The size of a cannabis plant will depend on the size of the pot it is grown in. Cannabis seeds grown in small pots will become small plants and large pots will encourage larger plants. Many growers choose to start their plants off in small pots and move them into larger ones later on.
Repotting plants like this has many benefits for the root ball and general development of a cannabis plant. That said, knowing the best time to transplant can make all the difference if you’re looking to grow a high yielding plant with a robust root ball. To grow the best medical marijuana everything must be taken into consideration, not just the parts of the cannabis plant you can see.
You can get away without repotting as well, but you’ll get better results if you re-pot them into something smaller first!
How Do Cannabis Plant Roots Grow?
A cannabis seed generally starts under ground in complete darkness, its root system will naturally grow into this darkness and start to fan out while growing down. If a root encounters any light or air as it grows, it will “air prune” itself and branch off into different direction. This process is a natural, healthy way to keep the roots in the dark and helps them in their search for nutrients. Air pruning will make the root ball denser as it happens more and more. Ultimately, the root ball will hit the bottom of the pot and start to spiral along the side of the pot.
Air pruning is a process that causes roots to multiply when they come into contact with air. Fabric pots like the one in this picture stimulate air pruning because the roots get more air.
The first root that comes out of a cannabis seed is the tap root, which extends downwards and anchors the cannabis plant into the medium. This helps create the support a cannabis plant needs to grow upwards. The tap root will also be the base from which many smaller roots grow.
The smaller roots that grow from the tap root are called fibrous roots and their job is to search for nutrients and water. Most air pruning occurs in these roots, making them more efficient in their search for nutrients and water. In the same way that you can pinch out or top a cannabis plant, you can make the root zone much denser using only air.
Why Do I Need To Transplant My Cannabis Plant?
As mentioned above, the roots of a cannabis plant will dig themselves down and push through the dark, moist medium searching for nutrients. This way the roots grow into the shape of the pot and eventually become root bound. Plant roots will always end up growing in the same shape as the pot. So whether you’re using a round pot or a square pot, the root zone will still grow to this shape. The only way to prevent root bound and struggling plants, is to transplant them once you are happy with the root ball. Ideally, the root zone is compact but not too dense to breathe.
This picture is a good example of a full, dense root ball. By filling out a root ball this densely you stimulate maximum nutrient absorption and optimal yields! Can you guess which shape the pot was?
A good example is to take a healthy, well rooted cannabis plant grown in a 5 litre pot and to transplant it to a 15 litre pot. Over time the cannabis plant will expand the root zone in every direction to find new nutrients and minerals and fill out its new pot. Starting cannabis seeds off in a 15 litre pot directly may not be as rewarding as it will not stimulate the root ball to become as dense. Another benefit of transplanting is that you actively break the roots apart, encouraging new root growth in every direction.
Re-potting a cannabis plant helps build a dense root ball that uses the entire pot. Without repotting the roots will shoot down and make less efficient use of the medium.
Is A Root Bound Cannabis Plant Always Unhealthy?
When growing medical marijuana from your own home or commercially, there are serious downsides to letting a cannabis plant become root bound. The most important negative effects include small stunted plants, root rot, slow nutrient uptake, under watered plants and low levels of production.
A cannabis plant can remain relatively healthy while being root bound, but will definitely suffer in terms of production. Healthy root bound plants will stop growing at a certain point as they simply can’t make more roots, this also causes water to be used up very rapidly. Using a certain size of pot can be a practical way to keep your cannabis plant at a desired size, but does increase your chances at running into problems.
This Fast Bud Outdoor plant was grown indoor in a 2 liter pot. You can “break all the rules” as long as you know what you’re doing, whether this is the most efficient or practical is a different story though.
If a root zone is so compact that it cannot breathe, it creates the perfect conditions for root rot. This condition causes cannabis plant roots to go from a bright hairy white to a yellowing brown. Root rot is difficult to fix and can easily kill an affected cannabis plant in only a few days. The root zone is the heart of a cannabis plant so an unhealthy root zone will lead to a sick plant and can easily kill freshly germinated cannabis seeds.
If you discover that you are suffering from root rot and see browning of the roots, you can salvage your grow if you get to it in time. You can use hydrogen peroxide to kill the bacteria that have been building up living in your medium thanks to the limited oxygen. After this, make sure your medium dries up properly so that the roots get air. Ensuring your root zone has good aeration and room to breathe will ensure that your cannabis plant stays happy and pathogen free. Root rot is usually caused by overwatering, but if the root ball has truly become too dense to breathe, your cannabis plant definitely needs a bigger pot.
The roots in this picture are not doing well, the brown color is a clear indication of root rot!
