Tag Archives: Roots

Air layering – an excellent technique for creating new bonsai

Of all the propagation techniques available for bonsai, air layering is surely one of the best. There are so many advantages to this practice! The main one is that it can be used on mature trees with large branches, so that your bonsai ends up with a large trunk from the beginning. If you air layer at an angle to the trunk, and/or at a junction with two or more branches, you can make it even wider again. If you select the right position for your air layer, you won’t need the trunk of the eventual bonsai to grow any more than it already has, which gives you a massive head start compared to other techniques.

We had a surplus-to-requirements magenta crabapple on our allotment which I have air-layered over the last two years – in the first year I did 20 air layers and in the second I did 10. Of these around 15 have survived. Some examples are below – the largest one I did was from a main branch and has a 12cm trunk. You can also see one where I did the cut at an angle to change the orientation of the resulting tree, and make the trunk wider.

Another advantage of air layering is that roots form from callus at the edge of the air layer, which thickens the trunk right down at the base and also creates nice radial roots for good nebari (assuming you’ve properly prepared the air layer).

And there’s more! An air layer is as old as it was on the original tree. If it’s old (or large) enough to have mature foliage, flowers and fruit, it will continue to do so once separated. As far as I can see, this is by far the most accelerated technique for creating bonsai with flowers/fruit and thick trunks (as opposed to cuttings, which also retain the age of the source material, but are usually not as wide).

So what is air layering and why does it work?

An air layer involves ringbarking the tree at the place where you want to separate it into two. By wrapping the bare strip of branch in growing medium, roots are encouraged to grow at this point, and after a period of time, the branch can be sawn through and removed – the section above the air layer will have grown its own roots and can be planted out just like any normal tree. It’s called an air layer because the roots are literally being grown in the air.

Below are some images from my crab apple air layer. On the left is the ringbarked section of trunk prior to applying the growing medium. You can see the matt texture of the wood – all the living cambium has been scraped off (I use the blade from a pair of scissors). In the middle is the separated air layer with its plastic wrap on, and the pot still in place around the roots. On the right is an example after it has been separated and the plastic covering removed (an old compost bag). You can see the roots have developed nicely, circling the pot which was being used to hold the growing medium in place (sphagnum moss).

How the heck does this work?

Air layering takes advantage of the ‘plasticity’ of plant cells. The meristems within plants can become all sorts of different cells depending on the hormonal signals they receive. In the case of air layering, the passage of phloem (the liquid which flows from the leaves and carries the sugars which are the products of photosynthesis) is interrupted. This causes the hormone auxin, which is produced by stems and leaves, to accumulate at the site of the cut. And where auxin accumulates, callus material develops, and roots grow.^ref

What types of trees and branches work with air layering?

The positioning of an air layer is more important than the size of branch. Personally I think if you are going to the effort of air layering you may as well choose the fattest, chunkiest branch you can. But it’s important to know that some leaves need to remain on the section of tree above the layer, to drive the air layer root growth, and some need to remain elsewhere with an unimpeded path to deliver sugars to the roots of the main tree. In the image above left you can see there is another branch on the lower left of the air layer, which can supply the roots. If one layer is being placed above another, each needs to have their own source of sugars (ie. leaves with a connection to the layer). The year I did 20 air layers on the same tree, I made sure there were enough branches to go around, so each layer as well as the roots would have an energy source.

That’s the sugar supply, but what about water? Water can still flow to all the leaves on the tree via the xylem, as the xylem layers remain in the outermost part of the trunk & branches. These are not removed when the cambium is removed, so they continue to transport water around the tree.

Now – you may have read elsewhere on my site about ring-porous and diffuse-porous trees. Ring-porous trees only use a small range of xylem cells around the outside of the trunk just below the bark – some grow a completely new layer every year before they let their leaves bud out (eg. oak and beech). I have a hunch that it may be harder to air these species as they are reliant on this narrower xylem band which might be damaged by the layering process. There is some evidence that this is the case – one study could not successfully air layer several American oak species^ref and a quick search of bonsai forums suggests similar anecdotal evidence.

It might be important with these trees to create the air layer after they have leafed out, to be sure they have xylem there for water transport before you remove the cambium. And to be extra careful when scraping off the cambium, to avoid removing the water-conducting layer as well. This won’t be relevant for conifers, which are all diffuse porous and should be air-layerable. I have successfully air-layered cypress as well as juniper and you can see both in the image below (cypress in front, juniper behind on the left):

For angiosperms, you can check whether they have diffuse or ring porous xylem on this website. From experience I can tell you that Acer japonicum and Malus air layer relatively easily.

I have found that on an older section of tree (where the cells may be less plastic and less amenable to becoming root cells), you can increase your chances of success by air layering at a junction with a younger branch. Layering at a junction results in a multi-stemmed tree, as well as larger more interesting nebari, but it also seems from the ones I have done that the presence of the younger branch encourages more vigorous roots.

How do you do a successful air layer?

The basic practice for creating an air layer is to remove a strip of bark around the trunk, with the top of the strip aligned to where you want the base of the trunk of your bonsai tree to be. The strip of bark needs to be completely removed – all the way around the tree – and the cambium layer which sits just underneath the bark needs to be scraped off (sometimes this layer is not very visible but once you start scraping, you will see it coming off). In effect this creates a ‘phloem dead zone’ by removing the cells in the tree which transport photosynthates (the sugars produced by photosynthesis). It’s important that there are no stray cambium cells left, and that the gap is wide enough that it cannot be bridged by any callus which grows.

