I had never heard of root exudates before creating this website, but in fact their production is so important to plants that they “invest up to 20–40% of their photosynthetically fixed C”ref in this process.
Root exudates are basically substances created by root cells and sent out into the nearby environment – known as the rhizosphere. These can be waste products which diffuse across the cell wall, or manufactured compounds which serve a specific purpose in response to the environment.ref
There are many different exudates produced by plants, including carbohydrates (sugars), organic acids (such as acetic, citric or malic acid), amino acids, flavonols (molecules which can have protective effects on cells), enzymes (such as amylase, which helps to digest carbohydrates), plant growth regulators (substances which stimulate cell growth, such as auxins), phenolic acids (which have anti-oxidant properties), flavonoids, terpenoids, tannins, steroids and an assortment of other substancesref1,ref2.
The roles they play are just as diverse, including:
producing food in the form of carbon metabolites to support beneficial bacteria (such as nitrogen fixing bacteria), fungi, nemotodes and protozoaref1,ref2, which in turn assist with nutrient uptake and the production of their own compounds such as phytohormones
producing phytotoxins (plant poisons) such as terpenes (also found in conifer sap) to repel pathogenic microbes, invertebrate herbivores and parasitic plantsref1,ref2
detecting ‘kin’ (related plants) and avoiding competing plants which are not relatedref
changing the soil chemistry to allow for better nutrient uptake – for example exuding chelating substances which allow for better uptake of metallic micronutrients, or organic acids which enable better phosphorus uptakeref
So it seems that the roots of a tree can act like a sort of pharmacy, creating compounds that protect and nurture the tree and its beneficial partners via the rhizosphere. In my opinion what this means for bonsai is that firstly you don’t want to damage the tree’s ability to produce exudates, and secondly you don’t want to remove too much of the soil from a healthy root ball.
You can damage the tree’s ability to produce exudates by underwatering/drought – and this may not be recoverableref, but also by failing to provide all of the nutrients and micronutrients needed for healthy growth (by not fertilising enough).
What’s in the bonsai pot is clearly more than just roots and soil – it’s an entire ecosystem delicately managed by the tree itself. So perhaps being less heavy-handed during repotting would be a good idea – replacing a good amount of the soil back into the pot along with its microbes, exudates and adjusted chemical makeup.
Roots exist in a their own ecosystem along with soil, chemical compounds, microorganisms and variations in pH, humidity and temperature. This environment is known as the ‘rhizosphere’, a term created by Lorenz Hiltner in 1904, using the greek word for root ‘rhiza’.
The term refers to the area around the roots, and is broken into three parts. “The endorhizosphere includes portions of the cortex and endodermis in which microbes and cations can occupy the “free space” between cells (apoplastic space). The rhizoplane is the medial zone directly adjacent to the root including the root epidermis and mucilage. The outermost zone is the ectorhizosphere which extends from the rhizoplane out into the bulk soil.”ref
The rhizosphere is FULL of microbes – this articleref estimates there are 1000-2000 times the number which are found in non-rhizosphere soil. These include endomycorrhiza and ectomycorrhiza as well as beneficial (and pathogenic) bacteria. Below is an estimate of the number of genes represented in a sample rhizosphere across each type of organism (a list of the species included are in the research paperref)
Rather than passively respond to the rhizosphere, roots produce ‘exudates‘ – substances released from their cells – which are used both to sense the environment (such as, where competing roots are located and the presence of beneficial microbes and nutrients) and to alter it to the plant’s benefit. So the rhizosphere is a very dynamic place, teeming with life and being constantly manipulated by the tree for its own benefit. Below is a great image illustrating everything that’s going on – different mycorrhiza, bacteria and the roots interacting in the rhizosphere.
‘Mycorrhiza’ are fungi which have a symbiotic relationship with roots – they each provide something of value to the other party. The word comes from the Greek words for ‘fungus’ and ‘roots’ so one should strictly call them mycorrhiza and not mycorrhizal fungi since the latter is an example of ‘RAS syndrome’ (redundant acronym syndrome, which itself is also an example of RAS syndrome).
