Bonsai blog

So, what is a slab planting?

A slab-planted bonsai is a tree that is grown on a thin slab of rock, or a slab of material that resembles a rock. Naturally occurring slabs of rock that are suitable for such plantings are few and far between, so they are often created using either ceramics, or mixtures of man-made products such as chicken wire, cement and glassfibre.

Why plant a bonsai tree on a slab?

Partly the reasons are aesthetic: a slab-planted tree has none of the ”man-made” straight lines associated with a tree planted in a traditional bonsai pot. It is possible for the whole of the bonsai composition to be made up from natural materials, thus giving a more harmonious effect.

There are also some very practical reasons for slab-planting a bonsai tree. Sometimes it’s easy to forget amidst all the watering and feeding that roots also require two other things; warmth and air. By placing the tree's rootball on top of a free-standing rock slab, warmth and air are two things that the tree will certainly get in abundance! The other major advantage of slab planting is the lack of future root pruning. The tree's roots simply grow out of the soil mass into the open air and the tips are killed off by dryness or the sun's heat. As the root tips die off, more roots are created deeper inside the rootball and voila! – constant root pruning is achieved. If the rootball has been correctly constructed, with occasional repairs it should be possible to leave the tree on the rock slab for many years, maybe fifteen or more, without repotting.

“inny” or “outy”?

One other aspect of slab planting that is often overlooked is the curvature of the slab. Flat slabs can look a little artificial and “man-made”, though they work perfectly well. Ideally a slab should be convex (an upwards “hump”) rather than concave, or dish shaped. If the soil mass is placed on top of a convex slab, the roots grow out of the soil and into the open air, achieving the desired effect of “air pruning.” If a concave slab is used, the roots will tend to grow into the middle of the soil mass without achieving the “air pruning” effect. It’s worth considering that an unsuitable concave slab can be changed into a suitable convex one by flipping it upside down!

A big thank you to Nobuyuki Kajiwara for permission to use these photos of him at work:

In this photo the root pruning has been carried out on the original root ball, and a new “retaining wall” of keto soil (see my previous blog post, “A short guide to bonsai compost”) is being built up to follow the contours of the root ball and slab.

The keto soil retaining wall continues to be built up round the freshly trimmed root ball.  The wires that can be seen pointing up around the rootball have been inserted through holes in the bottom of the slab, and will eventually be used to secure the tree.

The basic retaining wall of keto in place, the root-trimmed bonsai has been lifted out of the way. The retaining wires can now be seen more clearly.

A layer of large grained akadama is placed within the retaining wall of keto.
 

The tree is placed on the bed of akadama to check if it is properly centred, and at the right angle.

More loose akadama is added, and the keto wall is added to and extended upwards to retain the loose mix. Note that the retaining wires have now been used to secure the rootball in place.

More keto is added and sculpted until the rootball is fully enclosed.

Collected moss is planted on the surface of the newly enclosed rootball. Tweezers are being used to firmly press the edges of the moss into the keto.

A shot of the same tree a few months later.

- Richard -

An interesting project has recently come to fruition with the publication of the “The Journal of Wild Culture”. In July of 2012, Liam, Tom and Lee from the Journal visited the collection to take some pictures of Kew's bonsai trees for their debut issue. An impressive amount of technical gear was wheeled in, with bonsai receiving a “supermodel” level of attention. Powerful flashes were triggered, highlighting the trees and darkening the background.

Below are a few snapshots of the process!

First the expensive photographic gear has to be unloaded...

Lee, Liam and Tom unpack the photographic gear.

A raffia screen is set up to show the trees to their best. First onto the catwalk is Juniperus chinensis (Chinese Juniper)...

Juniperus chinensis (Chinese Juniper) under the spotlight

Once you start paying attention, it's fascinating to realise that light and shadow are coming from all directions; getting a good, clean image turns out to be much harder than you'd think...

Lee and Tom adjust the set-up

Even when the creative part is over, there's still the tidying up to manage! Luckily I have a bonsai truck all of my own to lend a hand...

Lee borrows the bonsai truck to ship the gear off site.