Once this has been done, the cut at the top of the strip needs to be packed with moist growing material and sealed. Many people will use sphagnum moss, but I have also successfully used half-moss/half-soil, and half-coco coir/half-soil, usually in a plastic pot which I have cut to fit the branch. The medium needs to be quite moist, and thickly packed above, below and around the cut. It has been demonstrated that adding IBA (Indole-3-butyric acid also known as auxin – found in rooting gels) can improve root growth speed and quantity.^ref

Once you have packed the cut with moist growing medium, it can be sealed in a plastic bag, or in plastic wrap (I also use a plastic pot under this). I have found it best to attempt to seal the wrap as best as possible, as this maintains the moisture within the air layer throughout the entire period. Moisture is critical for root development. Some people advocate leaving a hole for watering, but I think this just risks the layer drying out and is unnecessary extra effort. I use cable ties to secure a plastic bag around the base of the cut (on the bare trunk) and then wrap it several times around the layering medium before securing it around the top, leaving no gaps. If needed you can also tape up any loose edges with duct tape or similar.

It may be possible to do away with the growing medium altogether and to use a strip of aluminium foil instead. One study found that the reason why this exceeded the performance of moss/plastic on air layered radiata pine^ref was that the moss absorbed some of the auxin, taking it away from the plant and slowing down callus formation

People often ask how long an air layer will take to grow roots, but it’s very hard to answer this question. I would suggest give it a growing season – in the UK that could be creating it in March/April and separating it at the end of August or in September. If you unwrap it and the roots are not developed enough, it can be rewrapped and left for another season.

The obvious downside of using air layering is that it’s a lot more effort than taking a cutting or growing a seed, and you have to have access to good source material. Also that nobody will mind the presence of plastic bags and cable ties in the tree for the growing season! But the effort really is worth it when you consider the quality of material that can be created – here’s one of my favourites from the crab apple batch, only one year after separation:

To see a video of all the layers that succeeded, in their bonsai pots, please check out my Instagram @londonbotanica.

Mulch – relevant for bonsai?

Nitrogen-fixing and bonsai

Transpiration

I’ve talked about transpiration in quite a few different posts on this site, but a recent thread on http://bonsainut.com caused me to think maybe I should have a post dedicated to it, so here goes…

Transpiration is the evaporation of water from the leaves of a tree. It’s actually a critical process for trees, because excess transpiration is one of the few ways in which a tree can die; so-called ‘hydraulic failure’ has been identified as the most prominent cause of tree death.^ref Hydraulic failure – the failure to access enough water to replace water lost mainly through evaporation – causes cell death, xylem failure and a fatal reduction in photosynthesis. So it’s really important for bonsai practitioners to understand this process.

The main driver of transpiration is not – as you might think – to cool the leaves (although this is one reason for it). In fact transpiration is a by-product, or ‘cost’, of photosynthesis, and it happens because of the way that leaves obtain carbon dioxide. You may already know that plants have small pores called ‘stomata’ which open up to let air – and CO₂ – inside the leaf. But you might not have known that gaseous CO₂ in air needs to be dissolved in water before it can be accessed by chloroplasts and used for photosynthesis (this is explained in Vogel (Chapter 5 – ‘Leaking Water’). This means that water needs to be available on the surfaces inside the leaf – which means that when stomata open up, this water is subject to evaporation.

Vogel says that “only if the relative humidity is 100 percent will water not be lost…[and]…if the leaf’s temperature is above that of the surrounding air, then water can be lost even at that humidity.” He also says that for every gram of CO₂ used by a leaf for photosynthesis, it’s estimated that 125 grams of water is lost.

Smith et al (Chapter 4.10 Movement of Water & Minerals) explain that this evaporation causes a constant flow of water known as the ‘transpiration stream’. As water evaporates from the leaf cells, pressure in those cells is reduced, and this negative pressure causes water from the xylem to move into the space, due to strong mutual attraction between water molecules. That in turn pulls more water behind it and so on. This hydraulic mechanism is responsible for pulling water all the way up the tree from the roots. Actually this process is fundamental to the health of the tree, maintaining cell turgor (stiffness), transporting nutrients, metabolites & growth substances synthesised in the roots throughout the tree, and providing a source of water for the phloem stream which flows in the opposite direction providing energy to the tree. When there is enough water available, all of this works perfectly – when there isn’t, problems arise.

The extent of evaporation from the leaves of a tree is determined by several different factors, which can be divided into environmental, tree-specific physical factors and tree-specific response factors.

The main environmental factor which drives transpiration is the ‘vapour pressure deficit’ (“VPD”) – this is the “difference between the amount of moisture in the air and how much moisture the air could potentially hold when it’s saturated.”^ref VPD is a function of both heat and humidity, and provides a measure of how powerful the evaporative force of the air is with any combination of these.^ref

Occasionally while writing articles for this blog, I end up in the world of cannabis cultivation. Maybe because they are very motivated to keep their crops vigorous, cannabis growers and their equipment suppliers sometimes have the best data and charts out there! This is just such an occasion, see below for an excellent chart from ‘Ceres Greenhouse Solutions’ showing the VPD for a given temperature and humidity (you can download a copy here). The VPD is low in the blue section and high in the red.

https://ceresgs.com/wp-content/uploads/2018/10/VPD-Bioengineering-Chart.pdf

What you will notice is that the relationship between humidity and temperature isn’t exactly linear. Also, VPD increases with higher temperature and lower humidity. Since a higher vapour pressure deficit means there is more ‘pull’ on the water in leaves, increasing temperature and decreasing humidity both increase transpiration – and they reinforce each other, so dry and hot is a high transpiration combination.

Another environmental factor is wind. One study found that wind actually improves water use efficiency, because whilst it does increase transpiration, it also increases CO₂ uptake, and the net effect is greater water use efficiency and not less.^ref But for the purpose of this article, wind does increase transpiration.

