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Acer palmatum

Acer palmatum, the Japanese maple, has a wide distribution and grows at many elevations throughout its native habitat. In cultivation, it is seen everywhere as a garden plant and as bonsai.

Under good conditions, the tree grows from about 2.7m to 4.6m in a spreading or rounded, open-tiered shape, depending on the cultivar. The tree prefers light woodland conditions, with dappled shade providing some wind protection, to give it the best chance of performing well.

Young plants grow strongly, upwards and are multi-trunked. The branches tend to follow this shape when young; later they will either become more horizontal, or send out side branches, which assume flattish positions giving the tree its typical layered appearance. There is much variation in height and spread, but the palmate leaf form and dense foliage mass is common to most cultivars.

The leaf commonly has five or more lobes and is usually toothed along the margin, often with deep divisions between its lobes. The colour and size of the leaf varies enormously, but the strains favoured for bonsai usually have leaves of between 1.5-2cm in spread, with a bright green base colour edged by red; these are borne on deep red stalks, or petioles.

Acer palmatum ‘Shishigashira’ Claudia Heinzelmann (CH)

On such strains, the seasonal colour is usually interesting. From a pinkish orange and lettuce green in the spring giving it an overall subdued brown appearance like washed blood to the summer colour of deep green, it then develops the bright reds and yellows of autumn. The winter form consists of a periphery of reddish twigs, borne on apple green branches, sometimes with a blue bloom, springing from a trunk that deepens to a buff colour. This buff colour is a transitional stage and, as the tree ages, it’s replaced by thicker, whitish grey bark, often striped with buff.

Cultivars of the Japanese maple Red-leaved cultivars

Some of the most attractive are those maples that produce bright crimson spring foliage. There are two groups, both attractive in subtly different ways: Seigen and Chishio are more feathery and in the full spring phase are a pinkish orangey red in general appearance; Deshojo and Shin-deshojo have broader leaves and during the same period are a bluer-toned red by comparison. Seigen and Chishio are more dwarfed and softer in the spring than the other two. All red-leaved cultivars need spring protection until the first set of leaves has hardened.

Rough-barked cultivars

These varieties add a further dimension of ‘age’ with their texture of rough bark. There are two or three with a really rough bark, but the one that seems strongest is Arakawa. This cultivar develops a dark warty kind of bark like Kalmia latifolia. Its leaf colour is light green turning yellow in the autumn.

DWARF CULTIVARS

There are dwarf strains that lend themselves to bonsai culture. Among these miniatures, Kiyohime, Kashima and Yatsubusa are well known. They all have tiny leaves and twigs, elegant branches and trunks and strong-looking surface roots. With most maples, growers must work to increase density, but with these cultivars, deep grooming is often necessary to open the canopy of leaves. Autumn colour is yellow. In winter their tiny features create a truly beautiful bonsai image.

When training the dwarf types, remember that they tend to be base dominant and undue basal activity should be redirected by pruning back. This will channel energy back towards the apex. With these species particularly Kyohime is the weakest area. The apex needs shade to remain strong. The trick is to grow the trees in dappled shade to conserve the balance of moisture. They need air in their soil and die back when this is denied.

SPECIES

  • Common Name: Japanese Maple
  • Genus: Acer
  • Higher Taxon: Sapindaceae
  • Species: Palmatum
  • Skill Level: Beginners to advanced
A moyogi style Acer palmatum ssp Matsumurae, Height: 50cm, Owner: Jean Paul Polmans

HUSBANDRY


Placement is a key issue in Japanese maple culture. Japanese Maples grow best in light woodland cover so, for bonsai, a simulation of this proves ideal. In hot climates if the trees are grown inside a lathe wood cage with shade cloth at the sides, the heat can be substantially controlled. A mulch of firbark, or an equivalent, is a handy way of combating dry conditions. In some areas wind scorch is more often the enemy than the sun, but when the two combine, maples can become dried out and burnt in less than half a day if not protected. You need to understand local conditions intimately to know where best to site your maples.

Winter
Generally maples are quite hardy and can be helped considerably if a programme of winter preparation is followed. Feed the trees with a fertiliser NPK: 0-10-10 from late summer to ripen and harden the whole tree ready for winter. Place the tree in winter quarters early. I used to place maples under the growing bench on shingle and then draped heavy polythene in front of them so they were not freeze dried by sub-zero winds when the roots were frozen. Trees growing in the light soil mix l suggested are far less affected by the cold. Colder climates dictate a greater degree of frost protection but the principles are the same.


