Marijuana roots need room to grow and expand or they become rootbound. Rootbound means they’re pushed up against the walls of their container and begin circling the root zone, eventually constricting and choking the interior root mass.
A rootbound marijuana plant’s roots have a hard time absorbing oxygen, water, and nutrients, and aboveground growth is sometimes severely impacted. Rootbound plants may exhibit stunted growth, nutrients-related problems, weak stems, and stalks, and reduced production of bud weight, cannabinoids, and terpenoids.
Marijuana motherplants are the category of marijuana plants that most often become rootbound, but even marijuana plants grown just one season can become rootbound if initial pot size is too small.
In this article, we give you ways to prevent your cannabis plants from becoming rootbound, and what to do if they do become rootbound.
Until about ten years ago, almost all of us growing marijuana in containers used plastic containers with rigid, non-porous walls and the only openings were at the top of the pot and a few holes on the bottom of the pot.
When the marijuana roots hit the rigid walls of a plastic or other type of solid pot, they circle and begin to choke the rest of the root mass. They also attempt to grow through the holes at the bottom of the pot. When growers see roots emerging from the bottom of their pots, their usual tactic is to repot into a larger pot. This is a good tactic for most single-season marijuana plants, although repotting and transplanting is messy, and stresses the plants. However, sometimes you can’t upsize your containers because they’ll take up too much floor space or for other reasons.
For marijuana motherplants, the tactic of increasing pot size over time creates escalating repotting that can get out of control. I’ve seen growers who don’t root prune their motherplants, and the motherplants end up needing 50-gallon containers or even larger. It’s unwieldy and unnecessary. Once you master root pruning, you avoid that.
One way to avoid rootbound roots is to use cloth pots or other types of pots designed to do automatic “air pruning.” These pots are made of porous cloth or they have openings around the sides of the pots. When roots come into contact with open air and light penetration, they prune themselves. Instead of circling and girdling, they stop growing outwards, and don’t circle.
The most popular air pruning pots are known by the brand names Smart Pots and Radicle Bags. These cloth or canvas pots range from one gallon to several hundred gallons in size, often have handles, and are used by indoor and outdoor marijuana growers.
Some cloth pots are light colored, which is very nice if you’re growing outdoors, because black pots attract and hold heat that fries roots, especially if marijuana plants are growing in 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 hard to flush without making the root zone too soggy for optimal root health. When you water a cloth pot or other air pruning pot, the water can pour out the sides, purging excess nutrients salts more efficiently.
Not only that, but the air flow that comes in through the pot’s walls aerates 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 marijuana roots.
Manual Root Pruning
Even if you use root pruning pots, there may come a time when your marijuana roots are too dense and large for the container. This is almost always true for marijuana motherplants. The solution is manual root pruning. In manual root pruning, you use bonsai plant trimming tools such as root shears to trim the roots. This has to be done carefully or it damages roots and plant. Improper root trimming can create so much stress that it kills or severely harms a marijuana plant.
Timing is important. Don’t do root pruning of a marijuana motherplant just after you took cuttings from it, or just before. You want at least a 3-4 week buffer on each side. For example, wait at least 3-4 weeks after cuttings are taken to do root pruning on a motherplant, so the mother has had time to recover from the stress of having cuttings taken from it.
After you do motherplant root pruning, wait a minimum 3-4 weeks before you take cuttings. This gives the root-pruned motherplant time to recover from root pruning stress before it endures the stress of having cuttings taken from it.
Similarly, don’t do root pruning of a one-season cannabis plant 15 days or less before your plants into bloom phase, or during bloom phase. The stress of root pruning interferes with plant hormones and other physiologic processes, which can delay onset of bloom phase, and decrease floral production.
If I see rootbound single-season cannabis plants in grow phase, it’s usually around weeks 3-4. I do my root pruning, and/or increase container size, and then wait 15 days or more to start bloom phase.
Tools & Tactics For Root Pruning
Learning the physical procedures of manual root pruning is better seen on video than only described in words, which is why I’ve placed three root pruning videos in this article—please watch all of them. The general rules for marijuana root pruning are:
- Don’t root prune if your marijuana plants are already under stress. For example, if your plants have been recently suffering from nutrients problems, pests, diseases, high heat, or other problems, don’t root prune until they’re healthy and in ideal conditions.
- Don’t root prune less than 16 days before bloom phase starts, within four weeks of taking cuttings from a motherplant, or when a plant is already in bloom phase.
- Don’t prune more than 35% of total root mass. You see a higher percentage of root mass being pruned in some videos accompanying this article, but those percentages are targeted towards 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 motherplants and only once for one-season and rejuvenated plants.
You’ll notice that one of the videos shows root pruning in a deep water culture system. Growers using pure hydroponics systems such as deep water culture, aeroponics, and nutrients 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 marijuana roots are in. This is especially true in aeroponics systems.
When you view the root pruning videos, you see gardening tools most marijuana growers don’t have. These tools come from the bonsai tree and specialty gardening industry, and they’re a great investment for marijuana growers. They can be used for removing root balls from pots, for trimming roots, and some can be used for trimming and training marijuana plants aboveground.
After you’ve done root pruning, always feed your plants with B-52, a vitamins formula that helps plants deal with stress, and with Rhino Skin potassium silicate, a cellular strengthener that boosts healing and structural integrity.
If you’re growing autoflowering cannabis or run a sea of green grow op, your marijuana plants won’t be alive long enough to become rootbound, unless you have their roots in an ridiculously small container. But in situations where you have particularly robust plants whose genetics predispose them to rapid root growth, you use long grow phase durations, or you have motherplants, root pruning is an essential skill when you’re growing marijuana.