The cannabis plant root system

You only see half of a plant when you see it in nature. The underground part – the roots – is at least as big as the part above the ground. Then why do plants that grow indoors in soil or on rockwool have such a tiny root volume? That’s because we hand them everything they need on a silver platter.

Function of the roots

A plant creates its own building materials through the process of photosynthesis. The solar energy is caught by chlorophyll in the leaves and if then used to create chemical bonds, which are mainly sugars. The building materials for the sugars are made from CO2 and water. There’s usually enough water and light, so the CO2 is ultimately the variable that determines the rate of photosynthesis. The more CO2, the more sunlight can be caught and can be processed into sugars. This concept is valid up to a certain limit, after which more CO2 no longer makes a difference, since other parts of the photosynthesis can’t speed up any more.

Regular outside air contains 0.035% CO2, indicated as 350 ppm (parts per million). It’s possible that the concentration in a grow room is much lower, since the plants consume CO2. The CO2 therefore has to be supplied from outside, because a prolonged shortage of CO2 leads to slowed down growth and development of the plants.

The absorption of water and the absorption of nutrients are very strongly connected. If they roots absorb a lot of water, they’ll also take in a large amount of nutrients. This virtually happens by itself through osmosis, and doesn’t require any energy from the plant. The plant can also actively absorb most nutrients, through a pumping mechanism. Since this is an active process, it costs energy.

The motor of water absorption is evaporation. The leaves evaporate water through the stomata, while the roots absorb it, so the water is pulled upwards inside the plant, sort of speak. A healthy root system is therefore vital for the proper growth of a marijuana plant.

How to grow strong roots

To create a healthy root system, you have to take good care of your roots from the very beginning. First of all, they must have the opportunity to grow freely. So make sure you have airy soil with some perlite, rockwool or another known source. When growing outdoors you should be careful with clayey soils, because roots have a hard time growing through this and the clay has a suffocating effect.

Your roots need sufficient water from the start to prevent them from drying out. Especially seedlings die very quickly when experiencing water shortages, because they have no reserves whatsoever. Provide water at room temperature to make the absorption as easy as possible. Water with a temperature between 64 and 72 degrees is perfect. You could heat the water with a fish tank heater.

The temperature of the area is also important for your roots. The top of the plant can easily withstand heat or cold temperatures without suffering any damage. But the roots are very sensitive and can’t stand heat or cold very well. If you’re growing indoors, the temperature shouldn’t get below 64 degrees when the lights are off, and shouldn’t get above 77 degrees when the lights are on. Stay within these limits and the temperature at the roots will be fine.

The pH value around the roots has to be between 5.5 and 6.5 to optimally absorb all nutrients. The root hairs are very sensitive and could burn if the pH is too low. You should therefore regularly measure the pH of your soil or rockwool to see if the value is around 6.

Root problems

Root problems always show in the leaves. These can form yellow or brown spots, the edges can discolor and the leaf can completely shrivel and die. If you suspect root problems, you have to take the plant out of the pot to check and see if the roots are still white. Always check this part when you’re transplanting your plants. These are the most common causes of root problems;

Small plant in a large pot; a small plant in a large pot is easily overwatered. The roots won’t get enough oxygen this way, causing them to suffocate. Give a little bit of water around the stem if your plant is in a large pot.

Large plant in a small pot; if the root system becomes too big for the pot, the plant will stop growing. This is called rootbound. Check every other day to see if the roots are coming out the bottom of the pot yet.

Cold or heat; roots are very sensitive to cold or heat. They absorb fewer water and nutrients if they’re too cold and if the temperature differences are too big. You can place a electric heater with thermostat in your room to heat up the room when the lights are off.

Too much water; this is one of the most common problems among beginning growers. Too much water suffocates the roots, impairing their ability to absorb water and nutrients. Always make sure there are some holes in the bottom of the pots so an excess of water can always run out.

The wrong soil; buy good soil with for instance some coco peat and perlite. This makes the soil nice and airy, so the roots can easily grow through it. If you’re growing outdoors, it’s a good idea to first dig a hole and then put some pre-fertilized soil in there.

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Robert Bergman

Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations. His passion for growing lead him to develop his own Gold Leaf strain. Now, Robert is dedicated to sharing his knowledge with the world.... [Read full bio]

13 comments on “The cannabis plant root system”

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