flower. Because, if you understand what a marijuana plant uses water, light and CO2 for, you’ll also know how much you should give your plant, and you will also identify possible problems more quickly.
Converting light into energy
Just like humans and animals, a marijuana plant needs energy to grow and flower. A plant takes its energy from sunlight by catching it with its leaves. The leaves contain chlorophyll to absorb the sunlight. You can compare the leaves to large solar collectors. Luckily, there are manufacturers that developed special grow bulbs and flowering bulbs since we’re growing indoors.
These lights are not specifically for growing marijuana; instead, they are for all fruits and vegetables, that are cultivated in greenhouses. During cloudy days or in the winter, these bulbs can provide a greenhouse with additional light. It’s best to use an MH lamp during the growing phase and an HPS lamp during the flowering phase.
Besides light, a plant also needs water (H2O) and carbon dioxide (CO2) to create energy. This process is called photosynthesis. The light starts the following chemical reaction in the plant:
Light + carbon dioxide + water sugars + oxygen
The arrow in the middle indicates that the reaction can go both ways. The arrow to the right means that the CO2 and water are converted into sugars during the daylight period. The plant takes the energy required from the light, through chlorophyll in the leaves. The leftover oxygen is a waste product and is removed through the leaves. This is called the carbon assimilation process.
The longer energy is available, the longer the factory can run and the plant can grow or flower. You control the lighting period with your lamps, and there never has to be a shortage of water or nutrient. It would, therefore, be obvious to keep the darkness period as short as possible, or even skip it. It’s however not that easy.
At night, other necessary processes take place, such as the production of proteins. The energy required for this uses the same chemical reaction, only from the right to the left. The energy stored in the sugars and the oxygen are converted into water vapor and CO2. This is called dissimilation, or the respiration of the plant.
The plant uses this energy to create products such as proteins that aren’t formed during the assimilation during the daytime. For this reason, a marijuana plant needs a day period and a night period.
Research determined that a day period of 18 hours is optimal during the growth, and is also the maximum duration. In fact, it’s mimicking the longest day of the summer, when the plant experiences its most critical growing period.
During the flowering stage, the day period shouldn’t last longer than 12 hours. In this case, the fall is mimicked, when the plant experiences its all-important flowering period. Lighting your plant for more than 12 hours could, therefore, cause the plant to go back into the growing stage. So you only have to make sure that there’s enough light, water, carbon dioxide and nutrients available to your plants.
Absorption of nutrients
The absorption of water (and the nutrients that are dissolved in it) takes place through the roots of a plant in a process called osmosis. Osmosis is a physical process in which substances from one fluid, separated by a semi-permeable wall (the plant cell), can be transported to another fluid. This takes place from the side with the highest concentration of nutrients to the side with the lowest concentration. The movement of ions is referred to as osmotic pressure.
The same principle applies to a marijuana plant, which consists of about 80% water. The type and the amount of substances on both sides of the wall of the cell are therefore important. We’ll keep it simple and assume that the liquids in the plant (and in the water outside the plant) only contain N (nitrogen), P (phosphor) and K (potassium).
If the marijuana plant uses nitrogen ions, the concentration in the plant is lower than the concentration in the water outside the plant. New nitrogen ions will be sucked in from the water. If the plant has sufficient potassium ions – the concentration level on both sides of the cellular wall is the same – the K-molecules won’t move and stay where they are. If the plant has many more P-ions than the water outside the plant, the phosphor will be sucked out of the plant, creating a deficit in the plant.
Transportation of nutrients
The salts and oxygen in the nutrient water put a pressure on the roots while the water with the nutrients wants to go in. This process is further increased by the evaporation of water through the leaves, creating an underpressure in the plant. The high pressure on the roots and the suction force in the leaves enable the plant to transport water and nutrients upwards in the plant.
Even if all the right nutrients are available in the water, a humidity that’s too high can decrease the evaporation (suction power) in the leaves. The plant will absorb fewer nutrients because of this. The availability of water and nutrients and the humidity of the air around the marijuana plant, therefore, determine whether a plant can send the nutrients to where they’re needed.
Temperature can have the same effect. The evaporation of water through the stomata in the leaves has another function. Heat is required for evaporation to take place. If the temperature gets too high, the plant can open its stomata to evaporate more water and lower the heat. This can occur if the lamp is placed too closely above the plants.
When it’s cold, the weed plant retains warmth by closing the stomata to evaporate less water. This mechanism has no direct influence on the growth or the flowering of the plant but does help to keep the factory running optimally. Everything works together very closely in the metabolism of the plant. If you make sure you have the right temperature in your growing room and you keep it constant, the plant won’t have to sweat as much and it can use its energy for more important things.
Converting nutrients into building materials
During the assimilation, nutrients in the marijuana plant are converted into energy and building materials. These building materials are necessary for maintaining existing plant cells and creating cells. During its growth, a plant mainly produces leaf and stem cells. The plant needs slightly different raw materials for these than for making flower cells. This explains why you use different fertilizers during the growing and the flowering phase.
Like all chemical processes, temperature influences the rate at which processes take place. This is no different for marijuana plants: too warm means they’ll burn and too cold means they won’t sufficiently start the process. For assimilation, which takes place during the day, weed plants thrive at temperatures between 70 and 77 degrees. At night, during dissimilation, temperatures around 60 degrees will do fine.
You can find much background information on how osmosis, photosynthesis, stomata and many other things work in the category Plant biology. If you understand what processes take place inside your plant, you’ll also understand the needs of your plants. Read these articles if you have the time.