According to one study, “for efficient nutrient uptake, most land plants need to be associated with mycorrhizal fungi that supply minerals, increasing their productivity and conferring resistance to stress.”ref So these fungi are actually a critical part of life on earth, and necessary for healthy plant function.
Mycorrhiza are usually divided into two groups – endomycorrhiza and ectomycorrhiza.
‘Endo’ comes from the Greek ‘endon’ meaning ‘within’ – and endomycorrhiza (known as Arbuscular Mycorrhiza or ‘AM’) have hyphae (fungal threads) which actually penetrate the plant’s root cells and establish an intracellular symbiosis with the plantref. AMs scavenge for nutrients such as Phosphorus and Nitrogen released by saprotrophic microbes (ie. bacteria which feed off dead material) and make these available to the plant.ref
‘Ecto’ comes from the Greek ‘ektos’ meaning ‘outside’ – and ectomycorrhiza (‘ECM’) form a thick mantle around root tips from which clusters of hyphae extend beyond the root zone.ref They ‘mine’ Nitrogen and Phosphorus from the soil by producing enzymes which digest soil organic matter – they can then make these available to the trees in return for carbon sources such as sugars.
Whether a particular species of tree is associated with endo- or ectomycorrhiza is detailed in this site. The trees we’re interested in from a bonsai perspective fall in each camp: Associated with ECM are oak, beech, hornbeam, birch, hazel, alder (actually with both), tilia (lime/linden), chestnut and all of the Pinaceae family (including fir, cedar, larch, spruce, pine & hemlock). Associated with endomycorrhiza (AM) are grapevine, Prunus (cherry, peach, plum etc), pyrancantha, magnolia, Ilex (holly), Araucariaceae, wisteria, ficus, mulberry, ash, olive, all maples, horse chestnut, poplar/aspen, willow, buddleja, yew, camellia, elm, podocarps, flowering quince, hawthorn, apple, cotoneaster and all of the Cupressaceae family (including Cryptomeria japonica, cypress, junipers, redwoods and thujas),
Aside from this, azaleas are associated with a different mycorrhiza called ericoid.
Fungi aren’t the only microbes in the rhizosphere – it’s also teeming with bacteria – ‘rhizobacteria’. Symbiotic bacteria in the rhizosphere – known as Plant Growth Promoting Rhizobacteria (‘PGPRs’) deliver a raft of benefits to their host plants – some of which they literally could not survive without. They improve a plant’s resistance to pathogenic fungi, bacteria, viruses and nematodes as well as abiotic (environmental) stress like drought or heavy metal pollution, they also fix nitrogen into root nodules, convert organic nitrogen into inorganic forms (NH4+ and NO3–) which are available for plants, improve the availability of phosphorus and iron, control other nutrients including sulphur, iron and manganese, and synthesise plant growth regulators which improve plant growth.ref1,ref2This study has a table showing some of the positive plant responses to specific bacteria in research studies.
They achieve these outcomes for their host plant partly by going about their task of decomposing organic matter, but crucially also by producing substances including siderophores which make iron available, enzymes which degrade the cell walls of pathogens, volatile compunds such as hydrogen cyanide, biosurfactants which lower the surface tension of liquids, antibiotics which target pathogenic bacteria and phytohormones which promote plant growth processes; all of these go into the soil and into roots.ref Bacteria are also able to remove toxic metals from the soil through several different mechanisms and pathways.ref
This is such a fascinating area – bacteria turn out to be tiny bespoke pharmacies available to plants to help them thrive. And plants are not just passive recipients of bacteria – they create root exudates which attract bacteria they specifically need at a point in time, they are able to manipulate the rhizosphere to meet their needs.ref Plant genotype (ie. it’s genetic makeup) and the soil type are two main drivers that shape the rhizosphere microbiome.ref pH is particularly important, with studies showing that bacterial diversity was highest in neutral soils and lower in acidic soils.ref
The different bacterial species which are associated with different benefits for plants include the followingref:
On a final note, bacteria can produce ‘bad’ substances as well, particularly in anaeroic (no oxygen) conditions, when they produce phytotoxic nitrates and hydrogen sulphide. So avoid your bonsai substrate becoming too enclosed without aeration.