Coming onto tree-specific physical factors – these are all the attributes that relate to the size, shape, position and structure of the tree. In general the more foliage a tree has, the more it will transpire – so a large broadleaf tree will transpire significant amounts on a hot day – in one study they found a large canopy tree in the tropics (Eperua purpurea) transpired up to 1180 litres per day!^ref By comparison in the same study, smaller (presumably more shaded) trees transpired a lot less. Thomas (Chapter 2: Leaves the food producers) gives the following figures: “<100L/day in conifers, 20-400 L/day in eucalypts and temperate trees such as oaks, reaching perhaps 500 L/day in a well-watered palm and as high as 1200 litres per day in specimens of Eperua purpurea growing out of the top of the Amazonian rainforest canopy.”

The chart below shows the daily transpiration rate during the growing season for a sessile oak tree in Turkey which measured 18.5m x 34.5m – this maxed out at 160 kg/day (effectively 160L).

https://www.ewra.net/ew/pdf/EW_2017_59_34.pdf

As well as the volume of foliage, trees have different stomatal size and density (number of stomata in a given area) which are determined by genetics as well as environmental factors (such as intensity of light and VPD to which they are exposed when developing).^ref1 ^ref2 Low stomatal area (ie. density x size) will result in lower transpiration when compared to a tree with higher stomatal area. These researchers measured stomatal area for 737 plant species across 9 forests and at the lower end of the spectrum conifers such as Cunninghamia lanceolata (0.2%) and Picea koraiensis (0.4%) had 100 times less stomatal area than angiosperms such as Viburnum betulifolium (23.77%) or Quercus serrata (21.74%). You can download all their data here. Basically the more stomatal area which is open to the air, the more transpiration there will be.

Many trees have wax plugs in their stomata which reduce their efficiency, and transpiration at the same time. To copy a piece from my article on needle leaves, wax deposits in Sitka spruce stomata reduce transpiration by two thirds but photosynthesis by only one third.^ref One study found that 81% of the species they looked at contained such plugs and that wax plugs are particularly numerous in conifers.^ref

Another factor is the level of transpiration via bark. This isn’t due to stomatal opening but simply due to partial permeability of bark to air – also genetically determined and due to the presence of ‘lenticels’ – small channels which allow passage of water and air for the metabolism of living cells in the bark. One study on Pinus halupensis found that “Bark transpiration was estimated to account for 64–78% of total water loss in drought-stressed trees, but only for 6–11% of the irrigated trees.”^ref This is because bark transpiration is passive and unmanaged, unlike leaf transpiration which can be somewhat controlled by the tree (see below).

Also relevant for individual trees is their position relative to other trees and the sun. Shade will reduce the temperature at the leaf surface and reduce transpiration, a mass of trees together along with undergrowth may increase humidity, also reducing transpiration. A tree standing alone or above others will be exposed to higher temperatures and lower humidity, thus increasing transpiration. Different areas on a single tree will be exposed to different combinations of these factors as well, so rates of transpiration will differ even from leaf to leaf on a given tree.

The final category of attributes which determine transpiration relates to the trees’ ‘behaviour’. That is, how they react to different environmental conditions. As we all know trees may be sessile but they are also incredibly dynamic and can adjust a wide range of parameters of their own biology. The main issue they need to address in this case is losing too much water, which could lead to death. As a result, they change their physiology to manage evaporation as well as water intake at the start of the transpiration stream.

To manage evaporation, trees adjust their stomata based on water availability, changing their ‘stomatal conductance’ to reduce transpiration if not enough water is available.

They do this in a couple of different ways – ‘passively’ and ‘actively’.^ref The passive mechanism is where lower water pressure within leaves causes guard cells around the stomata to lose their stiffness, which has the effect of reducing the stomatal aperture. The active mechanism relies on the tree producing abscisic acid (ABA) – this “triggers efflux of anions and potassium via guard cell plasma membrane ion channels, resulting in decrease of turgor pressure in guard cells and stomatal closure”.^ref

A study on Metasequoia glyptostroboides found that in most conditions of water availability the passive mechanism was in play, and it wasn’t until prolonged or severe water stress was experienced that the active ABA-mediated mechanism came into play.^ref The article explains that different gymnosperm species use different combinations of these passive and active processes to manage a lack of water availability by reducing transpiration. Angiosperms by contrast use a more sophisticated and more recently evolved version of the active process, mediated by ABA.^ref

Thomas says that stomata usually close when it is “too cold or dark for photosynthesis” or when the leaves are in danger of losing too much water and wilting”. The consequence of stomatal closing is an associated reduction in photosynthesis – so when a tree is drought stressed, it won’t be generating energy at the same rate as when it was healthy. A study measuring photosynthesis versus stomatal conductance for Pinus radiata (see in the chart below) found there was a roughly linear relationship, as the stomatal conductance increased, so did photosynthesis.

https://www.researchgate.net/publication/8690701_Variation_in_foliar_13C_values_within_the_crowns_of_Pinus_radiata_trees

There are several other ways that trees manage their transpiration – by adjusting their root conductance (ability to draw in water), changing their leaf expansion so that there are fewer/more leaves which are smaller/larger in area, pointing exposed leaves downwards during hot periods of the day, changing the root/shoot ratio to match water source to water use and by operating a daily cycle of metabolism which optimises transpiration (eg. increasing their root hydraulic conductance at night when there is lower evaporation, and ‘filling up’ to deal with higher transpiration during the day).^ref So they are very much active participants in responding to and controlled their transpiration rate.

But what does it all mean for bonsai? The first thing is, if your tree has plenty of water availability, transpiration should not become a problem, but you need to remember that up to 95% of water use is evaporation^ref so trees need a lot more water than you might expect. The best way to avoid issues associated with excess transpiration is to supply your trees with all the water they need. This is achieved by regular and sufficient watering, and by using a medium which has some water retention to avoid drought stress – but is also well-draining. A well-draining medium allows you to water more often without the risk of waterlogging roots or creating conditions for pathogens to take hold.

