Tag Archives: Pots

Mulch – relevant for bonsai?

If you’re a fan of Gardening Australia as I *massively* am, you will have noticed they are always going on about mulch. Mulch (often in the form of bark or woodchips) gets added religiously to everything they plant whether in a pot or in the ground. This got me wondering whether mulch could be beneficial for my bonsai.

What is mulch? Well back to my expert source Gardening Australia in their article Mulch, mulch, mulch, it is a layer of materials such as compost, bark and woodchip products, and/or various grades of pebbles and gravels which are placed on the soil. The benefits they claim for mulch include water retention, weed control, protection from extreme hot or cold, reducing erosion, delivering organic matter and nutrients into the soil, and even – that it looks good!

Actually I want all of these things for my bonsai, so what does the science say about the effects of mulch?

The main benefit most studies seem to agree on is that mulch reduces weeds, and the thicker the mulch the more weed reduction.^ref In one study on container-grown Thuja plicata it was as effective as chemical weed control.^ref This finding is repeated across many other studies as well.

How about reducing hot root temperatures? Potted tomatoes with grass mulch showed a direct relationship between mulch depth, soil moisture and soil temperature (see the chart below.^ref Moisture was increased and temperature decreased with additional depth of grass mulch. I don’t think it’s realistic to add 10cm of mulch to a bonsai pot though!

In a winter study, chopped newspaper as well as other mulches moderated cold temperatures.^ref The Thuja plicata study by contrast found no soil temperature improvement by using mulch, and they blamed the colour of the pots (black) for this.^ref So it looks like there might be a positive effect on root temperature but not if you have black pots – and only if you put a decent amount of mulch on the soil.

What about water retention? A study using plastic mulch (ugh) on Japanese privet plants found that the water that needed to be applied was 92% less in mulched pots^ref but the Thuja plicata study stated that no change in water retention resulted. The researchers proposed that transpiration was the main driver of water use (and since this happens at the leaf surface mulch will not impact it).^ref An intriguing study in South Africa found that only a mulch of white pebbles was useful for water retention in the hot summer, but mulches of other organic types (bark & leaves) were also effective at reducing evaporation during the colder winter period. They were pretty brutal with their research subjects – potted Polygala myrtifolia – which only got a watering once at the beginning of the trial and then had to tough it out for 6 weeks without any more water being added! In the summer period of the trial only 7% of survived, and 50% of these had white pebble mulch. During the winter trial 92% of plants survived and in these circumstances mulch of any kind provided a 20% improvement in soil water content relative to no mulch.^ref So it looks like mulch provides some improvement in water soil content as long as it’s not a drought scenario (and you don’t have black plastic pots).

One thing I have noticed is that a layer of Melcourt propagating bark (2-7mm) on my bonsai seems to ‘suck’ the water into the pot in when I am watering. Several studies have found that a layer of mulch on soil increases water infiltration rates.^{ref1, ref2} This may be because the pieces of mulch are “able to absorb the kinetic energy of rainfall…[or watering]…and maintain soil aggregates longer” and result in “an increase in the tortuosity of water pathways due to the higher roughness”. A study on Holm oaks found that rock fragments were a good mulch for shallower root systems and improved soil moisture.^ref A rough-textured mulch might be useful if water is bouncing off the surface of your planting medium.

So should you use some form of mulch on your bonsai? If you want weed control, probably. If you have trees which are particularly prone to drying out or succumbing to the elements – for example they have very shallow or small pots, or are potted in medium without some form of water retention (such as coconut coir, vermiculite, bark or sphagnum moss) it might be worthwhile. It may also act like a form of insulation (as discussed in the post on frost) to protect roots from the cold. Finally if your medium doesn’t want to cooperate with the watering can or hose, and water bounces or flows off it, mulch might be a way to reduce overflow and improve infiltration.

What options are there for bonsai mulch? There are quite a few different types of mulch described in this article but not all of these would be practical for a bonsai pot, and many you wouldn’t use for aesthetic or ‘aromatic’ reasons. Only a mulch with a relatively small component size would be feasible – this could include a small-sized bark mulch, or even a layer of smaller medium such as akadama, pumice or molar clay. I’d love to be able to use seaweed but I don’t think it would smell good, and it’s not that easy to find in suburban London. Organic mulches will break down over time and add organic matter to your soil – which you may or may not want to do. So – maybe this is a practice you might want to consider.

