The Maple family consists of many different species, these all come under the genus of Acer, this includes some common varieties such as the Sycamore tree “Acer Pseudoplatanus”.
The Japanese Maple family mainly comprises the Acer Palmatum (Mountain Maple) and Acer Japonicum species which the Japanese call “Momiji”. The other is Acer Buergerianum (Trident Maple) with the Japanese name of “Kaede”. All these species have many different cultivars to them.
For Bonsai, the Acer Japonicum variety is not that often used because the leaves are quite large and the branch structure is quite coarse. Refinement is harder to achieve because of this. The Acer Palmatum and Acer Buergerianum cultivars are very popular to create into Bonsai, this is because they achieve very fine branch structure and naturally have quite a small leaf. Good branch structure is very easy to achieve, hence its popularity.
The natural habitat of the Japanese Maple family is generally growing under the protection of other trees in a forest condition; this is why for best leaf quality you should keep them in a dappled shade area within your garden.
Pinching
The Maple family have opposing buds at each node, so once the terminal growth has been removed it activates the dormant buds. By doing this regularly the internode length shortens and the leaves become smaller. The best way to achieve intense refinement is by pinching out the central shoot, with tweezers, once the first pair of leaves emerges.
Once you have done your first pinching you will get secondary budding and usually more adventitious buds around the growth ends. These should be thinned out to two opposing horizontal buds for a better branch structure.
The other consideration with pinching the growth tips, is to control the vigour within the tree to get balanced growth. This is achieved by pinching the dominant zones first leaving the weaker buds to emerge. Amongst the Acer Palmatum family there are variations in area dominance depending on the cultivar; for example Acer Palmatum Kiyohime has a natural bush habit, so its vigour is usually in the basal branches, therefore you should obtain an understanding of the cultivars habit before you consider pinching or pruning your Bonsai tree.
Pruning
The best time for pruning Japanese Maples is in the spring when they are just coming into leaf. There are two main reasons for this; the first is you can see the branch structure easier and, the second is if you need to remove large branches the tree will not bleed excessively and affect the health of the tree.
It is advisable to always seal the cuts you have made to the tree, to prevent transpiration through the cuts and contracting any diseases. The pruning of these trees for Bonsai is to improve the graceful pad structure and to defuse the dominant sap flow into the branch tips, which is created by selecting good tapering shoots within the side branches that do not conflict by crossing over one another. You do not want dominant vertical pointing shoots unless you are creating a secondary foliage pad, so it is advisable to keep only horizontal shoots and weaker tertiary vertical shoots to construct the foliage pad.
Defoliation
The main reason for defoliation (foliage removal) is to improve ramification, balance the vigour within the tree and to get light into dense foliage canopies to reduce weaker twig die back on the inner parts of its Bonsai structure. This is best undertaken in June when the tree has stored up enough energy within the tree to produce a second crop of leaves, this is why you should only defoliate healthy trees.
You should cut the leaves off at the leaf petiole (the stalk part of the leaf), so this will die back without affecting the dormant bud at the base. In order to regulate vigour within the Bonsai you should defoliate the vigorous zones first leaving the weaker until about a fortnight later before complete defoliation. If the tree has very weak zones it is possible to leave them completely in leaf while the rest of the tree is defoliated.
N.B: If the tree is not pinched leaving two leaves when the buds break after defoliation the ramification will not be improved.
Once the tree has been defoliated you should not fertilise the tree. Also be very careful how you water because the tree is not taking in any water while out of leaf, but once the buds become activated it will then require moisture.
Wiring
The best time for wiring Maples is when they are in active growth, this makes them more pliable due to the sap flowing through there branches, but you will have to be careful when applying the wire because some Japanese Maple cultivars branches can be extremely brittle so do take care! The wire has to be carefully monitored during the growing season because it can bite into the branches very quickly during vigorous growth. This can mean as little as just a few weeks before wire removal.
Feeding
As with most Bonsai you should feed gently with a slow release organic fertiliser such as the rapeseed cake, but apply sparingly on certain vigorous cultivars as the internodes will extend greatly and you will lose your branch refinement. You should feed always around the edge of the soil so it draws the roots to the edge of the pot, as this will improve the trees root spread.
Repotting
Japanese Maples should be root pruned and re-potted in the spring when the buds swell. The best soil medium for bonsai is pure Akadama (Japanese clay soil), sifted free from dust then graded out into varying particle sizes. The largest grade should be at the base of the pot working up to the finer grade at the top. If the pot does not have good-sized drainage holes you should use double red line harder grade Akadama to prevent the soil breaking down to quickly.
You should cover the freshly planted trees top roots with either the finest grade (1mm particle) Akadama or Sphagnum Moss to prevent your top roots from drying out. If you use Sphagnum it should be removed after a few weeks otherwise the roots grow upwards. The Japanese gently wash the soil off of their high quality Japanese Maple Bonsai so they can correct the roots, such as moving or eradicating crossing roots. It is very important that the roots do not dry out once exposed otherwise you will kill the tree, so if you mist the roots with water this will prevent the roots being affected. This is the same reason why once you have planted the tree into its new soil medium you should water the tree thoroughly to stop the roots dying back.