When To Re Pot Your Cannabis Plant
Most cannabis growers transplant their crop two or three times before starting the flowering phase. Ideally, a cannabis plant should be transplanted to a pot that is around 3-5 times the size of the previous container. Smaller will have you repotting too often, larger will make the root system much less dense.
The first pot on the left can contain up to 7.5 liters, the one next to it can hold 12L. Transplanting between these two pots would not add enough room to make it worthwhile.
Generally you will want to allow a cannabis plant to grow for around 3 weeks per transplant. The most common moments to transplant are after the seedling stage, during early veg and again a few weeks before flowering. The exact timings and the number of transplantations depend on the duration of the vegetative phase, medium, training and strain. The global instructions in this article will therefore work for virtually every grow, but can obviously be optimized once you’re more familiar with your setup.
You will have to decide the size of their final pot, as well as what medium you will grow in depending on the total amount of medical marijuana you need. Make sure you repot your cannabis plant shortly before switching it to flowering. The reason for this is because you should avoid shocking a cannabis plant during flowering at all costs. Repotting a cannabis plant during flowering will stunt it and can be very detrimental to your harvest. Just like the rest of the cannabis plant, the root zone will be grown and shaped mostly during the vegetative stage.
Most growers like to start their seeds in a coco, peat or rockwool plug, these provide enough room for root development for the first week or two. After the first leaves come out your seedlings will need some more room and food, so it’s time for their first transplant. The first pot is usually smaller then 1 liter in size, some growers like to go as small as 200mL. Let’s say we’re aiming for a 10 liter pot to finish flowering in, in this scenario a 500 mL would be good to start with.
These seedlings have developed their first leaves and are in the process of being transplanted. A 500mL pot filled with hydroton will provide enough room to grow for the next few weeks.
Two to three weeks after its first transplant, your cannabis plant is ready for its second transplant. This is the time it usually takes to develop three or four sets of leaves and the moment its first full sized leaves start to appear. Without transplanting, a young cannabis plant will become root bound and stop growing at some point after these first big leaves are developed. You can tell this is about to happen when you see a sudden burst of growth and the stem becomes thick enough to hold onto gently without snapping it. At this stage, where a seedling has truly become a small plant, it is time for the second re-potting. The second pot should be quite a bit bigger than the first one and should be large enough to last for most of the vegetative stage. A good fit for our root bound plant in the 500mL plastic cup would be at least a 2 liter pot, let’s say we’re using a 2.5 liter pot in this example.
These cannabis plants have developed a few nodes, and have just started making full-sized, 5-fingered leaves. Now that these stocky little ladies have moved past the seedling stage and grown a bit, they’re ready for their next pot.
Before actually completing the vegetative stage, it is time for the third transplant. For many growers, as well as in this example, this will be the final pot in which flowering stage is completed. By moving our cannabis plant from a 2.5 liter to a 10 liter pot, we’ll add plenty of space to grow roots during late veg and early flowering. We like to start flowering two weeks after the final transplant to make sure the cannabis plant will have completely adjusted to its new surroundings. A cannabis plant that has been grown, pruned and transplanted correctly during the vegetative stage will pay off during flowering and yield an enormous amount of medical marijuana.
After a few weeks of vegetative growth these ladies are ready for their final pot. With multiple nodes worth of full sized leaves, you can tell these are mature cannabis plants that are ready to flower
The sizes mentioned here are all based on our preferences and work well for soil and soilless media like coconut coir. The best sizes for you and your setup might be very different, so experiment as much as you can to see what works best for your grows. Make it your goal to be able to feed every 2-3 days throughout the grow without overwatering. If your cannabis plant is in an appropriately sized pot, it will be able to draw all the moisture out of the medium in a few days.
The Final Pot Size
Many medical marijuana growers are under the impression that the roots of a cannabis plant will fill out its pot during flowering. This is true, but the root zone will grow the most in the vegetative state so this is the time to shape them properly. If you want a cannabis plant with an optimal root system during flowering, you have to start by growing dense roots throughout the vegetative phase. Choosing the final pot size should be based on the roots having enough room to settle and then focus on feeding during the blooming phase.
These monster plants were vegged for over 4 months and have packed on more than a pound each during flower. This wouldn’t have been possible without multiple transplants and an enormous final pot.
The final transplant should take place 2 weeks before flowering your cannabis plant. This will allow time for the root zone to expand without becoming root bound for the rest of the cannabis plant life cycle. When flowering is induced, the root zone will continue to grow for the first 3-5 weeks, after which most of its energy will be geared towards developing heavy flower sets of medical marijuana.
We hope this article has been helpful and has increased your toolset for cannabis cultivation. We also hope to have given you some new ideas on how you can grow the best medical marijuana for your personal needs. As always, good luck growing and have fun with your cannabis plant!