Root development in Bonsai

Tree_roots_cross_section-e1351581137309-1024x451It pays to know something about the anatomy and physiology of roots when it comes to developing the root system of Bonsai trees. It is said that a tree mimics its above ground growth to what happens with root growth below ground level. This might be true for trees in nature, but with Bonsai it could be quite different due to the fact that we prune the roots and the roots also have to grow in a confined space.

When a root is studied under a microscope it is clear that there partsofrootsare different tissue types present in different areas of the root. On the outside is an epidermis layer consisting of cells. To increase the surface area, and therefore the absorptive area of the root, extensions of these epidermal cells grow into the soil. These are the root hairs. The main job of the root hairs is to absorb water and nutrients. They are very small and fragile and normally breaks off when a plant is pulled from the soil. This is a very good reason to be gentle when a plant is uprooted and when soil is removed from the roots.Root(cross_section)

The next layer under the epidermis is the cortex. These cells are loosely spaced to allow for the movement of gasses and dissolved nutrients to the core of the root. The middle part (core or stele) of the root consists of the vascular tissues, the xylem and the phloem. The xylem particularly is of importance here as it is through this tissue that the water and nutrients are transported to the rest of the plant. Around the vascular tissue is another layer of cells, called the endodermis and underneath this is the pericycle.


Lateral root growing from the pericycle.

This pericycle is of particular importance to the plant and Bonsai artists as this is where lateral roots originate from. The origin of the lateral roots is therefore situated deep inside the root structure and not like the branches or side shoots of the plant that originates from adventitious buds. The only reason for this that I can think of is that it provides better protection for the roots. It also means that the vascular tissue is in close contact with the lateral root.


Apply rooting hormone to the cut roots.

If the root is studied externally the branching of roots can be seen as starting a little bit further back up the root and not right from the root tip. The same mechanism that applies to pruning above ground is also evident here. When the apical meristem (in the root tip) is removed, the pericycle is stimulated to grow more roots. This is also due to the influence or the lack of this influence, of plant hormones. This is the science  behind root pruning. When a root is pruned, more roots will grow from that area. It will help if rooting hormone is applied to the cut area as this will help to stimulate the pericycle to produce more roots. Ensure that the rooting hormone is applied evenly for an even spread of new roots. There is some evidence that when the rooting hormone is applied to only one side of a root, that more new roots will grow on that side. I will experiment with this and report back on findings.


Air pruning.

The term air pruning has also been used in the propagation of plants. This method can be used for Bonsai as well, especially during the early phases of development where the emphasis is on the development of branches as well as the root system. It simply means planting the tree in a container with holes in it. The roots will grow to the edge of the container and either split or not grow any further.

Well-DrainedSoilsIt was mentioned earlier that the removal of the root tip (apical meristem) will stimulate the pericycle to grow more roots. This is also true when the apical meristem is damaged. Growing the tree in a soil medium with lots of granular particles can cause the root tip to split or damage it and this will in turn stimulate more lateral roots to grow.

Also of note here is that the majority of water and nutrients are absorbed through the root hairs. As these are usually in the vicinity of the root tip (directly behind it), it can be taken for granted that these root hairs will be removed or reduced when root pruning. The uptake of water and nutrients will therefore be compromised when root pruning takes place. Be vigilant. This is the reason why the tree should be watered well and placed in the shade after root pruning. Water loss through transpiration must be avoided. Factors that increase transpiration are heat, wind and leaf surface area. It is therefore a good idea to reduce the leaves after root pruning in the case of deciduous trees. This might be another reason for re-potting and root pruning in spring before the leaves are fully grown.

938277CE-FFB2-4BE6-B1C5-98D83AA77553As roots age, they undergo secondary growth or secondary thickening. This is basically when the softer tissue becomes woody and bark is formed. This happens faster with exposed roots. The main thing to remember here is that older roots that have gone through this process will not absorb water and nutrients. When root pruning, this needs to be taken into consideration. Always leave some of the younger, finer roots to provide the plant with water and nutrients.

Nebari – The Root of the Matter


These roots have grown over time, dropping of the side of a cliff.

No, we are not talking about the Nebari fictional alien race from the planet Farscape, but the Japanese term generally used to indicate the roots and specifically the root flare at the bottom of the trunk where the root region starts. It also indicates the visible surface roots.

Wikipedia refers to it as: “Also known as “buttressing”, nebari is the visible spread of roots above the growing medium at the base of a bonsai. Nebari help a bonsai seem grounded and well-anchored and make it look mature, akin to a full-sized tree.”


My idea is to write three or so Blogs focusing on Nebari and the development of roots. There is no doubt in any Bonsai artist’s mind that the roots of a plant is where it all starts. Healthy roots = healthy plant (generally). A plant absorbs most of its water and nutrients from the soil through the roots. We therefore have to know something about soil and its water retention and draining properties. Not all soils are created equally. We also need to know something about the physiology and anatomy of roots. This knowledge will help us to understand many of the concepts around root pruning. This will come in later Blogs.

Autumn bonsai images 2014 (17)

A tree growing in a woodland area. These roots definitely add to the character of the tree and indicate that it has been there for a while.