Also – a tree’s ability to handle water loss varies widely depending on the species – Thomas gives the examples of eucalypts and alder as species which cannot control transpiration effectively, and some oaks as species which can. So each tree in your collection will be different.

But let’s consider all the factors explained above that increase transpiration: high vapour pressure deficit (high temperature and/or low humidity), wind, lots of foliage, high stomatal area, clean (unwaxed) stomata, passive bark & leaf evaporation, a sunny/solitary/high position, and a lack of water availability to the roots which activate stomatal closure.

Some of these are adjustable for bonsai. If it’s going to be a hot, dry, windy day then your trees are going to transpire a lot more than normal and if their roots can’t keep up, you need to improve their environment; newly collected and recently root-pruned trees or trees in particularly small or shallow pots will be most affected. You can help them by providing shade (reducing the temperature), increasing humidity, and moving them out of the wind – and obviously by watering. For a temporary period, on a very hot day, it might even make sense to sit pots in water (do not do this for an extended period).

Transpiration can also be a problem in the winter as trees do continue to transpire, albeit at lower levels, even if they are deciduous. As such, they do need water to be available which means you need to keep an eye on moisture levels in pots. If they get dry, water them. If the medium is frozen, this will lock up water and can have a dehydrating effect so in this case you need to also water, ideally when it’s above freezing. Mulch is suggested to avoid hydraulic failure for trees in the ground^ref, a similar approach can be used for bonsai in pots, to reduce freezing and make more water available to roots. Even at night it is not the case that transpiration completely stops – typically it is 5% – 15% of daytime rates.^ref

Balancing the amount of foliage with the roots when repotting or pruning is another important way to help your trees manage their transpiration rates, so that there is enough root mass to meet transpiration demands. Root pruning in the heat of summer should be avoided unless a comparable foliage reduction takes place. If you’ve gone a bit far with the root pruning, use the approaches above – provide some shade, increase the humidity and maintain a watering regime. This is where the bagging method for collected trees comes from – it reduces transpiration by increasing humidity and can be used for trees struggling to recover from a severe root prune.

Anti-transpirant is a product that some bonsai aficionados use. This does what it says on the tin – it is a “film-forming complex of polyethylenes and polyterpenes that when applied to foliage will reduce the moisture vapor transmission rate”^ref The active substance is derived from conifer resins. In reducing transpiration these products also reduces photosynthesis, which is a consideration. I’m personally not a fan of disrupting a plant’s natural processes in this way, and successful use of the product depends on the individual tree and product selected (read more here).

Hopefully you can see from all of the above that transpiration is an extremely important concept to understand as a bonsai geek, but one which can be managed, as long as you are aware of the factors at play. Here’s to helping our trees avoid hydraulic failure!

Live Veins on Bonsai – do they exist?

Most bonsai enthusiasts will have come across the term ‘live veins’ in the context of bonsai. Live veins are areas of living bark surrounded by deadwood. They are often seen on juniper bonsai, where a section of bark twists around the tree in a dramatic contrast to the white deadwood (Sierra juniper are particularly amazing). But how does this actually work and is it a ‘vein’?

The bark layer on a tree contains the phloem, which is responsible for transporting photosynthates (sugars) and other molecules around the tree – it sits just at the base of the bark next to the sapwood. As new plant organs develop, a connected line of phloem cells is created so that sugars can be transported from these organs (if they are leaves) or to them (if they are sugar consuming organs like roots).

Phloem cells in the leaves connect to phloem cells in the branch, then to phloem cells in the trunk. They are long tubular cells with the main connection point for sap flow at the end of the tube, and minor connections in the sides. Below is an vertical image of sieve tubes of Cercidiphyllum japonicum (Katsura tree) – you can see the sieve tubes in blue, and their connections at a diagonal in dark blue. The brown cells are companion cells which help the sieve tubes to function. In this example there are some connections between the sides of tubes, but most of the connections are end to end.

https://scholarlypublications.universiteitleiden.nl/access/item%3A2951200/view

Phloem & sugars preferentially flow along this natural end-to-end route. One research study looked at what happens when you block a phloem path by girdling. It was observed that initially sugar flow to roots from that branch stopped, then resumed partially by finding another route (probably laterally through the sides of the phloem cells), then the tree grew new phloem and resumed sap flow.^ref

So what does this mean for bonsai? Basically – live veins (or more accurately, ‘live strips’) of bark can supply sugars to roots as long as they have phloem connections to sugar producers (leaves) and to sugar consumers (roots). What is really important is that we work with the orientation of the phloem cells when creating deadwood. Cutting across the grain of the phloem would sever the sap connections and be a form of ringbarking. Instead leaving a strip which goes along the grain of the phloem will provide a leaf to root connection. The phloem tubes will always be aligned lengthwise along a branch or on the trunk – ie. heading down to the roots. The variation you might see is that some phloem & bark spirals around the trunk and some goes straight down. This should be obvious from the bark pattern.

It’s also important to ensure there is enough foliage at the top of the live vein to meet the needs of the tree (or scope to grow more foliage). It’s useful to know that sugar from a leaf is prioritised for use local to that leaf. Leaves provide sugars for the developing shoot apex nearest to them, and flowers or fruit on the same branch; so from an energy perspective, as soon as it can be, a branch is self-sustaining. Lower leaves on a branch typically are the ones exporting sugars to the roots.^ref This might be useful when thinking about deadwood creation and possible options for live veins.

