How restricting pot size can help grow bonsai

cannabis bonsai pot in pot technique

One technique we’ve been experimenting with over the last year or so is the pot-in-pot, or double pot technique, an indoor adaptation of a bonsai practice that can help thicken the trunk in relation to the canopy.

Why use two pots?

The idea behind the double pot technique is to create an inner pot with holes only on the lower sides, and rest it a few inches deep into another larger pot which simulates the ground and allows your plant to grow rapidly and gain thickness. Because small pots limit the size of a plant through its sensory roots, this technique allows the inner pot to contain the majority of the rootball and the outer volume of soil to prevent the roots from sensing a small pot , so at the end of your grow you can remove the inner pot and repot it in a nice presentation pot for outsized flower relative to the pot size. In short just for the purposes of experimenting with these techniques!

Autoflowers

When used on autoflowers, the pot is best placed in the second pot early on or even at the start. The benefit is simply the ability to grow a large canopy by sending the plant signals to grow larger, while containing the rootball in a way that allows it to be transferred into a presentation version of the same pot (glazed ceramic or terracotta) toward the end of its life. Even a few extended exploratory/auxiliary roots will signal that it can grow as big as if it was in the large pot, and build its ability to extract more water and nutrients from its larger root network.

Photoperiods

With photoperiods, this technique becomes an important step in the long term process of thickening the trunk and branches by growing the canopy out and then cutting it back. Photoperiods can be placed in another pot when entering a vegetative growth cycle, whether young or after revegetation. Once the plant grows out its canopy and fills the lower pot with its roots you can remove the inner pot and cut back some of the canopy and the roots extending out of the sides. This process reduces leaf size, increases trunk and branch thickness, improving their thickness-to-height ratio while in the vegetative phase. During flowering you can leave to flower in the set and transfer to presentation after, or flower after removing from the pot-in-pot. Placing the pot into another during flowering can detract attention from the flowers to growing roots which may affect flower development negatively.

This autoflower cannabonsai in a contained pot we grew in Fall 2018 serves as a control to compare different sizes of the plants.

Another autoflower contained in its pot shows the natural stunting that takes place in small volumes of soil/medium.

cannabonsai pot in pot
This autoflower grew unconstrained and untrained within another large pot through Summer 2020 showing that even a modest auxiliary root system will cause the plant to grow far larger.

Origins of the pot-in-pot technique

We’ve seen techniques in a few hobbyist bonsai youtube videos where holes in a bonsai pot are used to grow out larger set of roots to thicken the trunk. The first was from Peter Chan at Herons Bonsai where he walks through his nursery lifting plants up off the ground, and another from a bonsai cultivator sitting in his backyard demonstrating his pot techniques. 

Peter’s video demonstrates that during periods of regrowth, you can place a bonsai pot on gravel or soil to allow the roots to extend outside of the pot. The second video showed how you can cut holes into the sides of a pot open to allow lateral roots to expand while containing the rootball. We’re still looking for those videos so if you’ve seen them let us know and we’ll add the links!  

Adapting the technique

We thought the technique would be great to adapt to cannabis so we started experimenting with the technique in our indoor cannabonsai grows to learn more.

Thick branches, small pot

A desired characteristic of many bonsai trees is a thick trunk in proportion to the rest of the plant, making it look aged beyond its size. A central technique to the practice is letting the plant grow vigorously, followed by several rounds of cropping the canopy back, then stabilizing, and then regrowing to get closer to ideal proportions.

Plants have exploratory roots

Root systems perform a number of functions. Among them are sensory roots – these are roots that are sent out early in a plant’s life to determine the size of the space the plant has to grow in. A cellular mechanism informs the plant which helps it determine how big to grow to avoid overextending itself with a large, burdensome canopy where there isn’t enough soil volume to begin with.

This mechanism is also what many bonsai enthusiasts use to limit the metabolism of a plant to slow its growth. If there’s less room for the roots to grow, naturally the plant will stay a smaller size. Small pots make for small plants.

“Doubling pot size increases plant size by 43% on average”

A 2012 study I read at the time went into detail on this phenomenon – Pot size matters: A meta-analysis of the effects of rooting volume on plant growth by Hendrik Poorter et al. (PDF download here and a read link here.) After scanning literature on the subject of pot-size restriction over the last 100 years, they drew these conclusions along with new MRI scans of active root networks in different pot sizes.