After all the fussing with lights I was quite surprised to see the results which are rather dark - very dark - very very dark, in fact! See what you think in The Journal of Wild Culture: Training Beauty. Thanks to Lee, Liam and Tom for showing me my trees in a whole new light - or shade! It was a fascinating experience. 

- Richard -

Having covered the basics of repotting last year, this time round I’ll give an overview of the various different types of soil that I use for the bonsai.

Akadama

Akadama, or Akadamatsuchi (in Japanese; “Red ball earth”) is the largest constituent of most of the composts that I use at Kew. It is a naturally occurring hard soil that is surface mined in Japan then sifted and bagged. It comes in different qualities or grades, with the harder grades (which come from deeper below the surface) generally costing more. Akadama will usually hold its granular structure for several years before the actions of frost and the growth of a tree's roots break its structure down. Akadama is a good all-round growing medium, as it holds plenty of moisture without getting too waterlogged, and also acts as a good “nutrient buffer” (taking up nutrients and minerals and releasing them slowly).

A 14 litre sack of “Double Red Line” hard grade Akadama bonsai compost

Before I use akadama I pass it through a sieve that has interchangeable screens with varying mesh sizes (a picture of a bonsai sieve can be seen at my previous blog “The Tools of the Trade”). This leaves me with three different grades of akadama: large grains, which are used for larger deciduous trees and most evergreens; medium grains, used for all the other deciduous trees and a few of the smaller conifers; and small grains which are used for dressing the surface of pots and potting up seedlings and cuttings. There is also a small quantity of dust which is discarded.

Akadama bonsai compost, sieved out into three different grades.
Top left is large grain, top right is medium and bottom is the fine grain

Keto

Keto, or Keto tsuchi (tsuchi in Japanese simply means soil or earth) is a sticky, usually black or dark brown soil formed from rotting plant material in boggy areas. Its sticky consistency is somewhere between peat and clay which makes it ideal for slab and rock plantings. It can be kneaded and sculpted like clay but does not dry out so readily. If it becomes dry, it will re-absorb water more readily than clay would. I usually try to mix in about 15-20% Akadama and 15-20% pumice before using keto for slab plantings. As far as I know, there is no viable substitute for keto available.

Using keto to build up the first stages of a “retaining wall” for a slab planted bonsai

Using keto to construct a slab-planted bonsai. The loose, large grained soil under the tree's rootball has been enclosed and held in place by a thin shell of keto soil. More about this later!

Pumice

Pumice is a naturally occurring volcanic rock. Its foamy structure can be up to 90% air so it will often initially float on water. I mainly use it in soil mixes for pines and junipers where I will use between 30% to 50% pumice with the rest of the mix being made up of large grain Akadama.

Pumice, with a pound coin for scale

Kanuma

Kanuma is another naturally occurring volcanic rock, which only seems to be available from Japan. Though similar to pumice it is a lot lighter and softer and can usually be ground to dust between two fingers. Kanuma is most usually recommended for the growing of azaleas and rhododendrons due to its reputation for having a higher acidity than Akadama.

Top left is kyodama, top right is kanuma and bottom is a moler clay
based cat litter. Pound coin in the centre for scale

Kyodama

Kyodama is similar to pumice. Recently I have been adding about 25% to my deciduous soil mixes. I believe it is UK-produced so will have a smaller carbon footprint than imported soils. I also think that I am right in saying that it is a waste product from the glass industry so even more kudos to those of us trying to save the planet! Adding these harder, slightly abrasive materials to soil mixes is reputed to improve the fine root structure of trees.

A 9kg bag of kyodama

Cat litter!

Some brands of cat litter are made from a type of “diatomaceous earth” (soil containing fossils of algae or “diatoms”) called Moler clay that is mined in Denmark. Though I have not carried out extensive tests, I have grown some Zelkova bonsai in cat litter for a number of years without the trees showing any ill effects. As you may appreciate, moler clay-based cat litter is significantly cheaper (and more readily available) than akadama soil, though the granules tend to be rather small. Moler clay is used as soil improver for sporting grounds, golf courses, parks, etc under the trade name “Terramol”. There is also another product available named “Biosorb” which appears to have similar characteristics. A form of moler clay is often provided in “spill kits” for mopping up spilled fuel or chemicals.