If you are aggressive with your live vein creation, and remove a lot of bark, it’s likely that some roots will die. One way to minimise this is to maintain a reasonable bark/phloem coverage around the base of the tree, and to start the deadwood further up.

One final word – there isn’t really any such thing as ‘finding’ a live vein. All phloem/bark is live until you create deadwood above or on it. It’s more about creating the deadwood and leaving the live vein (or ‘live strip’) behind.

Choosing a pot

Of course your choice of pot has a lot to do with the aesthetic vision you have for your tree, and I’m certainly not going to get into a debate about ‘feminine’ and ‘masculine’ trees and pots (hint – I’m not a fan of gendered bonsai!) Or glazed/unglazed (etc).

The pot for your bonsai is more than just its physical receptacle, it is also a life support system, holding water, soil & microbes and providing physical support. There are some physical attributes of pots which promote or inhibit a tree’s health, including materials, volume, width/depth ratio, drainage and even colour.

To start with materials. I had quite an unsatisfactory experiment with concrete in the form of hypertufa, for a while I was trying to save money by making bonsai pots using this material. Hypertufa is a combination of cement, sand, an organic material like moss or coir, and perlite/vermiculite. It worked well for making pots, but I found that they dried out really quickly; further reading told me that cement and particularly hypertufa is very porous. It also can leach calcium & silicon, which may or may not harm/benefit your tree depending on how much comes out.

A study on tomatoes showed better results for plastic pots in winter, and clay pots in summer, which related to the temperature of the pot.^ref Clay/ceramic pots did not heat up to the same degree as plastic. As an illustration of this is below – a thuja occidentalis has suffered heat stress damage on one side of the pot.

https://www.researchgate.net/profile/Avner-Silber/publication/332235948_Chemical_Characteristics_of_Soilless_Media/links/625e92ad709c5c2adb86e7cb/Chemical-Characteristics-of-Soilless-Media.pdf#page=324

This comes down to the principle of temperature buffering – or, the ability to withstand temperature variations without transmitting these to the roots. Buffering is improved when the pot is larger, and when the surface area at the top of the pot is reduced. On the other hand, some species respond well to having warm (not hot) roots.

Be aware that a dark or black pot will get hot out in the summer sun. In one study, a black pot caused the growing medium to be up to 10 degrees C higher than the air temperature.^ref In a sunny or hot locale, this could prove deadly to roots if maintained for too long. Where the pot is positioned and the foliage of the tree in it will affect how much sun hits the pot. One study found that plants grown in white pots had 2.5 times the root density of those grown in black or green pots.^ref

The geometry of the pot affects evaporation rates, since more surface area provides more space for evaporation to take place. You can see the differences in the table below. Yellow highlights show two pots of similar volume. The 7cm radius pot with 5cm of depth has a similar volume to the 9cm radius with 3cm depth. But the second pot’s surface area is 1.7 times larger than the first. This will significantly increase evaporation. It’s probably no great surprise to anyone who has bonsai that a shallow wide pot requires more careful attention to watering.

Another aspect to consider in choosing a pot is the geometry relative to the tree being blown over. Whilst a heavy pot can compensate for geometry somewhat, a tree wired into a pot is effectively a giant lever with the fulcrum at the edge of the pot. Wind coming sideways onto the tree will push the lever and if the pot is too narrow relative to the height of the tree, the tree will fall.

You can calculate the force needed to turn a tree over based on the fulcrum position of the edge of the pot. The larger the difference between the pot radius and the centre point of wind force on the tree, the less force is needed to push it over. I’ve done some calculations on a 2.5kg tree with a 30cm-ish diameter foliage canopy, and you can see below that once it gets to 50cm tall with a 20cm wide pot, the wind needed to push it over becomes much lower.

If the centre point of wind force on the trunk moves upwards, the surface area of the foliage increases, or the pot width decreases, you can soon end up with instability. My suggestion is to test this when you’ve repotted, push the tree at the point where you think the wind will be centred, and see how much force is needed to push it over.

Root Food Storage (or, can I root prune before bud break?)

One piece of advice often given to bonsai enthusiasts is that root pruning should be avoided until bud break – usually the advice says you should wait until the buds are just about to burst and then you can repot to your heart’s content. But is there any scientific basis to this? The rationale for the advice is the belief that trees store energy for bud burst in their roots, which translocates prior to bud burst and is used to power bud swelling and opening.

Below is a chart showing non-structural root carbohydrate levels through the year for Prunus avium – these include sucrose, glucose, fructose, sorbitol, raffinose & inositol. FB indicates when the tree was in full bloom, and H was the fruit harvest. As can be seen, the root carbohydrates don’t deplete until after bloom has happened (this species flowers before leafing out) and then builds up again after leafing, is depleted at fruiting and then builds up again. So in this case the tree has used the majority of root carbohydrates after blooming, and they were built back up again once the leaves were out.

https://journals.ashs.org/downloadpdf/journals/hortsci/25/3/article-p274.pdf

Labelling studies use radioisotopes to track where carbon has moved over a period of time. These have shown evidence that carbohydrates from roots are translocated to the first formed leaves and flowers in apple, cherry, pecan & grape.^ref This study also confirms that “In broadleaf deciduous trees, non-structural carbohydrates are depleted during winter dormancy and at the onset of spring growth, then replenished during the growing season”, however “in evergreen conifers non-structural carbohydrates accumulate in the crown in late winter and gradually decrease during the growing season”.^ref In evergreen angiosperms (Eucalyptus in this case) it was found that root carbohydrates did vary somewhat between a peak in summer and a minimum in spring, with starch being the major storage molecule – not only that, the researchers also found a lot more starch in the roots than in the lignotuber which is commonly believed to be some kind of storage organ (but apparently isn’t).^ref

So in general it is correct that trees are using their root-stored carbohydrates to flower and leaf out – although it would appear that they use these for actual leafing and not just to get to the bud stage. So theoretically it may be better if you are doing a major root prune to do this once the leaves are out (taking care not to remove so many roots that the leaves can no longer access the water they need).