Unlike animals, plants do not have a digestive system, although the sustainable food trust makes a good argument that ‘soil is the collective stomach of all plants’ref Trees synthesise all of the substances they need to live and grow from 17 nutrients. It’s important to understand that plants don’t ‘eat food’ in the sense of consuming sugars, fats or proteins like animals do. Aside from oxygen, carbon and hydrogen (which come from air and water), trees absorb nutrients through their roots.
Water and nutrients are transported around trees via the xylem, a network of narrow dead cells which act like a kind of pipe. Nutrients are dissolved in the water (‘solutes’) and travel with it in the form of ions (charged molecules). To get into the xylem in the first place, water is absorbed into the root tips.
In many species this is done through the root hairs. Root hairs are “long tubular extensions of root epidermal cells that greatly increase the root surface area and thereby assist in water and nutrient absorption.”ref According to Thomas most live only for a few hours, days or weeks, and are constantly replaced by new ones as the root growing tip elongates. Some conifers do not have root hairs and rely on mycorrhiza instead to assist nutrient and water absorption.
In order to absorb water, the root tips need to be in physical contact with it, so having root hairs that reach into the soil provides contact with more water (and nutrients). Nutrients in the form of ions are ‘pumped’ into root hairs (or cells, if the species has no root hairs) using a process called active transport, which uses some of the energy from photosynthesis. Because the root cells have dissolved nutrients in them, water is then attracted into the space by osmosis.
From the roots tips, water and solutes make their way to the ‘stele’ – this is the central part of the root which contains the vascular system (xylem & phloem, shown in blue and red respectively in the left hand diagram below). Surrounding the stele is the endodermis – seen below in orangey-brown cells with red lines through them.
The red lines represent cells known as ‘Casparian strips’. They are full of lignin and other hydrophobic molecules, which basically plug any gaps between the endodermis cells. This forces any water or solute to pass through the endodermis cells. After this they travel through the root parenchyma cells into the xylem.ref
The existence of Casparian strips leads to a pretty important insight, which suggests that most molecules entering the xylem from the outside world are actively invited in, and have to be able to traverse a cell membrane. So the tree can theoretically control or at least limit what can enter. Vogel says “the sap that rises up a tree trunk has to be nearly free of dissolved material. So much water gets transpired that the accumulation of dissolved solids, coming out of solution as water evaporated in the leaves, would make big trouble as the growing season advanced.” So this implies there aren’t a lot of non-nutrients dissolved in xylem sap. But in fact, xylem has a microbiome (it’s part of the endosphere) and literally thousands of dissolved molecules in it (described more in xylem), so obviously the Casparian strips are not a 100% barrier.
It’s not all down to the root hairs or root tips though, symbiotic fungus known as mycorrhiza play an important role in enabling root function, read more about this in The Microbiome and Symbiotic Microbes.
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?
they absorb water from the soil to meet all the tree’s needs (both for photosynthesis and transpiration)
they absorb all the nutrients that the tree needs from the soil (using a different process to water, hence a separate point)
they transport nutrients & water up to the above-ground parts of the plant, and photosynthates (the products of photosynthesis) down to the root tips
they produce exudates (secretions) which sense and control the rhizosphere (the environment in which the roots are growing)
they produce plant growth regulators for signalling and enabling growth within the plant
they store food for later use
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.
You’ve probably heard of rooting hormone powder, or auxin, or gibberellins – these are all ‘Plant Growth Regulators’. Plant Growth Regulators used to be known as ‘phytohormones’ which means plant hormones. This has been quite a contentious topic among plant biologists.
