Tag Archives: Phloem

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 speciesref 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 pineref 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.

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.


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.


The roots of your tree are *just* as important as the above-ground parts, with a lot of responsibilities which aren’t immediately obvious. I’ve summarised the main ones here but there is a lot more detail in separate posts with links provided below. So why are roots so important?

  1. they absorb water from the soil to meet all the tree’s needs (both for photosynthesis and transpiration)
  2. they absorb all the nutrients that the tree needs from the soil (using a different process to water, hence a separate point)
  3. they transport nutrients & water up to the above-ground parts of the plant, and photosynthates (the products of photosynthesis) down to the root tips
  4. they produce exudates (secretions) which sense and control the rhizosphere (the environment in which the roots are growing)
  5. they produce plant growth regulators for signalling and enabling growth within the plant
  6. they store food for later use
  7. they provide structural strength and stability for the tree by attaching it into the soil

Points 1 and 2 are fundamental to the health and growth of the tree – the roots are the mechanism for the tree to obtain all of the water and nutrients it requires (despite the mythic popularity of foliar feeding, this is only a way of augmenting nutrient absorption and not a primary mechanism). The mechanics of how they do this is described in more detail in how roots absorb water and nutrients – in summary it’s the fine roots and their root hairs which do the majority of the absorption since they have the closest and most expansive contact with the soil. There needs to be enough root surface area to supply the stems, shoots and leaves with the water and nutrients they require.

Point 3 reflects the fact that roots are part of a tree’s vascular system, that is to say, they transport the fluids necessary for growth around the tree. Above the ground the vascular system is present in stems, shoots and leaves, and below the ground it is present in the roots. Water and nutrients are transported up from the roots through the xylem and photosynthates (the products of photosynthesis) are transported down from the leaves and other storage organs in the tree via the phloem, to provide the energy and nutrients for the roots to grow and function. Do roots grow all year round? Find out here: when do roots grow?.

Points 4 & 5 show that roots are very much an active participant in tree growth and not simply a set of supply pipes. They produce both cytokinin and auxin (read more in the post about plant growth regulators), they also produce a huge variety of substances known as exudates which both sense and control the rhizosphere (the environment in which the roots exist). Researchers believe that roots use exudates to “regulate the soil microbial community in their immediate vicinity, withstand herbivory, encourage beneficial symbioses, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species”ref. Read more about exudates and how they are produced in root exudates.

Point 6 reflects the fact that roots are used to store food, in the same way that the trunk and branches do this aboveground (throughout the ‘woody skeleton’ (Ennos)). I was going to tell you that a lignotuber is an example of this and show you a lovely picture of my eucalyptus, but then I read “contrary to common assumptions…the lignotuber in young eucalypt trees did not appear to be a specialized starch storage organ. Rather, the lignotuber resembled an extension of the stem because its starch concentrations and temporal fluctuations mirrored that of the stem.”ref How roots store food and how much of a contribution to the plant’s overall storage capacity they make is debated. More on that in Root Food Storage (or, can I root prune before bud break?)

Finally as per point 7, the roots are responsible for physically holding the tree steady and stable against wind and gravity. They do this in many ingenious ways by adopting different root architectures – combining vertical taproots, lateral roots & sinker roots, creating ‘buttress’ roots, sending roots far from the trunk when needed and managing new root development in ways which stabilise the tree. More about this in root structure and architecture.

What all of this means from a bonsai perspective is that you need to pay just as much attention to the health and care of the roots of your tree as you do to the above-ground parts. Never mind developing a strong nebari for aesthetic purposes, you need to ensure that even though the roots of your bonsai trees are squashed into teeny-tiny pots, they are still able to perform the vital functions outlined above. Neglecting the roots will negatively affect the overall health of your tree.

Practically speaking, this is why you should aim to develop a well-ramified fine root ball, to provide the tree with lots of root surface area for nutrient & water uptake – taking into consideration the amount of biomass above-ground as this will determine how much root mass is needed.