Another study looked at the age of sugars in the woody and fine roots of different tree species. They found a big difference between those of ring-porous vs diffuse-porous trees – remember that ring-porous trees have a smaller and more defined ring of conducting xylem – and in some of these trees the xylem completely seizes up during the winter and a new conducting layer is grown every year. In the chart below ring-porous trees are on the left and diffuse-porous (which includes all conifers) on the right.

https://academic.oup.com/treephys/article/40/10/1355/5861906

In both types of trees, the youngest sugars are in the smallest coarse roots, suggesting these are being used as a sugar supply within a season. The sugars in the larger roots are aging with the tree, suggesting that the tree has obtained enough carbohydrates by other means (from photosynthesis or other storage tissues such stemwood) and hasn’t needed to tap the coarse root food storage.

The obvious difference between the two is that ring porous trees have younger sugars in their fine roots as well. It looks like ring-porous trees, which probably have a higher energy requirement since they need to regrow conducting xylem as well as buds & leaves, are tapping the fine roots for energy as well as the small coarse roots. Diffuse porous trees on the other hand do not appear to be using fine roots for this purpose.

But how much are roots contributing relative to other storage tissues in the tree? One study looked at a range of different trees in Harvard Forest near Harvard University in Massachusetts in the USA.^ref See below for the data showing the change in total non-structural carbohydrates throughout the year starting at January and going through to December for five species. What’s obvious is that root storage plays a different role depending on the species – and is least important in the white pine.

https://nph.onlinelibrary.wiley.com/doi/full/10.1111/nph.15462

What’s also interesting is that the only gymnosperm in the study (white pine), has a different peak – in June (midsummer when the sun is highest in the northern hemisphere). The other species peak in October after a season of photosynthesising.

Why do we care about this as bonsai enthusiasts? Well, stored energy helps to power processes within the tree, so whenever we prune storage tissues such as branches, stem & roots, we are removing energy reserves. So ideally we’d prune these when stores are lowest. When this is depends on the species but the above chart would indicate that actually August is a good time to remove roots – which goes against the advice often provided. Using the same chart would suggest that April pruning is best for branches. Which maybe suggests that bud break is being driven more by branch stored carbohydrates than root stored carbohydrates.

Ramification of Roots (lateral root development)

Lateral roots are ones which branch off from the main root – just like lateral or axillary buds aboveground. Lateral root development is how roots branch and ramify – similarly to stems, which ramify through bud initiation and stem growth. Encouraging strong lateral root development is a goal in bonsai, because we want to create lots of fine root mass to provide maximum exposure to water, nutrients and enabling symbiotic partners (fungi & bacteria); it also helps to stabilise the tree in the pot.

The below image is a nice one illustrating the development of lateral roots. The root grows from the tip – known as the root apical meristem (RAM), where new cells are created and the root tip is constantly extending. Above the apical meristem are pericycle cells which are preparing to initiate lateral roots, and above that these have been initiated and are starting to grow. At the top a lateral root emerges. The pale beige section in the centre labelled the ‘central cylinder’ is the location of the vascular bundle containing xylem and phloem.

https://images.app.goo.gl/E112RndafnE5jdNz5

I have been astonished while researching this post to discover just how many things can affect lateral root growth. My post on Ramification of Branches and Foliage had just three ways to improve ramification – dividing the apical meristem, pruning (in various ways) and applying cytokinins. In this post there are no less than nineteen different mechanism for encouraging root ramification! I’ll list them (x) as I go.

Before we look at each one of these, let me tell you about the ‘root clock’. The root clock is an oscillating cycle in roots which determines where lateral roots are formed – the spacing between them is dependent on the cycle time of the clock.^ref The way this works is through the oscillating expression of genes in a region close to the tip of the primary root, called the oscillation zone.

As with anything growth-related, our friends the plant growth regulators (along with genes) are involved. Auxin (1) promotes the development of branching lateral roots as well as adventitious roots (such as on air layers) whereas cytokinin (2) opposes these effects.^ref The formation of lateral roots involves both shoot- and root-derived auxin with the root tip responsible for lateral root initiation (the first step of creating a new root), and auxin from the shoot responsible for lateral root emergence (the elongation and growth of the initiated root).^ref The root clock is involved here because the back and forth gene expression causes programmed cell death at the root tip to happen periodically, which releases auxins back up the root and initiates a lateral root.

Ethylene (3) inhibits root growth, and brassinosteroid (4) and abscisic acid (ABA – some species only and in small amounts (5)) stimulate lateral root growth and elongation.^ref1 ^ref2 In fact there are complicated interactions between genes and plant growth regulators when it comes to roots with different hormone levels detected in different parts of the root, based on the differing roles they are playing in each stage of root growth. For more check out this article about tap roots which has a good diagram showing plant growth regulators (fig 2).

