Category Archives: Keeping Bonsai Healthy

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.

Biochar

(Thanks to Dr. Karen O’Hanlon of Probio Carbon for answering some of my questions about biochar).

Biochar is a product which has been advertised as a beneficial component of bonsai soil over recent years. So what exactly is it?

Biochar is basically charcoal which has been “produced from organic waste using pyrolysis technology under temperatures ranging from 400C to 700C where oxygen is either absent or depleted”.^ref Pyrolosis means decomposing carbon-based materials through the application of heat.^ref So a feedstock (source material) is acquired and heated in the absence of oxygen for a given period of time to create what you would probably recognise as charcoal. The structure of biochar is shown in the image – as you can see, it has many, many holes in it.

Scanning Electron Microscope image of biochar
https://www.rhs.org.uk/soil-composts-mulches/biochar

So why would you add biochar to your bonsai soil? There are a few good reasons. It has been proven to improve water availability^ref, act as a fertiliser reducing the need for chemical fertilisers^ref and increase microbial biomass^ref (ie. it attracts beneficial microbes).

An experiment conducted in Colchester, UK by the Bartlett Tree Research & Diagnostic Laboratory amazingly found that ash trees treated with biochar did *not* get infected by ash dieback disease over a period of 4 years even when the disease was present in adjacent trees on the same site. They believed the reason for this was that the biochar enhanced the trees’ immune system and improved root growth.^ref

The microbe aspect of biochar is really interesting – in one study it was found that microbes living in it were able to ‘mine’ the biochar pores for phosphorus. So it appears to have synergy between its composition (with nutrients for plants) and its attractiveness to microbes which can help get those nutrients into plants.

One of the key physical properties of biochar is that it has a massive surface area, relative to its size – in one study on malt spent rootlets (a residue from brewing) it was 340 m² per gram!!^ref That’s larger than the size of a tennis court for every gram of biochar.^ref This increased surface area along with the physical structure of biochar having lots of tiny pores, results in greater water retention in the soil.^ref

Biochar can be made from basically any organic material, from forestry to food production to agricultural by-products and this source material is the main determinant of its chemical properties.^ref So when choosing a biochar for your bonsai soil, you want to know what it has been made from, and what this means in terms of its properties. Some of the properties which vary significantly include pH, surface area and cation exchange capability/electrical conductivity. For bonsai I would say you want high surface area & pore volume (to assist with water availability) and high microbial mass. The fertiliser aspects are probably a nice-to-have. Looking at the table below this means probably biochar made from a wood-based source material is best.

There is quite a bit of research out there on different biochar properties, which I will summarise here for you to read through. Unfortunately I haven’t found any research which looks at volume of microbes for each feedstock, but I would expect this to be positively associated with surface area.

Biochar Feedstock	Properties
Wood	Highest surface area (leading to better water retention) and highest pore volume (a factor of 10 higher than manure) Lowest cation-exchange capability Largest amount of C Contain less plant-available nutrients More electrical conductivity Lowest ash content (associated with lower pH) Micro-nutrient content mixed (see table here) Total bioavailable nutrients mixed (see table here)
Crops & grasses	Highest average particle size Highest K content Lowest calcium carbonate equivalents Micro-nutrient content mixed (see table here) Total bioavailable nutrients mixed (see table here)
Manure	Lowest surface area and lowest pore volume Highest cation-exchange capability Highest calcium carbonate equivalents Lowest average particle size Highest ash content (associated with higher pH) Greatest N, S, P, Ca, and Mg concentrations Highest micro-nutrient content (Fe, Cu, Zn, B, Mn, Mo, Co, Cl) Total bioavailable nutrients mixed (see table here)

Source: https://link.springer.com/article/10.1007/s42773-020-00067-x/tables/1

The temperature at which the biochar is created makes a difference too. Increasing pyrolisis temperature leads to “increased biochar C, P, K, Ca, ash content, pH, specific surface area (SSA), and decreased N, H, and O content”^ref

Like many things in life though, you can have too much of a good thing. In some studies, too much or the wrong biochar in soil has led to phytotoxicity^ref You might also be wondering why it doesn’t just remove all the nutrients in the soil like activated carbon, which is used in aquariums and drink bottles to remove metals, chlorine and contaminants. When asked this question Dr. Karen O’Hanlon at Probio Carbon said it was because biochar is not ‘activated’ to the same degree as activated carbon. Reading more about this, the absorbent properties of biochar are “1/6th to 1/12th that of high quality activated carbons”.^ref Activation forces more pores and surface area into the charcoal, this is done by varying the temperature and pyrolysis process. So whilst there probably is some nutrient absorption, it’s not going to be at the same level as activated carbon and can be compensated for by the nutrients within the biochar themselves and the increased microbial activity.