- Richard -

A few weekends ago I was able to attend the Swindon and District Winter Bonsai show, so thought I would show some pictures of "non-Kew" bonsai trees for a change. My apologies for poor picture quality. The show is held at a leisure centre and, while the lighting is fine for a game of netball, it’s not conducive to bonsai photography!

It’s good to see that the Swindon and District Bonsai Society hold their show in the winter. Whilst some might find it strange that the deciduous trees are being displayed in their leafless winter state, this is when bonsai shows are traditionally held in Japan. It allows the viewer to fully appreciate the fine form and structure of the trees without the distraction of leaves!

For more information about Bonsai shows, based both in the UK and abroad, take a look at the “diary dates” page at the website of the Federation of British Bonsai Societies

Many thanks to Amelia Williams from the Swindon and District Bonsai Society who has provided assistance in identifying the trees and their owners:

English Elm owned by Reg Bolton of Swindon & District Bonsai Society
and purchased originally in mid 1990’s from Kevin Willson
 

This unusual Weeping Willow by Simon Tremblett won the
“Best Deciduous Tree” and “Best in Show” awards. Simon also created the pot.

This “shohin” sized “Itoigawa” Chinese juniper (Juniperus chinensis itoigawa) owned by John Armitage was displayed on the British Shohin Association stand

A "root over rock" style Trident Maple (Acer buergerianum). This tree was originally imported from Nagoya, Japan in 2007. The tree had originally been "field-grown" for 5 years to develop the roots and trunk before being potted up. It was then worked on at the grower’s nursery for 35 years to develop and refine the branches, after which it was sold to its current owners. The pot was made in Tokoname, Japan by Seizan (Reihou). In the photo, the tree stands on a slice (a “Jiita”) of Trident maple.

A '"literati" style Scots pine (Pinus sylvestris) displayed by the Wirral Bonsai Society

A Siberian Elm (Ulmus pumila) owned by Mike Jones, pot by Walsall Ceramics

There is step-by-step history of this tree at bonsai-passion.co.uk

This English, or European yew (Taxus baccata) won the “Best Tree/Pot combination” category at the show. The tree was originally collected from the wild in Italy, and has been developed over a period of nine years by it's current owner. The pot was created by John Pitt.

English Elm owned by Reg Bolton of Swindon & District Bonsai Society
and purchased originally in 1996 from Kevin Willson.

- Richard -

Related links

• The Swindon and District Bonsai Society
• Federation of British Bonsai Societies

Welcome back. With spring fast approaching it’s time to start on the first major job of the bonsai year; repotting. It’s going to be a rather image heavy blog so I’ll start with a why, when and how:

Why:

Bonsai trees do not require the large, woody, anchoring roots that trees in the wild need to prevent them from being blown over. It is necessary to develop a mass of fine ‘feeder roots’ close to the bottom of the trunk which will help the bonsai to take up water and nutrients more efficiently.

• Repotting provides the tree with fresh soil, including fresh minerals and nutrients which may have been leached out by the watering process or successfully used by the plant
• Repotting removes inefficient woody roots that have become too long and are taking up essential space in the pot
• Repotting improves the air circulation around the roots by removing unnecessary roots and old compacted soil
• When repotting we can work on and improve visible surface rootage

When:

Repotting should be carried out every three to five years, dependant on the age and vigour of the individual tree; younger trees can be repotted more often. NEVER carry out unnecessary repotting of trees, as it is stressful to the plant, and could kill it.

You should consider repotting a tree when its vigour starts to drop off i.e. if the tree sets less buds in the autumn, or if twigs or branches start to die off unexpectedly. Other signs that repotting is required are difficulty in watering a tree; water fails to penetrate the root ball due to the air spaces between the soil particles filling up with old root material and broken down soil grains. Root pruning can be carried out in the late autumn before a tree enters dormancy (usually for conifers and other evergreens), but is best carried out in early spring, just as it is beginning to break dormancy.