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.

Bonsai Tree Growth Stages

Most bonsai trees progress through stages of development, each with a different objective. In general the progression is thicken trunk -> achieve branch & root structure -> achieve branch, foliage & root ramification -> reduce leaf size -> evolve as branches grow/fall off. The faster we can move through the first few development stages, the faster we will have beautiful, well-proportioned bonsai – harnessing the tree’s natural growth is a way to speed this up. We also want to avoid doing things which slow down a tree’s growth during these phases, as this will mean it takes longer to get the tree we want. Read about how trees grow before starting at #1 below. Also consider what do old trees look like?

1. Trunk

Some bonsai enthusiasts collect mature trees for bonsai specifically so they can start with a thick trunk, following a collection process which minimises damage to the tree. The alternative is growing your tree’s trunk. Once a tree has its roots and foliage reduced in size in a bonsai pot, it won’t generate the energy needed to make significant sapwood additions and its girth will only increase by small increments every year. So you really need to be happy with the trunk size first before you stick it in a tiny pot. But – how big should a bonsai tree’s trunk be?

2A. Branch Structure & Overall Shape

Arranging the branches is what gives you the canopy and overall foliage shape that you’re after and the first step in this process is growing (or developing) the branches you want in the positions they are needed. Growing a branch starts with a new bud, which, unless it’s a flower bud, becomes an extending shoot and eventually a new branch. So firstly you need to work out where new buds will grow on your tree and then deal with the extending shoots as needed to get the required internode length.

You may need to remove some buds and shoots if they don’t help achieve the shape you are looking for – this should be done as soon as possible to avoid wasting the tree’s finite energy reserves. You have a trade-off to make here because leaving more foliage on the tree will provide more energy overall which contributes to its health and ability to recover from interference. However, growing areas of the tree which won’t be part of the future design is a waste of energy. You don’t want to remove so much of the tree’s foliage that it struggles to stay alive or develop the areas that you do want to grow out.

When you are creating your branch structure, often you will need to reposition branches – this is done with a wide range of different tools and techniques. A more advanced technique for adding new branch structure is grafting.

Sometimes the trunk itself or larger branches need a rework, to make them more interesting or to make them look more like old trees – for example adding deadwood or hollowing out the trunk. Usually this is achieved through carving.

2B. Creating a Strong Root System

The trunk thickening and branch structure phases both work best when the tree has lots of energy and so letting it grow in the ground or in a decent sized pot during these phases will get you there quickest. This also allows the roots to keep growing, but you want to understand about the role of roots, and root structure & architecture even if you still have your bonsai in a training pot. Particularly in this case, knowing about how to foster the the rhizosphere will help your tree stay vigorous. To maximise the roots’ exposure to nutrients and water you want to encourage Ramification of Roots (lateral root development).

Eventually it’s time to move the tree into a bonsai pot. This requires cutting back the roots, but as long as the roots are balanced with the foliage in terms of biomass, the tree should be OK. Root growth is usually prioritised outside of times of stem/foliage growth, and above 6-9 degrees C. So repotting might be best conducted at times that meet this criteria. Your growing substrate/medium is an important consideration.

3. Ramifying Branches & Foliage

Ramification is when branches subdivide and branch, giving the impression of age and a full canopy – and a well-ramified tree is a bonsai enthusiast’s goal. There are some techniques for increasing the ramification of branches and foliage. But not as many as there are for root ramification.

This stage also involves ongoing branch selection and reshaping (see 2A above). Another consideration is whether to keep or remove flower buds.

4. Reducing Leaf Size

An end stage in the journey to bonsai perfection is leaf size reduction. In nature, leaf sizes reduce relative to the biomass of the tree as it ages but since bonsai are small this effect doesn’t translate since the biomass never gets large enough. The tried and tested method for reducing deciduous tree leaf size is actually to practice one of the various methods of defoliation. A couple of others are covered here in reducing leaf size.

When to conduct these various activities depends on when the tree can best recover from them – which is a function of the Tree Phenology (or Seasonal Cycles).

5. Evolving Branches

Trees are not static organisms – they obviously continue to grow which is what we harness in the above steps. Part of this is that eventually branches may become too large for the design, or they may fall off (Peter Warren notes that Mulberry are known for this). As bonsai artists we want to have this in mind so that branches are being developed which can take their place in the future. This is an ongoing version of step 2A.

Roots

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.