The growing medium plays a huge role as well – this is your tree’s rhizosphere. It should provide the water, nutrients and micro-organisms the tree needs as well as (some) oxygen for root cell respiration, and ideally should not be disrupted so much so that exudates and microbes (fungi or bacteria) are lost. The risk associated with bare-rooting a tree (or excessive repotting) is that it destroys the rhizosphere all at once, leaving the tree vulnerable to pathogens and forcing it to regenerate exudates it has already created (which can use up to 40% of its stored carbon).

Your tree’s roots need to have a regular supply of nutrients, so they require fertiliser of some kind. Even if good compost is added during potting, the small size of bonsai pots will mean the nutrients won’t stay in there for very long. Trees will need added fertiliser – either home-made (for example, regular doses of diluted compost leachate), or purchased. And obviously – watering is critical. Given the role of symbiotic partners (such as fungi & bacteria), you can also add these to the soil – if your tree senses their presence and wants them to stick around – it will probably produce exudates to achieve this.

Vascular Cambium

The cambium – or more precisely the vascular cambium – is a layer of cells underneath the outer and inner bark and outside the wood of a tree. It’s officially defined as a ‘meristem’ – that is, a region of cells capable of division and growthref. You may recall the ‘shoot apical meristem’ in the post about How Trees Grow – this is the part of the shoot which is actively dividing and creating new cells at the tip of the shoot (known as primary growth). The vascular cambium does something similar – it divides to create the vascular system – a layer of xylem cells on one side and a layer of phloem cells on the other. The vascular cambium is where part of the secondary thickening of a tree takes place, as the xylem layers become the wood of the tree, and the phloem layers become the inner bark (the outer bark has another meristerm – more here). Always in between there is a single-cell thickref layer of vascular cambium. See below for an image of the cambial zone, phloem and xylem cells.

From The Plant Stem: A Microscopic Aspect 1st ed. 2018 Edition, Kindle Edition
by Fritz H. Schweingruber & Annett Börner

Two types of cells exist in the cambium – vertically elongated ‘fusiform initials’ and horizontally oriented ‘ray cell initials’. The fusiform initials produce xylem and phloem cells and the ray cell initials produce rays (which cut across the tree connecting xylem and phloem).ref The ray cells can create gum and resin channels, which can also be activated when the cambium is wounded. You can see a resin duct cavity in the image above, as well as a ray.

Like a lot of growth in a plant, the activity of the cambium meristem involves plant growth regulatorsref. Auxin levels peak in the middle of the cambial zone, where cells are dividing, cytokinin peaks in the developing phloem cells and gibberellin peaks in the developing xylem cells. A study in Populus found that increased local biosynthesis of cytokinin led to increased trunk biomass and radial size (width).ref ‘Local’ biosynthesis in this case meant a tree which had been transgenically modified to produce more cytokinin.

The vascular cambium slows down or stops completely during winter in temperate zones, this depends on the tree’s phenology. More detailed information about the vascular cambium can be found in this book.

From a bonsai point of view the main takeaway is that the vascular cambium tissue underneath the bark is critical for your tree’s growth so avoid damaging it.


The word ‘phloem’ comes from the word for bark in ancient Greek. It is a parallel system to the xylem which transports water and nutrients up from the roots, but instead transports the products of photosynthesis (‘photosynthates’) from the leaves to the rest of the tree. A big callout to The International Association of Wood Anatomists for the images in this post, contained in this open-access publication.

One of the main photosynthates produced by trees’ leaves is sucrose (maple syrup anyone?), but others found in phloem include fructose and glucose, sugar alcohols and the raffinose family of oligosaccharides (RFOs). A sugar alcohol known as ‘D-pinitol’ has been found in substantial amounts in gymnospermsref and is believed to be the main carbon transport molecule for Scots pine. In addition to sugars, the phloem system is used for signalling and defence throughout the tree (as is the xylem), so plant growth regulators (including auxin, cytokinin and salicylic acid), proteins, minerals and RNA travel in the phloem sap as well. If a foliar insecticide/herbicide/fungicide has been applied and is able to penetrate the pores or stomata (see foliar feeding), and is able to get into the phloem vs staying inside adjacent cells, it will translocate throughout the plant.ref As a result I would not be eating non-organic maple syrup (previously paraformaldehyde was used to reduce microbial attacks on maple trees for syrup product, but this was banned by 1989).ref