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.

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.

The Rhizosphere

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 article^ref 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 paper^ref)

https://academic.oup.com/view-large/figure/90643206/fmr12028-fig-0001-m.jpeg

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.

Plants, Mycorrhizal Fungi, and Bacteria: A Network of Interactions
Paola Bonfante and Iulia-Andra Anca
Annual Review of Microbiology 2009 63:1, 363-383

‘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 plant^ref. 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, ^ref2 This 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 following^ref:

Plant Growth Promotion (supporting plant health & growth): Pseudomonas, Bacillus, Rhizobia, Achromobacter, Azotobacter, Arthrobacter
Biocontrol (fighting pathogens): Pseudomonas, Bacillus, Serratia, Pantoea, Acenetobacter, Xanthomonas, Alcaligens
Bioremediation (removing pollutants): Pseudomonas, Bacillus, Alcaligens, Arthobacter, Achromobacter, Azospirillum, Pantoea

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.

How roots absorb water and nutrients

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.

https://onlinelibrary.wiley.com/doi/10.1111/jipb.12534

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.

Foliar Feeding

Some products advise spraying them on the leaves of your trees – a process known as foliar feeding. At first glance this makes no sense, as plants synthesise everything they need from nutrients obtained from the soil and air and these nutrients come up with water through the roots and xylem. And leaves haven’t evolved for nutrient uptake, they have evolved for photosynthesis.

But could this actually work? Well, in order for the nutrients in foliar feed to be useful to plant cells, they would need to both penetrate the leaf and enter the cell.

Can substances on a leaf surface enter the leaf itself? For the most part they can’t as leaves are covered by a protective waxy layer known as the cuticle (described in the post Leaf Structure). One of the main roles of the cuticle is to stop pathogens and other environmental stressors entering the leaf.

But as so often happens with systems of mind-bending complexity like plants – it’s not that simple. For one thing, we know leaves have stomata which allow gas to enter the leaf. But it also turns out that the rest of the cuticle isn’t completely impregnable. The cuticle has tiny pores in it at the base of trichomes (hairy projections) and glands – these range from 0.45 to 1.18nm in diameter^ref. One study did indeed find that dissolved nutrients can enter the cell through these pores in the cuticle: “penetration of ionic compounds can be fairly rapid, and ions with molecular weights of up to 800 g mol(-1) can penetrate cuticles that possess aqueous pores.” The key term here is ‘aqueous’ – the pores need to be wet in order for nutrients to enter through them. For example carnivorous plants use this process to bring nutrients in from their traps via the pores in glands^ref.

A great article summarising the physics of nutrients entering a leaf is here – they conclude that it’s easier for positively charged ions (calcium, magnesium, potassium, ammonium-form nitrogen) to enter via the cuticle pores whilst it’s not as easy for negatively charged ions (phosphorous, sulfur, nitrate-form nitrogen). Similarly smaller molecules or those with a smaller positive charge are easier to translocate around the plant – including ammonium, potassium, and urea. Larger molecules will stay close to their point of entry, including calcium, iron, manganese , zinc and copper. Another study states that younger leaves are less able to transport nutrients out and so applying foliar feed to developing leaves may result in the nutrients staying within the leaf (which perhaps is an effect one might want to achieve?)^ref

So it seems that some amount of foliar feed may be able to enter via the cuticle’s aqueous pores, and a subset of this may be able to move around the plant.

But what about the stomata? Previous studies have said that “the combination of cuticular hydrophobicity, water surface tension and stomatal geometry should prevent water droplets from infiltrating the stomata”.^ref (ie. water can’t get through stomata) but apparently dissolved ions can in some circumstances, because the ions change the surface tension properties of the liquid. This study ‘confirmed the stomatal uptake of aqueous solutions’^ref; but also said this depended on whether the aqueous solution was chaotropic (reducing water tension) or kosmotropic (increasing water tension). So it’s easier for the ions on the left to enter via the stomata, and harder for those on the right.

from: https://water.lsbu.ac.uk/water/kosmotropes_chaotropes.html

But once in the leaf, can nutrients be used by plant cells? It seems so, in some cases, but the evidence is extremely varied and there are many different variables to untangle.

