Richard, Kew’s bonsai specialist, and renowned bonsai expert Nobuyuki Kajiwara look into the process of creating a “slab-planted” Japanese white pine bonsai.
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 -
About Richard Kernick
Richard Kernick is the Bonsai specialist at Kew Gardens. He has worked at Kew since 2004, caring for and improving the bonsai collection while also working part time for the Alpine unit, helping to maintain their collection of woodland plants.
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