A hormone in an animal is a chemical messenger, a substance which acts as a signalling or control molecule to cause an action to take place. “Hormones carry out their functions by evoking responses from specific organs or tissues that are adapted to react to minute quantities of them”ref. In animals, hormones are produced at a specific site (a gland, like the pancreas), work at low concentrations, and have a predictable dose-response. That is, an increase in hormone will result in the more of the related action (eg. more insulin leads to more sugar being taken up by the liver).
There *are* substances synthesised by plants which are involved in regulating growth – plant growth regulators – but they don’t work in the same way as animal hormones. It’s said that the assumption that they did sidetracked plant researchers for decades.ref Plant ‘hormones’ are synthesised in multiple sites in a plant (and potentially in every cellref), have multiple actions on different cells (they don’t act on just one organ or tissue), don’t exhibit predictable dose-response behaviour like animal hormones and are involved in significant interactions or feedback loops with each other.ref
What this means is that it’s quite hard to unpick what they do and how they work. The roles and mechanisms of plant growth regulators are still very much current research topics, as can be seen at two of the research groups at Cambridge University’s Sainsbury labref1,ref2 Some of the early theories about them were comprehensively demolished in a seminal article by Anthony Trewavasref (in particular the theory of auxin-derived apical dominance which was later proven wrong as explained below).
We now know that plant growth regulators act in concert with genes and the proteins they express, and not as an independently acting substance (one of the genes involved in cytokinin synthesis is known as LONELY GUY…).
So what are the plant growth regulators, where and how are they made? There are nine main plant growth regulators you may come across in your reading:
Auxinref – classically called ‘the growth hormone’ and a signal for division, expansion, and differentiation throughout the plant life cycle – involved in root formation, branching, the tropic responses, fruit development, shoot meristem function, the development of cotyledons and senescence. The most common form is Indole-3-acetic acid (IAA). Auxin acts in a ‘ying-yang’ relationship with cytokinin (see below)ref as well as with gibberellins. More about auxin below.
Cytokinins (CK)ref1ref2 – involved in cell division, shoot initiation and growth (including maintaining the stem cell niche), nutritional signaling, root proliferation, phyllotaxis, vascular bundles, leaf senescence, branching and nodulation, seed germination, nutrient uptake, and biotic and abiotic stress responses. 6-BAP or 6-Benzylaminopurine is a synthetic cytokinin which is used in micropropagation and agriculture. Coconut water (not milk) has been found to be a natural source of cytokinins.ref
Brassinosteroidsref – involved in a wide spectrum of physiological effects, including promotion of cell elongation and division, enhancement of tracheary element differentiation, retardation of abscission, enhancement of gravitropic-induced bending, promotion of ethylene biosynthesis, and enhancement of stress resistance.
Gibberellins (GA)ref – involved in multiple processes including seed germination, stem elongation, leaf expansion and flower and fruit development.
Strigolactones (SL)ref – induce hyphal branching of arbuscular mycorrhizal fungi and are shoot branching inhibitors.
Abscisic Acid (ABA)ref – involved in the induction and maintenance of seed dormancy, stomatal closure, and response to biotic and abiotic stresses.
Jasmonates (JA)ref – shown to be inhibitors of growth but also involved in development of flowers and defense responses against herbivores and fungal pathogens
Salicylic Acid (SA)ref – associated with disease resistance
Ethyleneref – multiple effects on plant development including leaf and flower senescence, fruit ripening, leaf abscission, and root hair growth.
Slightly maddeningly none of these substances do just one thing – they’re all involved throughout the plant!