There still seems to be quite a bit that’s unknown about how phloem actually works – an article published in 2014 said “Because of the difficulties in measuring phloem function, particularly in trees, we lack a basic natural history and phenomenology of tree phloem”ref and another published as recently as 2021 said “phloem loading strategies in gymnosperm trees have been only tested in three species: P. sylvestris , Pinus mugo and Ginkgo biloba.”ref

But the basic principle is that sugars are created by the process of photosynthesis, ‘loaded’ into the phloem cells (with assistance from adjacent cells) and transported to places in the plant where they are needed, then ‘unloaded’ (but even the mechanism for transportation of sugars in phloem is debated – a famous theory involving ‘osmotically generated pressure gradients’ has dominated but many recent articles point out the lack of data to support it.ref) According to one account, sugars are loaded from leaves into phloem companion cells by active transport (a process which consumes energy) and then diffuse into the sieve tube elements through the plasmodesmata (cytoplasm which is shared between cells via small pores between them). Water then moves by osmosis into these cells (creating the phloem sap), and sugars translocate (move) when sinks (areas of the plant consuming energy) remove sugar and reduce its concentration in the phloem sap.ref

Phloem is also believed to translocate (move from one place in the plant to another) sugars even when photosynthesis is not taking place – eg. in winter in deciduous species.ref In this case the sugars are coming from storage tissues in the rays and roots.

The cells which make up the phloem system in gymnosperms are different to those in angiosperms (similarly to the difference in xylem), but the basic structure for both is that tubular cells, known as sieve cells (gymnosperms) or sieve tube elements (angiosperms), are connected together via pores in their end walls, and the phloem sap ‘flows’ through these sieve cells/tubes.ref

Below is an image of pine sieve cells. The side and end walls are structurally similar, unlike the sieve tubes of angiosperms. The phloem sap flows from cell to cell downwards, through the pores. Many studies reference the fact that sieve cells & tubes contain material which would appear to create a barrier to flow, which calls into question the abovementioned ‘osmotically generated pressure gradients6’ theory.ref


If you’ve read the post about the cambium, you’ll know that there is a constant process of creating new xylem and phloem cells, and in the case of phloem, the most recent does the conducting.ref The conducting phloem usually lasts for one season, but can remain ‘functional’ for one-two years (ie. the cell is still alive, even if it’s not conducting phloem any more). Like xylem, phloem rings are created – see the image to the right of pinus strobus – all of the dark cells are the annual phloem sieve cells which are now non-conducting. The conducting cells are in the lower purple region.


A key difference between xylem and phloem is that phloem cells are living cells. This means that phloem sap must pass through living cells and their membranes in order to flow and this articleref suggests that this mechanism provides a high degree of control for the plant in managing what gets into and out of the phloem system. The phloem passes through holes in the sieve cells known as sieve plates (see pics below both of ficus species, the left hand side shows a transverse section and the right hand side a lateral section).)


In order to create the space for the phloem sap, sieve cells and tubes are missing quite a bit of the normal cell machinery, including a nucleus, vacuole and ribosomes – so they can’t control their metabolism or make proteins. Although they still have some specific proteins (P-proteins – apparently previously known as ‘slime’!ref), mitochondria, endoplasmic reticulum, and sieve element plastids.ref Both types of sieve cells have helper cells alongside which metabolise on their behalf – companion cells in angiosperms and Strasburger cells in gymnosperms.

Since phloem is full of delicious sugar-rich fluid, it can be a magnet for insects, which in turn introduce microbial pathogens including bacteria and viruses.ref Plants produce metabolites to defend themselves against these pathogens, and also induce sieve plate occlusion – basically blocking up the sieve cell or tube where the pathogen is located to avoid it spreading.ref

Both the active phloem and the old phloem which no longer transports photosynthates are together known as the inner bark. Outside these phloem layers is the ritidome or outer bark. You can read more about bark here.

For bonsai there’s really not a lot you need to worry about with respect to phloem, unless you are wiring super tight and cutting off the phloem (but by then your wire will be well embedded in the outer bark).