“Root growth is known to respond directly to impedance. Impeded roots stay shorter whereas the initiation and growth of side branches increases (Bengough and Mullins 1991; Falik et al. 2005). Furthermore, Young et al. (1997) showed that within 10 min of increasing the impedance to root growth, leaf expansion rate is reduced. This suggests that some kind of signal may regulate shoot growth when a large proportion of the roots are impeded. The actual signal for such a response remains as yet unknown. Possibly a reduced sink strength of the root system could cause a direct negative feedback on photosynthesis (Paul and Pellny 2003). Alternatively, a specific root–shoot signal is involved (Jackson 1993).”

Poorter, Hendrik & Bühler, Jonas & Dusschoten, Dagmar & Climent, J. & Postma, Johannes. (2012). Pot size matters: A meta-analysis of the effects of rooting volume on plant growth. Functional Plant Biology. 39. 839-850. 10.1071/FP12049.)

In hindsight, this study was one of the core pieces of knowledge that inspired this project years later when we started trying to create bonsai with Cannabis in 2018. When we started we got a lot of questions about the size and doubts regarding its possibility but three years later it has become something we get asked about less often.

Starting bonsai in the ground

A traditional bonsai technique used to thicken trunks is to plant the bonsai in the ground first and then pulling out the root ball, trimming it, and transplanting it into a bonsai pot. By planting directly into the ground, you are letting the plants roots feel like they are able to grow without limits. It can take years for bonsai trees started in shallow pots to thicken its trunk, but planted in open soil it can speed up the process by months.

Planting a pot in the ground

We’ve seen this technique in some bonsai youtube videos where they put it on the ground with holes in the pot. Peter Chan was walking around, lifting pots that were rooted into the ground through the bottom of the pots to show how much faster the trees grew when outside the pot over different time periods. 

The other bonsai grower opted for only cutting holes in the sides of the pot, burying it a few inches into the ground. This achieves a similar effect, where the rootball is contained and the exploratory roots allowed to roam free. Containing the rootball helps reduce the plants stress or risk of dying when trimming the roots, as the long and fine exploratory roots are not as core to the plants function as say cutting or damaging the taproot.

Adapting technique to cannabis

Because it’s easier on the plant to remove and downsize a pot when the rootball is contained, when you’re removing and downsizing pots there’s less damage to the root system. Given the delicate nature of roots, we wanted to try this technique with our cannabonsai to test the effects and see if we can achieve a thicker trunk.

We drilled holes at the bottom of the pot and prepped to bury them completely into the second one at different times of the grow, with different varieties of cannabis.

Experiment 1: Untrained Autoflower Pot-in-Pot

Our first experiment was with an auto from Barney’s Farm to see if the roots will be drawn into the larger pot below and how the root ball would compare to the roots that extended into the outer pot.

cannabis autoflower
autoflower cannabis pot in pot

We left the plant untrained from bonsai techniques to see how the plant naturally grows in this pot-in-pot technique without any other factors that may influence its size.

cannabonsai pot in pot
cannabonsai pot in pot

We’ve grown a number of bonsai in the smaller pot, but of course this plant is much larger. In fact it’s probably 4 times in size and weight than ones we grew in the smaller inner pot!

The roots that explored outside of the pot and expanded the reach of the plant were much thinner and longer than those in the rootball, visibly extending and reaching around the volume of the pot to determine how big it can safely grow. It’s clear the bulk of the root volume existed within the inner pot, and while these exterior roots were quite long, their mass wasn’t quite as full as it would’ve been with more time.

Experiment 2: Root trained Autoflower Pot-in-Pot

cannabis bonsai cannabonsai pot in pot technique
cannabis bonsai cannabonsai pot in pot technique in action where the flower finished

Later in the following year we worked on a pot-in-pot roots-on-rocks bonsai where we chased the roots from the top down into the lower pot below. This allows the soil volume to remain stable while the roots are being trained rather than simply subtracting from them without anywhere to expand into.

This auto was hardly trained on the canopy and we focused mostly on the roots. It was an autoflower, which don’t really like transplanting, so naturally it didn’t have a ton of time to expand into the lower pot, but we found a few small roots extending downward. Based on the size of this one, the root training and small outer pot didn’t help the plant grow too well. We’ll run the experiment in our new and improved set up and without any transplanting or root training for better controls.

Lessons here are that not all pot-in-pot techniques will work, but given the room to explore, the plant will have a much larger upper bound to its size. The goal would be next to try it with a photoperiod strain and see how it goes over a few more months! It’s simply a technique that may produce interesting results when removed from the outer pot at the very end of its cycle for presentation.

Double pot challenge

We’ll keep adding new pics to follow up with the experiment but give it a try and let us know how it goes, we want to add to this with other features as people try out similar experiments. Use hashtag #potinpot #doublepotchallenge and tag us @cannabonzai!

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