So how and where they are made in a plant? This isn’t simple either. In fact local biosynthesis is thought to be critical for plants, whereby plant growth regulators are made at the site where they are needed. For example both auxin and cytokinin are synthesised in leaves *and* in rootsref, and can be made by chloroplasts and mitochondria, organelles which occur throughout the plant.ref Chloroplasts can make precursors to auxin, abscisic acid, jasmonates and salicylic acid.ref
Even though plant growth regulators don’t act in a predictable dose-response way like animal hormones, they still have a role in shaping plant growth in tissues which are sensitised to respond to them. Theoretically by understanding these responses we can manipulate a tree’s growth. And this is what they do in plant tissue culture (more on that below).
You may have heard the theory of auxin-controlled ‘apical dominance’, which holds that auxin produced by leaves at the apex inhibits lateral buds. This theory was strongly criticised by Trewavas in 1981: “The only hypothesis of apical dominance which has retained some measure of support is the nutritional one. A number of plants placed under conditions of reduced nutritional status adopt a growth pattern of strict apical dominance.” His point of view was further supported by a 2014 study which found that “sugar demand, not auxin, is the initial regulator of apical dominance”.ref The researchers found that after removal of the shoot tip, sugars were rapidly redistributed over large distances and accumulated in axillary buds within a timeframe that correlated with those buds releasing. But auxin didn’t travel fast enough to be responsible for bud release. So basically they found that apical dominance arises because the main shoot is greedy for sugars, and due to its position at the end of the vascular system it can prevent lateral buds from taking the sugars needed to release and grow.
Auxin does play some role though, and the theory is that its role related to the fact that it’s the only plant growth regulator which displays polar transport. That is – it moves from the apex to the base of the plant, via the phloem, and can travel the entire length of the plant, ending up in the roots. This gives auxin a special role related to the spatial aspects of growth, and auxin ‘maxima’ (locations where auxin accumulates) are sites where new buds, flowers or lateral roots emerge. In fact, auxin and cytokinin work in concert throughout the plant, from shoots to roots, with apparently opposite effects in each location “like yin and yang”.ref
An excellent reference point for this subject is the world of plant tissue culturing. This is where small pieces of plant tissue are sterilised and cultured in a medium containing plant growth regulators, which cause the tissue to grow into a ‘plantlet’ (sometimes in a test tube, if the source material is small). Further steps multiply the plantlet into several plantlets, which are then encouraged to create roots, multiplied again and/or planted out as seedlings to harden off. This process is used for industrial plant cloning where large numbers are required, and in the aquarium trade to avoid contamination with snails and other microbes (see Tropica’s website).
In plant tissue culturing, plant growth regulators are used to induce the relevant growth stage, which ones work for each species in which stage is documented in the ‘protocol’ for that species. In all cases specific ratios of cytokinin:auxin (and sometimes gibberellins) lead to different developmental stages – shoot growth, lateral shoot growth and root growth.ref1, ref2 To give you a bonsai-oriented example, one study determined a protocol for the micropropagation of Prunus Mumeref. They were able to multiply fresh prunus mume shoots in a petri dish using a 4:1 ratio of cytokinins to auxins, and then root them using auxin.
So – apologies for the rather long read, it is quite a complicated subject! What can we take from all this for our bonsai practice? Firstly we can stop the brain-bending trying to understand how auxin controls apical dominance because it doesn’t – access to sugars does this instead.
Also we can use the yin-yang rule – high cytokinin:auxin encourages buds & shoots, and high auxin:cytokinin encourages roots. So I’m going to start adding some auxin rooting gel into my air layers and cuttings. Cuttings have never worked for me in the past so maybe this will be the secret sauce I need. I’m also going to try some cytokinin gel to encourage lateral budding on my trees.
If you are looking for products to give this a try, make sure the product actually contains the plant growth regulator you want. For example, Clonex contain auxin, and some of the orchid budding pastes such as Keiki paste contain Kinetin (a cytokinin). Many other ‘rooting hormones’ or plant hormones products on the market have no ingredient list at all, so avoid those. You can also find these products online in shops dedicated to hydroponics, where cloning and plant tissue culturing is a technique used by practitioners, or in lab supply shops such as microscience or Phillip Harris in the UK. You can even make your own hormone gels following these instructions.