Getting back to the Nebari, the main aim of establishing a root flair at the base of a tree is to indicate age. A wider base caused by a flaring just above the roots, helps to establish the taper of a tree. One way to develop taper is to increase the circumference of the tree at the base and to keep the top of the tree relatively thin. Young trees do not have a huge flair at the base, but older trees show this without fail. In saying that, there are trees that are used for Bonsai that do not show a big root flare. Junipers come to mind here.


The roots are spread over a flat surface and tied to this surface.

There are multiple methods to improve the Nebari of a Bonsai tree. The first one is the continuous root pruning of downwards growing roots. For the health of the tree it is advised that this is done over time to enable the tree to grow new roots in the place of the removed roots. A second method is done through air layering. This is simply done through preparing the bottom of the trunk as one would for air layering and heaping up the soil or packing sphagnum moss around this area. Roots will over time establish themselves in that area. I have read about a method where a wire is bound tightly around the trunk just above the roots. The flow of nutrients will be impeded and that will cause new roots to grow from above the wire. I have not tried this method myself before and cannot comment on how successful it is. Other methods include drilling holes through flat tiles and tying the roots of the tree in a horizontal direction to the tile with wire through the drilled holes. It could also be done through grafting.

In the end, the Nebari plays a vital role in the aesthetics of the tree. It is crucial thfotolia_2553306_XSat some sort of Nebari be established as it indicates age, shows that the tree is balanced and sturdy and has been there for a long time.

The next Blog will address the science behind root development.

Plant Hormones and Bonsai

Plant hormones have an affect on aspects of plant life. Every cell in a plant can produce plant hormones and these can act in that specific cell or it can be transported somewhere else. There are five major plant hormones; auxins, cytokinins, gibberellins, ethylene and abscisic acid.

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Auxins are referred to as growth hormones. These chemicals stimulate plant cells to elongate. They are mainly found in the tips of stems and branches. with this accumulation of auxins in the tips, growth in other parts are subdued. During pruning of Bonsai these tips are removed and the excess of auxins in these areas are reduced. This in turn will allow growth to take place lower down on these branches. This is the so-called apical dominant display. When the auxins are not there, the dominance is reduced and back budding will be allowed to occur. It will also allow branches lower down the tree to develop more. Auxins are also produced in roots.IMG_1888

The phenomenon of phototropism is directly attributable to auxin concentrations. Auxin moves away from light. It accumulates in the cells further away from light and will stimulate those cells to lengthen. As this growth is only on one side of the stem, it “pushes” the stem towards the light. This growth of plants towards light is what phototropism is all about. When we wire stems and branches and bend them in a specific direction, no role is played by auxins. We manually do what auxins will normally do. This also allows us to do things against what the normal plant hormones were programmed to do. We can as an example, bend a branch to force it to grow away from light.IMG_1248

Another practical implication for Bonsai enthusiasts is to regularly turn the trees on the benches as it will stimulate growth around the plant and not only on one side.

Cytokinins promotes cell division, in other words, growth. Where auxins will inhibit lateral buds, cytokinins will promote growth all over the tree. In the absence of auxins, cytokinins will cause lateral buds and dormant buds to grow. Pruning, candle removing and pinching of new growth will all aid in reducing auxin levels and promoting more lateral growth.

Gibberellins are responsible for shoot elongation, seed germination and fruit and flower development.

Absisic acid promotes seed dormancy. It also assists with the opening and closing of stomata.

Ethylene is a gas produced by ripe fruits.


Wisteria – repotting

Wisteria – Just for flowers?

Most articles and books refer to the fact that Wisterias are basically used as Bonsai just to show the flowers off. In contrast to this, Colin Lewis in Bonsai Basics (2008) makes the statement that Wisteria can make interesting trees even without flowers, at the same time acknowledging that it is the flowers that make them spectacular. I find that these plants (climbers) do need to be taken care of in exactly the same way as most other deciduous Bonsai. The tasks of potting, re-potting, pruning, root work, wiring and display is just as important for this species as for any other species of Bonsai.


I have had a nursery bought Wisteria in a smallish pot for about two years now and recently decided to repot in a larger pot. This is mainly due to the fact that the roots have developed nicely and I am afraid that the smaller pot will not balance and weight the tree down enough when it comes into bloom again. An article in Bonsai Today (96:2005) states that Wisterias reacts well to rootpruning. It also says that you can be quite fearless about cutting thick roots. This is exactly what I did when it came out of the nursery bag and as can be seen from the photo below, it has certainly grown a lot of finer roots. This article also warns against allowing the roots to grow too long as they easily become rootbound.

I have decided to plant it in a hexagonal pot as it is difficult to determine a “front” for the plant. The context of this specific species changes as it goes through its growing season. In winter it is the bare branches, then the flowers and then the leaves with rampant, long branch growth, that needs to be kept in check.  Herb Gustafson in The Bonsai Workshop (1996: 37) shows a Wisteria in a hexagonal pot that resembles my Wisteria quite well. In Bonsai Identifier (Owen, 1998:114) it is mentioned that pots for Wisteria will normally be glazed, sometimes decorated and heavy. With all of this information, the repotting session starts.

The roots as they came out of the old pot (left) and after some pruning (right).


The plant is tied down and soil added and well worked into the roots with the aid of a chopstick.


The finished job and ready for watering. Now it is a case of wait and see what it does when it is flowering time.