Other substances produced within plants which promote lateral root development include salicylic acid (6) and melatonin (7).^ref

Aside from plant hormones, the nutrients in the soil also affect the level of lateral root development – for example nitrogen (8): “in low-nitrate soils, patches of high nitrate have a localized stimulatory effect on lateral root development in many species, however where nitrate levels are globally high (i.e. not growth limiting), lateral growth is inhibited”^ref A phosphorus (9) deficit “favours a redistribution of growth from the primary roots to lateral roots”.^ref A sulphur (10) defiency “leads to the development of a prolific root system, usually at the expense of shoot growth…roots elongate faster than those with sufficient sulphate, with lateral roots developing earlier, closer to the root tip and at a greater density.”^ref

Lateral root formation is restricted when water availability is low (11), and somewhat surprisingly, when there is a lot of salt (12) in the soil more lateral roots form.^ref

You might not think that roots need light (13), but in fact light above ground is necessary for maintaining the oscillating signal of the root clock and for the formation of sites where lateral roots can branch off; an absence of light has a strong inhibiting effect on root elongation and branching.^ref This may be related to the point below about sucrose – light drives photosynthesis and the creation of photosynthates like sucrose.^ref

This intriguing study found that drenching roots in sucrose solution (14) “significantly increased lateral root branching and root formation compared with non-sugar supplemented controls.”^ref This may be because of effects on the rhizosphere vs the roots themselves. Just beware what you might attract if doing this in your bonsai garden as a sucrose drench sounds like an insect’s dream come true.

Several studies have determined that root pruning (15) encourages lateral root formation ^ref1, ^ref2 and that this happens most likely due to a surge in auxins after the root is cut. Similarly, synthetic auxins applied to roots of scarlet oak resulted in six times the number of adventitious roots compared to a control, and resulted in longer roots as well.^ref

Another way to increase lateral root formation is via encouraging arbuscular mycorrhizal fungi (16). Interestingly, the mechanism for this is that the plant root detects the presence of chitin in the fungi cell walls.^ref Chitin (C₈H₁₃O₅N) is “the most abundant aminopolysaccharide polymer occurring in nature, and is the building material that gives strength to the exoskeletons of crustaceans, insects, and the cell walls of fungi.”^refResearchers found that any source of chitin (17) had the same effect – including chitin derived from shrimp shells^ref Chitins can be found in many living creatures, including crustacean shells, insect shells (beetles, grasshoppers, cockroaches, blowflies), bat guano, resting eggs of Daphnia species, spiders, green algae, zooplankton (krill), phytoplankton and fungi.^ref1 ^ref2 ^ref3

On a similar theme (and maybe a repeat of 14) the presence of Pseudomonads bacteria has been shown to increase adventitious root development in tobacco.^ref

While reading a study about plant growth substances recently I came across (18) – apparently you can physically bisect a root apical meristem and it will become two autonomous RAMs.^ref

And lastly let’s look at two types of pots which affect root ramification: (19) if you use a white pot instead of a black, green or other dark colour, roots will be up to 2.5 times denser^ref, and if you use an air pot (20) they will have fewer circling or malformed roots but also less root mass overall.^ref

So let’s summarise all the different ways you can practically encourage root ramification in your bonsai:

Root pruning
Encourage auxin & sucrose production via photosynthesis (leave some leaves and give the tree good light)
Apply exogenous (from outside the plant) auxins – for example from compost or compost leachate
Drench roots in sucrose solution
Add and nurture mycorrhizal fungi and friendly bacteria
Add a source of chitin (as a vegetarian I can’t recommend any of the animal sources, but some other ideas include scooping the algae from your garden pond (or similar) or adding some mushrooms to your compost and adding that (or its leachate) to your pots
Bisect the root apical meristems (ie. cut them down the centre with a clean sharp blade)
- Use a white pot

When do roots grow?

Europeans have rather a binary view of when plants grow and when they don’t – growth in spring and summer, and none in winter. Deciduous trees give this impression, but as someone who did not grow up in Europe, I never had a sense that there were such specific times of growth – all around me were evergreen trees which seemed to me never to stop growing! So what actually is the truth for roots?

Obviously deciduous trees don’t photosynthesise when they don’t have leaves, but evergreens can and do photosynthesise throughout the year – albeit less intensely during winter. In a study on Picea abies “photochemical activity was high during early fall and then declined from November until April. Photochemical activity was at a minimum in April and then increased quickly to high values in May”^ref. A study in Idaho looked at four evergreen coniferous species and found that their photosynthetic output varied with temperature but did not go to zero^ref (shown in the chart below where the start of the chart is 7th September 2001)

https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/j.0269-8463.2004.00909.x

So do roots also show growth activity during winter? Some studies have looked into this question:

In established Scot’s pines, root growth has been shown to accelerate in early spring, then to back off while there is strong shoot growth, then to pick up again after this has finished in later summer and autumn.^ref Root growth did occur during winter but not at the same levels.
In 15 & 20 year old Sitka spruce, root growth up to 0.5 m from the stem base had a minor peak of activity preceding and a major peak following shoot elongation in the spring, while further than 0.5 m from the stem, root growth was frequently restricted to the period following shoot extension (effectively – summer and autumn).^ref
In 3-year-old Sitka spruce and Douglas-fir, height growth ceased in late September, but roots continued to elongate until mid-November, and the peak in root elongation occurred after height growth had stopped in one-year-old Douglas fir.^ref

The data points above suggest that root growth happens when resources are not prioritised for shoot growth. Temperature is another driver that has been identified – in one study covering six species, 85% of all new roots were grown in soil above 9 °C, and 6 °C was identified as the likely soil temperate threshold for root growth – with “a sharp increase of root growth between 6 and 9 °C, followed by a marked attenuation from 9 to 16 °C”^ref (the species in the study were Alnus viridis, Alnus glutinosa, Picea abies, Pinus sylvestris, Pinus cembra, Betula pendula). The root growth in this study was 40% less than the root growth in controls at warmer temperatures (16-23 °C). A similar dropoff temperature for root growth was also found for alpine species in a different study conducted in the Swiss Alps.^ref

What this suggests is that regardless of being deciduous or not, trees are able to use their stored energy to grow roots at temperatures above 6 °C. Looking at ‘Central England’ data from the UK Met Office (“representative of a roughly triangular area of the United Kingdom enclosed by Lancashire, London and Bristol”), during 2021^ref there were 332 days which went above this level at some point (ie. the max temp was greater than 6) and 200 days when it was above 6 degrees for the whole day with nearly 20% of these occurring between December and April. Most months other than January & December had a majority of days exceeding 9 degrees at some point, but the only months for which this was the case for the whole day were June – September. So in Central England there will be some root growth throughout the year, but not as much as during the summer months.