A research study was conducted by ‘Christmas Tree Specialist’ Chad Landgren for the Oregon Department of Agriculture in 2009 comparing foliar feeding to other forms of nutrient application^ref. They tested a range of approaches on blue spruce, Atlas cedar and four varieties of fir (abies), in pots and in the ground, using application methods including “helicopters, mist blowers and various backpack sprayers”. Their conclusions were: “Each conifer species and site are potentially different with regard to nutrient needs and response. Blue spruce appears rather “immune” to foliar application… Nordmann fir appeared to pick-up some of the foliar fertilizer… on other sites, no treatment (soil or foliar) appeared to move the foliar nutrient content levels.”

In another paper^ref the author concludes that “foliar application of particular nutrients can be useful in crop production situations where soil conditions limit nutrient availability.” and that fruit can benefit from direct sprays, but also that “applying fertilizers to leaves (or the soil) without regard to actual mineral needs wastes time and money, can injure plant roots and soil organisms, and contributes to the increasing problem of environmental pollution.”

And then of course it’s not just the leaves themselves. We now know that there is a phyllosphere – a symbiotic community of microbes in and on the leaves which perform a whole range of functions for their hosts, one of which includes producing cytokinins
that can be bioactive within the plant. If foliar feeding increases these bacteria, there may be effects throughout the plant not just on the leaf.^ref

The message from all of these seems to be that foliar feeding may work for leaves or fruit with specific mineral deficiencies which need to be corrected in-situ, if the nutrient in question can get through the cuticle or stomata. Or for plants which have environmental reasons for not being able to access nutrients through their roots (like pH?). But there needs to be a specific requirement in a specific location on the tree for it to make a difference – and it will be dependent on the species, environment, nutrient etc. In most cases I would say it would be better to provide the roots with the requisite nutrients instead.

Phloem

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 gymnosperms^ref 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

https://search.library.wisc.edu/digital/AVCQSJHVTUYFUP9D

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.^refThe 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.

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

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 article^ref 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).

Biogold

Biogold is another popular bonsai fertiliser, which may or may not be cagey about its ingredients since the packaging is all in Japanese which I cannot read. Deploying google translate on their website, and searching online yielded some information:

It’s a fertiliser with N:P:K ratio NPK 5.5:6:3
It contains micronutrients iron 0.12%, copper 50mg/kg, molybdenum 27 mg/kg, sulfur 0.5% and also magnesium and calcium (in unspecified amounts)
It contains chicken manure fermented using bacterial processes

Chicken manure isn’t used directly on plants because the organic matter will ferment and generate heat, usually it is fermented separately along with plant matter such as straw, leaves, cardboard etc, and requires turning or mixing to ensure exposure to air (this is aerobic fermentation which requires oxygen).

So it’s likely that Biogold contains some other kind of plant matter which is unspecified. When looking at other products, plant matter (particularly green or coloured leaves or skins) provided substances which helped the microbial communities in the soil flourish, enhancing root growth and nitrogen uptake. One study found a “positive effect of BioGold® and Compost in increasing the soil microbial population by providing nutritive sources for the growth of soil microbes”^ref

Chicken manure is a good source of nitrogen, contains humic acid^ref, which is a concentrated form of organic matter (also contained in the coal precursors leonardite and peat), and was found to have better growth potential for plants than cow manure^.ref

A researcher growing coconut compared BioGold with other fertilisers in this study^ref – and found “There were no significant differences (P> 0.005) between treatments in any of the growth parameters tested after a period of six months after planting.” ie. the plants tested had similar outcomes from all the fertilisers tested (which included inorganic fertiliser, cattle manure, ‘Kochchikade biofertilizer’ and compost).

So overall, hard to say, this product appears to be a good fertiliser with micronutrients and humic acid/concentrated organic matter. But since it doesn’t disclose all the ingredients it’s impossible to fully assess it.

What is organic fertiliser?

The word ‘organic’ in terms of fertiliser does not mean the same thing as ‘organic’ when it comes to food.

Organic food follows principles of production which in general do not permit soluble fertilisers and synthetic pesticides^ref to be used during the food production.

Organic fertiliser means “any substance composed of animal or vegetable matter used alone or in combination with one or more nonsynthetically derived elements or compounds which are used for soil fertility and plant growth.”^ref This does not imply that the animal or vegetable matter itself was not produced using chemicals or is organic in a food sense.

As an example, rapeseed meal can be called organic fertiliser if the oil has been extracted using cold pressing methods, but this doesn’t mean that the rapeseed itself was grown using organic farming methods. If the oil has been extracted using a solvent, I think it’s doubtful that this rapeseed meal should be called organic.