Cannabis roots need ample room to grow and expand, or they become root bound, also commonly referred to as pot bound. When plants become root bound, roots push against the walls of the container and begin circling the root zone, eventually constricting and choking off the interior root mass.
A root-bound cannabis plant eventually has difficulty absorbing oxygen, water and nutrients, oftentimes severely inhibiting aboveground prosperity. Root-bound plants may exhibit stunted growth, nutrient-related problems, weak stems and stalks, and reduced production of bud weight, cannabinoids and terpenoids.
Marijuana mother plants are often in danger of becoming root bound, but even plants grown for as little as one season can become root bound, especially if the pot size is too small.
Now, let’s explore methods of preventing your cannabis from becoming root bound, and what to do if this does occur.
Using Smart Containers To Prevent Cannabis From Becoming Root Bound
Until about 10 years ago, almost all cannabis growers used plastic containers made from rigid, non-porous, synthetic material. The only openings were at the top of the pot, with a few holes on the bottom.
When the roots hit the rigid internal walls of a solid pot, they circle the pot’s perimeter and begin to choke the rest of the root mass, also attempting to grow through the holes at the bottom of the pot. When growers see the roots emerging from the base of their containers, their usual tactic is to repot to a larger bucket.
This is a good tactic for most single-season marijuana plants, although repotting and transplanting can be messy and cause stress to the plants.
However, sometimes you can’t afford the floor space to keep on upsizing your containers, or the larger pot is taller than previous pots, taking away valuable vertical space, which impacts distance between grow lights and canopy.
When it comes to mother plants, constantly increasing pot size can quickly get out of control. I’ve seen growers who don’t root prune their mother plants, which end up needing 50-gallon containers, sometimes even larger. It’s unwieldy and unnecessary.
Once you master the art of root pruning, you can avoid that outcome.
Cloth Pots For Air Pruning
Cloth pots are designed for automatic air pruning. These containers are made of durable and porous cloth, or they have openings around the sides. When roots come into contact with open air and light penetration, they prune themselves. So, instead of circling and girdling, they stop growing outward.
The most popular air-pruning pots are made by Smart Pots and Radicle Bags. These cloth and canvas containers range from one gallon to several hundred gallons in size, often have handles, and are used by indoor and outdoor marijuana growers alike.
Some cloth pots, like this light-colored one from iPower, are great for outdoor growing, because black pots attract and hold heat that can fry roots, especially if marijuana plants are growing under full sun.
Not only do cloth pots create self-pruning roots, they also facilitate better water drainage, and allow the root zone to breathe in and out from all sides. This brings more oxygen and air exchange into the root zone; the added oxygen is absorbed by roots and utilized by your plants.
The fact that drainage can take place from the sides of the container, not just the bottom, is also a benefit. Fertilizer salts can easily accumulate in root zones and are tricky to flush without making the root zone too soggy for optimal root health. When you water a cloth pot or other type of air-pruning pot, the water can pour out the sides, purging excess nutrient salts more efficiently.
The air flow that comes in through the pot’s walls oxygenates the root zone so it isn’t soggy and stagnant. This allows roots to breathe easier, and removes wet conditions that are an ideal environment for pathogenic root rot organisms that destroy cannabis roots.
Check out the below YouTube video of how to root prune a potted plant, courtesy of Erich Maelzer:
Manual Root Pruning Of Cannabis
Even if you use root-pruning pots, there may come a time when your cannabis roots are too dense and large for the container. This is almost always true for mother plants. The solution to this problem is manual root pruning.
The best tools for manual root trimming of cannabis are bonsai plant-trimming tools, such as root shears.
These specialty gardening tools might not be utilized by many cannabis-specific cultivators, but they are indeed a great investment for growers. They can be used for removing root balls from pots, for trimming roots, and some can even be used for trimming and training marijuana plants aboveground.
This has to be done carefully, though. Improper root trimming can create so much stress that it severely harms, even kills your plant.
And timing is important. Do not root prune a cannabis mother plant right before or after you take cuttings from her. You want at least a 3–4-week buffer after cuttings are taken, to ensure the mother plant has had time to recover from the stress of having cuttings taken.
After you root prune your mother plant, wait a minimum of 3–4 weeks before you take more cuttings, in order to give her sufficient time to recover.
Similarly, don’t root prune a one-season cannabis plant less than 15 days before bloom phase, or during bloom phase. The stress of root pruning interferes with plant hormones and physiologic processes, which can delay onset of bloom phase and subsequently decrease floral production.
If I see root-bound single-season cannabis plants in grow phase, it’s usually around weeks three or four. I do my root pruning, and/or increase container size, and then wait 15 days or more to start bloom phase.
Best Tools And Tactics For Cannabis Root Pruning
For easy explanation of manual root pruning, check out the below YouTube video courtesy of CenCal Weed Cultivator, who discusses the Current Culture Undercurrent System:
The general rules to follow for cannabis root pruning are:
- Don’t root prune if the plants are already under stress. If your plants have recently suffered from nutrient problems, a pest infestation, disease or extreme heat, don’t root prune until plants are healthy and in ideal conditions.
- Don’t root prune less than 15 days before bloom phase starts, within four weeks of taking cuttings from a mother plant, or when a plant is already in bloom phase.
- Don’t prune more than 35 percent of total root mass. While you may see a higher percentage of root mass being pruned in some instructional online videos, those percentages are targeted toward plants other than cannabis.
- Root prune the outside perimeters of the root mass, and do it evenly to create a symmetrical root profile.
- Always use clean, sterilized root-pruning tools.
- Root pruning should be done no more than three times per year for mother plants and only once for rejuvenated plants or young plants no older than one grow season.
Growers using pure hydroponics systems such as deep water culture, aeroponics or the nutrient film technique often find that the root-boosting benefits of those systems creates huge, pure-white root masses that can quickly exceed the size of the container or tube that the roots are in. This is especially true in aeroponics systems.
After you’ve root pruned, always feed your plants with B-52 B-vitamin anti-stress tonic, and with potassium silicate, a cellular strengthener that boosts healing and structural integrity. Also, feed them beneficial bacteria and fungi hydroponics supplements found in Piranha, Voodoo Juice, Tarantula and Microbial Munch.
Used together, these supplements form a beneficial symbiosis with your cannabis roots, which will help them heal from the pruning, while also increasing their ability to intake water, oxygen and nutrients.
If you’re growing autoflowering marijuana or running a sea of green grow op, your cannabis won’t be alive long enough to become root bound, unless you have their roots in a ridiculously small container.
However, if you have particularly robust plants whose genetics predispose them to rapid root growth, or you use long grow-phase durations or mother plants, root pruning is a valuable skill that will keep your plants healthy and strong.