From a bonsai enthusiast’s perspective, this information tells you that roots grow at different times to shoots and often later in the season. Also – if you were wanting to encourage root growth you could consider having your trees in a greenhouse or sitting on a heated horticultural mat, as this would provide the higher temperatures needed.

Root structure and architecture

So we know what roots achieve for a tree, but how are they structured? To start with tree roots are either woody or non-woody. Woody roots have undergone secondary thickening and are long-lived, like the trunk and branches, and provide the structural framework for the tree.^ref

The ‘root collar’ is the area on the tree’s trunk where the roots join the main stem, and where there is typically a root flare (the root collar is still part of the trunk though, which is why it shouldn’t be buried in soil).^ref At the base of the root collar, there are usually five or more primary structural roots that “descend obliquely into the soil before becoming horizontal within a short distance of the trunk” and these taper rapidly within 1-2m of the trunk.^ref These are known as lateral roots since they grow in a lateral (horizontal) direction.

In his book ‘Trees, Their natural history’, Thomas says that trees develop a root plate, which is wide and shallow (vs the commonly held view of a root ball, which is only applicable to certain trees). Having a wide root plate helps trees achieve two of their main goals – to support and strengthen the tree against wind & weather, and to access waster and nutrients which are concentrated in the top layer of soil.

According to Thomas, root systems are more variable than shoot systems because the underground environment is more variable than aboveground. When roots encounter an obstacle underground, they fork, and as they fork and expand underground the main lateral roots can fuse into each other. This creates a criss-crossing of roots, which provides greater structural strength than if the roots were not connected. Roots can also connect to other trees’ roots (and even detect if they are ‘kin’ or not).

Structural lateral roots can develop into buttress roots, which have been found to provide tension strength in high-wind situations^ref – as a little girl growing up in Australia the best fun could be had climbing over the huge roots of the Moreton Bay Figs (Ficus macrophylla).

*Ficus macrophylla* in Kings Park, Perth Western Australia
http://skyfarming.com.au/public_html/great/firstrow/KPFig.html

In addition to lateral roots, most bonsai enthusiasts will have encountered the dreaded tap root. A tap root is the root generated by a new seedling (Thomas), which grows downwards and becomes a thick structural root. The tap root can become dominant in the root system and be a total pain for bonsai – it often generates its own lateral roots, creating a second root plate and makes it hard to get the tree into a bonsai pot. But luckily according to Thomas and others (and personal experience) the tap root isn’t necessary and can be removed. This is always best done sooner rather than later so that energy is not diverted to its growth vs the roots you do want to keep.

As well as tap roots, other structural roots trees create include sinker roots which go deeper into the soil (often to find water), can set up a secondary root plate, and also grow back upwards to create a ‘root cage’ (Thomas).

Susan Day et al^ref say “although structural roots comprise most of the root biomass, they account for a small percentage of total root length and root surface area.” The remainder of the root surface area is comprised of fine roots, which are the main mechanism for the tree to extract water and nutrients from the soil. Connecting the main structural roots to the fine roots are a network of tapering roots which branch off the structural roots.

A study of nine North American tree species found that in eight species roots <0.5 mm in diameter accounted for >75% of the total number and length of roots assessed.^ref Thomas quotes a study on Douglas fir estimating that 95% of the total root length comes from roots <1mm and about half less than 0.5mm.

As noted above the fine roots are non-woody and don’t undergo secondary thickening – this means they die and are replaced by new roots. It’s quite hard to measure this and there is differing information about fine root lifespan, but the above study found the average fine root lifespan to range from an average of 153 to 359 days. This is also expressed as a ‘fine root turnover rate’ and based on this data table fine roots of gymnosperms turn over more slowly than angiosperms (some Pinus species 20% per year vs beech 100% per year).

The fine roots are concentrated in the top part of the root plate, where most of the nutrients and water are located (20-30cm of soil, and the leaf litter & humus if present). Like the stems aboveground, the roots are constantly developing and growing, with new root tips being created by the root apical meristem (RAM) (this is described below). How the root goes about absorbing water and nutrients from the soil is covered in this post: How roots absorb water & nutrients.

These fine roots are what we are trying to encourage in bonsai as they enable the tree to extract the most water and nutrients from their environment, while still fitting into a small pot. What we want in the fine roots is lots of branching and ramification – just like aboveground – read more about encouraging this in ramification of Roots (lateral root development).

The below diagram shows the ratios of leaf, stem and root biomass to total tree mass for a data set including 3700 ‘woody’ plants (ie. trees!)

https://nph.onlinelibrary.wiley.com/doi/full/10.1111/j.1469-8137.2011.03952.x

As you’ll notice, the larger the tree gets, the more the stem (trunk) represents of the total biomass. However the ratio of roots to total biomass stays within a range from 16% to 40%. By comparison the ratio of leaf mass has a much wider range all the way from 60% down to 2%. So there is a certain baseline amount of root biomass needed to maintain a tree.

This mass is mainly made up of the structural roots, as although the fine roots comprise the vast majority of the root surface area, they are very light in comparison to the woody roots.

So bonsai nerds, what to make of all this? Key info is the fact that fine roots die and regrow on a regular basis – and – kill that tap root! Help your tree be more stable by encouraging a root plate of connected structural roots, and you won’t need a deep root ball or a tap root. Nebari and root mass should be around 20% of the mass of the tree for an old tree look.