By the time you read this newsletter we will be at the end of January. I hope you all had the Christmas and
New Year you wished for.
I started this article before Christmas after being asked what we should be doing with our bonsai this year, so
here we go!
We can look at some of the basic bonsai training and grooming techniques and consider how and why they
work. The basic techniques which are used to shape bonsai and keep them small are, limiting root volume,
root pruning, branch (hard) pruning, shoot (soft) pruning or grooming, shaping by wiring and leaf size
reduction by increasing twig density, by control of light supply and defoliation.
The supply of water and dissolved plant nutrients from the roots to the leaves is the main factor which limits
tree growth, so keeping the roots of bonsai confined in a restricted soil volume helps to slow down growth
and keep them small. However, for the tree to survive, its roots must function properly and to do this they
must keep growing so as to be able to produce new absorptive root hairs. This means they need fresh soil
space to grow into. A young bonsai will be vigorous and will fill the available soil with roots quickly, while
an old mature one will have slowed down, and will take longer to fill its pot with roots and this is why root
pruning and repotting is necessary every year or two for young bonsai and less often for old ones. It allows
room for fresh root growth in the bonsai pot and replenishes old soil.
Without it all of the soil becomes pot bound and will die.
The tree’s need is for lots of fine new feeder roots and so this is why it is advisable to cut back any thick
roots hard, stimulating the production of many new roots from the cambium around the wound edges.
If you are developing a root over a rock style, the rock and trees should be left completely covered by soil
until such time as the required size is reached. This is because roots can expand and grow in thickness more
easily without the constricting effect of a thick bark layer, and under moist dark conditions the outer
covering remains thin, only developing into a thick bark when exposed to the light and air.
Despite the confinement of the roots, bonsai do continue to grow and so we need to control this growth and
shape them by pruning unwanted growth to create and maintain the desired structure for the chosen style.
The general aim of this structural pruning is usually to maintain the balance of branches in the crown of the
tree, thinning the crown and keeping the apex lighter than the base, diverting vigour to the lower part of the
tree and developing increasing twig density. The way this works is that the larger buds at the end of the
branch produce a hormone that inhibits bud development back down the branch so removal of the branch tips
removes this hormone source and the dormant epicormic buds on older wood are allowed to develop into
Also, during the development of most species as bonsai, even if you have soft pruned continuously through
the growing season it will be necessary to shorten the branch growth made each year, cutting them back,
sometimes quite severely to a suitably placed bud. This is one positioned to give growth of the branch in the
desired direction next year.
The timing of branch pruning is important and late winter, when the tree is dormant, is the best time to remove any of the larger branches, particularly in the deciduous species.
This is because most of the stored energy material (starches) in the tree are laid down in special cells in the medullary rays of the oldest wood, deep in the trunk and heaviest main branches.
This stored Energy is mobilised as sugar in the spring to allow the buds to develop new shoots and leaves before the tree can begin to produce more sugar by photosynthesis. If therefore, you remove any of the surplus branches in spring, when the energy-rich sap has started flowing into those branches, their removal will also remove and waste part of the tree’s stored energy resources which it needs if it is to get off to a good start.
Branches which are removed in the dormant season should be cut to leave a short stub. Trees have the ability to internally seal off any damaged areas and stubs will die back and dry off and can be cut off flush later in the summer, when growth is at a peak. Then because it is actively growing, the tree will quickly cover the wound with protective callus tissue. If for any reason you decide to remove a branch in the summer, cut it flush with the trunk, and the wound will soon begin to callus over. If you leave the stub in summer it is quite likely that adventitious buds will develop around its end and it will produce more unwanted new shoots.
The process of wound healing by callus formation naturally occurs faster when the tree is in active growth. This is why it is recommended that major pruning cuts be made just prior to growth in the spring. The process of wound healing around pruning scars uses lots of energy and so it is aided by a supply of sugar rich sap. This flows best vertically downwards in the trunk and not so well laterally, so a long vertical cut interferes less with sap flow then a broad horizontal one and thus usually heals quicker.
Trunk wounds just below large branches will heal faster due to the good supply of sap from above and if you get an adventitious shoot developing from the top side of a large callus, leave it for a season or two as it will nourish the callus tissue and speed up healing. You will however need to remove it before it becomes too large or you will also have a second scar to hide.
New callus tissue is not constrained by an outer layer of bark, so it naturally bulges and swells out over the wound. This natural appearance of a raised callus on the trunk can be an attractive indication of age in some trees, but in order to get a flat callus surface where you need one it is necessary to make a hollow into the centre of the woody part of the wound. The new callus tissue will then grow out from the cambium on the wound edges and will roll over into this hollow, leaving it level. If the process of callus formation stops before all exposed wood is covered, just rewound the edges of the callus slightly and the trauma will stimulate more cell division from the cambium.
Larger trees can cope with damage by compartmentalising it and sealing it off from the healthy tissue, so do not need wound sealants. However with bonsai it is a good idea to cover large injuries with a wound sealant or soft clay. This acts as an instant callus and its main benefit is that it reduces drying of the exposed cambium tissue at the wound edges. This is vital as it keeps the cambium cells alive so they can form callus and adventitious buds. If the edges of the wound dry and shrivel, new shoots will not be formed there. They will arise further back from the wound, and this is no good if you want to develop a broom style by chopping off a nice straight trunk, when it is important to get the new shoots arising right from the edges of the cut trunk, not halfway down it.
Sometimes you find that an otherwise attractive bonsai has a thinner section of trunk near the base, detracting from its otherwise ideal taper. Such localised depressions can be encouraged to thicken up by gently but regularly beating! A few light taps with a blunt instrument through the spring growing period wound the underlying cambium slightly, releasing the hormone which stimulates callus formation and causing localised thickening just where you want it.
Sometimes it is advisable to delay the removal of unwanted branches which are superfluous to the final design. These surplus branches carry leaf area which will feed the tree, making the trunk thicken up and mature much more rapidly. The best ones to keep are those which arise low down on the trunk, as the effect of the sugar rich sap that they produce will be greatest below their point of origin, and so they help to thicken the trunk base, helping to create good taper from the base upwards. All unwanted branches should of course be removed before becoming so large that their eventual removal would leave a large obtrusive scar. Leaving the terminal buds on the lower branches so that they extend unchecked throughout the growing season means that they carry many more leaves then they would if pruned. This induces much greater caliper (thickening) growth in the base of those limbs and in the trunk below them. So if branch thickening is the main aim the selected branch may be left to extend fully for a whole season, even if it gets to be several feet long after which it can be cut back in the dormant season to a suitable placed bud closer to the trunk.