Another trick you can use is that gibberellic acid can be used to break dormancy in seeds, if you really don’t have the patience to wait for natural dormancy to break. Or give coconut water a try, this has been found to have a similar effect in a range of species. For hard coated seeds in particular, usually it’s best to search Google Scholar for a researcher who has experimented with different approaches, since what works is very species-dependent.
Ah repotting, such a fertile subject for ‘bonsai lore’! Any new bonsai enthusiast is soon taught (particularly in temperate locations), that all repotting should be completed in the spring, just as the buds are starting to leaf out. Here is some of the advice provided on popular bonsai websites:
“In general, it is best to repot right before your bonsai begins growing vigorously. In most cases this is spring.”
“The best time to repot a Bonsai is early in the spring, while trees are still dormant, and the buds begin to swell. At this stage trees are not sustaining full-grown foliage, so the damaging effect of repotting will be minimized.”
“Bonsai cannot be repotted at any time of the year; for the majority of species, there is a small period of time during the Spring where the roots can be disturbed and pruned with reduced risk of danger to the tree’s health.”
Unfortunately there isn’t any evidence that I can uncover to support these claims, and scientifically there may be good reasons to repot at other times of the year. But let’s start from first principles. Why repot in the first place?
Bonsai enthusiasts repot to avoid their trees becoming pot-bound – ie. the roots filling the pot. Why? There aren’t many research papers on this subject but luckily the eminent Australian research organisation CSIRO performed one studyref as a meta-analysis of 65 other studies to which they had professional access. They found what might appear to be the bleeding obvious – that increased pot size resulted in increased biomass – that is, the plants grew more when they were in bigger pots. More growth led to more leaf mass, greater levels of photosynthesis and more leaf nitrogen. In one experiment, doubling the pot size increased photosynthesis rates by 30%.
They also found that neither nutrient nor water availability nor higher temperatures could (fully) explain these pot size effects on photosynthesis and growth, and hypothesised that root confinement per se may cause growth retardation, with reduced photosynthesis as a consequence. Well – this is actually one of the benefits of keeping bonsai trees in small pots – it does reduce growth in both stem and root.
But in bonsai we need to find a balance. We want our trees to be healthy, we need them to develop and grow so that we can continue to refine them over time. If their roots take up 90% of the pot space, there is less space for nutrients, air and water. In one study on tobacco plants, pot-bound plants experienced premature senescence (their leaves fell off early), photosynthesis markedly declined as did the activity of Rubisco (a key enzyme involved in carbon fixation).ref
If we repotted all our trees into larger pots every time they got pot-bound, we’d be living in a potted forest and there would be no bonsai to be seen. Bonsai enthusiasts root prune to achieve the same outcome; root pruning creates space in the pot for soil, nutrients and water, and gives the remaining roots the opportunity to grow. This allows us to keep trees in small pots without halting their growth.
So it seems clear that root pruning is beneficial for bonsai in terms of longevity and growth (root pruning also encourages ramification). So if you are going to root prune, what negative effects might result? There are a few key ones:
You might cut away too much stored food which the plant might need to grow
You might not leave enough root mass to supply the leaves with water for transpiration – or another version of this one is that the plant might not have enough time to regrow roots in order to meet its needs
You might expose cut roots to damaging microbes
The first point is covered in my post Root Food Storage (or, can I root prune before bud break?). Whilst roots do hold carbohydrates they are by no means the only place where these are stored, with branches and stems also storing significant amounts. Furthermore, the point at which they are most depleted (which is when one would theoretically prune them, to avoid losing carbohydrates) is the end of summer (see the post for charts for different species). Pruning roots in spring just before leafing out actually deprives the plant of those carbohydrates for the leafing out or flowering process.
The second point is concerned with ensuring there is enough water uptake to meet the transpiration needs of the foliage. This can be managed by pruning foliage to reduce transpiration, although it’s tricky in pines. Any other technique which reduces transpiration can help – reducing the temperature or wind, increasing humidity (for example by putting a plastic bag over the tree, a practice which is used when trees are collected).