How:

Try to dry out the soil in the pot slightly before repotting. It is not important to add 'crocks' or other drainage material in the bottom of the pot – this simply uses up valuable space – there is little enough soil in the pot to start with! Prepare an ample supply of soil or compost before you start; this will minimise the stress to the tree (and yourself) should you have to stop halfway through the repotting process to prepare more. Soil should be sieved to remove dust, and it is often a good idea to prepare different grades (grain sizes) to use in different areas of the pot; larger grains towards the bottom of the pot, smaller towards the surface. If you are looking to change pots, or it is the first time you have repotted your particular tree, it may be an idea to prepare an extra pot (or two, if available) just in case the root ball does not fit in your new pot.

Refrain from fertilizing for up to a month after repotting. Appyling fertilizer can 'burn' weak new roots.

So that’s it for the whys, whens, hows, dos and don’ts. Below is a step-by-step guide to how you should go about repotting a bonsai tree. Kind thanks go to Mr Nobuyuki Kajiwara (Nobu) for granting permission to use the images I took of him repotting one of the Kew bonsai, a trident maple (Acer buergerianum) several years ago.

Carrying out the initial spring pruning. Pruning away any excess branches and buds at this point will reduce the 'workload' on the pruned roots when the tree comes out of dormancy.

 Many bonsai pots feature an in-turned lip which can make it almost impossible to easily remove the tree for repotting. A 'root sickle' is used to cut the roots and soil around the edge of the pot to release the tree easily (for more on tools and their use, see my previous blog Tools of the Trade).

After using the root sickle, the base of the pot is thumped to loosen up any roots that may still be gripping the sides of the pot.

The tree and root ball can now safely be removed from the pot.

A 'scribe' type tool is now used to tidy up the surface of the soil (a chopstick will do). This reveals any large roots that may require remedial work. The surface of the soil can be brushed down with a hemp brush to remove any old moss and debris.

The roots around the edge of the rootball are combed or raked out using a root hook (for more on tools and their use, see my previous blog Tools of the Trade).


Once the roots around the edge have been loosened up, the same process is applied to the base of the rootball.

Now, the excess roots can be removed. Roots should be cut at a 90 degree angle, leaving as small a cut surface as possible. Where possible, the cut surface of the root should point downwards.

Remedial work is now carried out on thicker, woody roots. A branch-splitter is being used to sever the roots.

The root pruning process is now carried out around the edge of the rootball.

The tree now undergoes a “test-fitting” in the pot to see how much more of the root ball still needs to be removed.

At this point, any changes to the trees position in the pot need to be carefully considered; a little to the left, further towards the front or back, or rotation to better show the front of the tree? Stand back and consider the planting position carefully, as you’ll have to look at it for the next few years..!

More roots are combed out from one edge of the root ball with the root hook and trimmed off to better fit the tree in the pot.
 

A thin layer of akadama soil is poured into the bonsai pot

A thin layer, less than a centimetre, of dust-free (sieved) akadama compost is now added to the bottom of the pot (I will write a future blog about various different bonsai composts and their uses). A small mound of compost is built up in the centre of the pot to fill the concave area under the root ball that has resulted from the pruning off of the large roots.

Retaining wires have been added through the drainage holes to securely wire the root ball in place (short wires to the back of the pot, longer ones to the front)

More akadama compost is now added round the edges of the bonsai’s root ball and carefully worked in with a chopstick to prevent any air spaces.
 

The periphery of the bonsai pot has been filled with soil, and more has been added to thinly cover the rootball’s surface. The retaining wires now hold the bonsai in place.

The tree is thoroughly watered

The tree is now thoroughly watered in. Water should be applied until it runs clear through the drainage holes, washing away any akadama dust that sieving failed to remove.

The surface of the soil has been coated with chopped, sieved, sphagnum moss; this helps to draw the moisture within the pot up to the surface, the area which is most likely to dry out quickest. Roots are more likely to be active in this top layer of soil, where there is a ready supply of water, air and warmth.  The top-dressing of sphagnum moss will also help natural-looking green mosses to colonise.
 

A small trowel is used to gently press the wetted moss into the surface of the soil.

Well, that's all from me (and Nobu) for now. I hope it proves helpful during your repotting endeavours!

- Richard -