Of course, a tree can grow new roots – and when they do so is covered in another post When do roots grow? I was interested to find that roots grow *after* leaves have had their growth spurt. So if you were trying to optimise root growth straight after pruning, the end of summer, beginning of autumn would be the best time.
So based on points 1 and 2 actually the end of summer or early autumn would appear to be the best time to root prune, depending on the species. The main risk with this approach is that of frost damage to newly grown roots if you leave it too late. But since this is when most root growth happens anyway, I’m not sure it’s really a risk.
A maxim I have is ‘the right time to do something is when you have time to do it’. Personally I have repotted trees in every season because I have a day job and a family and I certainly don’t have days on end to be repotting every tree I own at the same time in Spring! Unless you are being extremely brutal with your root pruning (in which case, do something to reduce transpiration), probably you can do it whenever it works for you.
Which brings us to the ‘how’. You might think that the choice of pot is purely aesthetic, but there is some science to it as well: see choosing a pot. Simply, you want to secure the tree into the pot without damaging its roots (sometimes harder to achieve than it sounds), fill the pot with growing medium making sure to get it into any open spaces, and give your tree a good water. Maybe add some mycorrhizal fungi (depending on the tree species), bacteria and slow-release fertiliser, then let it recover from repotting for a while and avoid constantly fiddling with it (hard I know)!
The term phenology is used to describe the life cycle of a biological organism like a tree. Phenological events for trees include bud development, bud break, flowering, fruiting and leaf & fruit drop, as well as other unseen changes such as sap rising, seed development, root growth, cambial activity or hardening off of tissues for winter.ref
Tree phenology is entwined with the environment in which the tree lives. As there are a very large number of different climates and micro-climates within them, there are accordingly many different nuances in tree phenology, according to the location and environment. Even the same species can show widely different phenology between two different places (at least from a timing point of view).
So to really understand how phenology would play out for your own trees, you need to understand the species phenology and how it varies based on location. You’ll often find bonsai articles are specific to the location of the author which won’t always be relevant to you.
The main phenological events relate to a tree’s growth and reproduction. For example, roots stop growing below 6°C, buds break when the tree detects a low chance of frost in the future (which might damage the tender buds and shoots), photosynthesis, energy production and growth is highest when there is the most sun, and reproduction happens in conditions which most favour seed survival.
In the boreal forests – “high-latitude environments where freezing temperatures occur for 6 to 8 month”ref phenology is mainly driven by temperature, affecting the timing of the start of the growing season and thereby its durationref
Temperate-zone forests are located between the tropics and the boreal forest zone – they have hot summers and cold winters with high temperature variationref, and their phenology is also mainly driven by temperatureref
Mediterranean coniferous forests are mainly driven by water availabilityref
Australian ecosystems are extremely diverse and also subject to irregular events such as fire, drought, cyclones and flooding, which can affect phenological events, but a key driver is water availability.ref Where evergreens dominate in this ecosystem, flowering is the main phenological event.
In tropical forests which have less variation in temperature and usually high water availability, leaf shedding and growth is continuous, but reproduction (flowering and fruiting) demonstrates ‘mast’ timing effects associated with drier than normal conditionsref (ie. all trees fruiting at the same time every seven years)
In boreal and temperate areas the phenology is described in this article and summarised in the images below. But if you’re keen to understand the specific phenology for your tree in your area, you could consult google scholar.
The chart below shows the proportion of Eucalyptus loxophleba flowering at any given time in a seed orchard in the southwest of Western Australia. The highest proportion of flowering happened in spring (Sept-Nov in Australia) but a significant portion also happened in winter (June-Aug). Flowering fell to zero in the hot, dry summer (Dec-Feb).
This all seems a bit confusing given how many different variables there are, but there are some basic principles you can use from a bonsai perspective:
Trees in their growth phase (usually when there is plenty of sun and water) will be able to recover more easily from significant damage (such as large trunk chops or carving wounds) and fight any pathogens which might seek to take advantage of these.
Similarly leaf pruning during active growth will result in more buds activating.
Trees which are in a strong vegetative growth phase (growing leaves and stems) deprioritise root growth. Root growth gets a turn after the leaves establish.
Trees which have set buds but haven’t flowered yet – if you prune indiscriminately – you will lose flowers! There is a way to identify flower buds on your tree but it involves a bit of effort. Flower buds differentiate from vegetative buds at a certain point prior to flowering/leafing out. You can identify different looking buds on your tree, then remove one example of each. Cut it open and look at it under a loupe or microscope and you will be able to see which one was the flower vs the leaf or shoot. Or if you’re both patient and organised, take a picture of some your tree with buds and then with flowers – and you should be able to see what the different bud shapes are.
Storage of carbohydrates to storage tissues will take place during growth phases, and these will be used in turn when less photosynthesis is happening, to drive respiration and other processes requiring energy. Read more about how storage varies in roots here: Root Food Storage (or, can I root prune before bud break?)
If you’re a fan of wiring, doing this before a stem hardens off will allow you more bendability (although watch out for growth around the wire)
Depriving a tree of resources (water, nutrients) will mimic ‘hard times’ and cause it to respond accordingly phenologically – drop its leaves earlier, produce less flowers/fruit or not flower at all, or push out emergency growth (like adventitious buds/suckers)
I think it’s important to say that although the term ‘dormant’ gets used in relation to trees, this is a little misleading. Trees are living organisms and still need to maintain their metabolism even during winter. This includes respiring (using oxygen and stored energy to maintain metabolism), photosynthesising (for any tree with green areas remaining including evergreen trees but also deciduous trees with green stems), transpiring (even deciduous trees still transpire during winter, although a lot less than when they have leaves and in particular they take up water to swell the buds prior to bud breakref), and taking up nutrients through the roots. As I’ve written elsewhere in this site, root growth can happen above 6 degrees C, so your tree may well be more ‘alive’ than you think during winter.
I know there will be people saying at this point – just tell me what happens when!! For those people here are some general guidelines for temperate zones.
You can expect conifers to cease xylem production in autumn and root growth in winter, and to pick these up again between 2-7 degrees C (cambium) and 6-9 degrees C (roots). Buds will burst from early spring onwards depending on the species and latitude and pollen cones will release their pollen. Seed cones will start maturing, which can take just one summer (Picea, Tsuga) or one or more years plus the summer (Pinus, Cedrus). Next year’s buds and future years’ seed cones will form in late summer, and old needles (2+ years depending on species) will drop in late autumn. Mature seed cones will drop or release seed from late autumn onwards. ref1ref2ref3 Hardening leaves for the winter also happens in late autumn.
The main differences for angiosperms in temperate zones revolve around xylem production, leaf growth and senescence within the season, and flowers & fruit. In spring xylem creation will commence – in diffuse porous trees buds can break earlier but ring porous trees need to create the new season’s xylem layer before budding. Some trees will burst bud based on temperature and others on photoperiod (or a combination of the two).ref Whether flowers or leaves come first depends on the species, and the timing of flowers is hugely variable (Frank P Matthews has a list of flowering times for ornamental trees in the UK). The leaves of deciduous trees start a structured senescence process in the autumn, when they remove cholophyll and other molecules from the leaves for storage and recycling (hence the colour changes). After this has been completed the tree creates a cork layer at the base of the leaf causing it to drop off. Fruit develops throughout the growing season and depending on the species will drop off from early summer through to winter.
There’s one more phenological domain which I haven’t covered in this article – the phenology of the microbiome. This is a whole other kettle of…microbes…and might be the subject for a future post.
Finally, the fabulous In ‘Defense of Plants’ podcast has covered phenology in this podcast episode.