Spring
Spring placement is critical to maples and many factors come into play. All winds and frost must be guarded against and every response made to local conditions. Plants must receive good light and l used to roll up the polythene frost curtains each day and lower them each night ,as l hung on the weather forecast. Larger plants that are inconvenient to move around are probably best placed on a trolley or barrow so they can be moved under cover, or they can be grown in a polytunnel with the ends left open during the day.

The green forms of Japanese maple usually leaf out without the need for too much cosseting provided care is taken to protect them from wind and temperature fluctuations. Red leaved and dwarf forms really need early spring protection, as they are so tender. Maintain frost protection until the danger is well past.

Summer
Protect the maples by growing them under netting. Pay attention to watering and never let them dry out.


Pruning: Pinch or prune back to two pairs of leaves after new growth has made about three to four pairs. Any large individual leaves can be trimmed off any time they go oversized during the growing season. Gradually they are replaced with finer foliage, which will also produce finer twigs. The leaves become very dense and will need periodic grooming and thinning. Remove growth close to the trunk, dangling leaves below the branch line and any new ascending shoot lines. Just these three simple steps will open and refine the tree quite profoundly. If the tree still looks heavy, check for opposite shoot lines and prune away one pair alternating left and right. The tree will soon look considerably lighter.

Excessive shoots like these can be pruned in June
In spring when the tree buds, pinch the middle leaf to leave a pair
Leaving only two leaves results in finer ramification and a lessening of the internodes

If heavy branch reduction is planned in those years when re-potting is scheduled, the two may be combined safely if the following sequence is observed. Always re-pot the tree dry and do not water until branch pruning is completed; seal cuts and then water. This prevents weeping. In other years, branch prune in late July.


Repotting: Follow standard re-potting procedures using the light soil mix. Take about a third to half of the root mass, depending on the vigour of the tree. More vigorous trees benefit from additional pot space and may have alternate slices of soil removed between the main surfaced roots. Dwarf types with their dense roots benefit from this treatment. This is normally carried out only on well-established trees with a solid root pad.Young trees are re-potted every two to three years. Older trees can probably go to every four years. Watch the vigour and let that decide. Due to their dense growth, dwarf types should be re-potted every two years or so. Re-pot as the buds swell in spring.

A moyogi style maple, Acer palmatum ‘Deshoyo’


Wiring: Use aluminium wire and apply in summer. Maples are brittle, so use great care. Dwarf maples are even worse than brittle the branches are fragile. If you let such maples dry a little, the tissue will relax a bit allowing you some latitude! What works is my ‘hand clamp’ technique. I place both hands together slightly overlapped, and then squeeze the wired area, closing my fingers so that the whole area is supported. This spreads the load and makes bending so much easier. Avoid making small sharp bends. Shade the tree for a couple of weeks after wiring.

Acer palmatum ‘Kiyohime’ Daniel Schmidt (CH)


Soil: Japanese maples grow best in a light, well-drained soil with sand. The sand should consist of a mix of sharp and round particles from pinhead to match-head sized. The mixture should have a preponderance of round particles as the roots pass through this rapidly, encouraging gentle growth patterns in the top growth. If sharp sand predominates there will be more divisions and irregularities in the tree, as the root tips become sliced and divided. There is a very real relationship between the nature of the branch and root system. 80% round sand to 20% sharp should make up an ideal mix.

The organic content may be varied to suit local conditions, but should not exceed five parts by volume of the total mix. Ensure all ingredients are dry and sieved carefully, remove all fine powder, mix thoroughly and grip-test the mixture: it should feel like gritty sponge, with little adhesion. The use of topsoil should be kept to an absolute minimum as it holds a lot of moisture and lighter soils are less likely to freeze. Maples grown in the loam-based mixtures traditionally recommended for them are definitely at risk in cold climates. The lighter soil l recommend harbours less moisture and is safer. During hot weather, simply shade the tree.

Soil Mix:
5 parts sand
2 parts composted peat; 2 parts leaf mould
1 part good sieved topsoil
(Alternative soil mix: Akadama and kiryu (80%-20%)

A broom style maple, Acer palmatum ‘Seigen’, Height: 16cm, Owner: Jos Corstjens


Fertilisation: Feed every two weeks from leaf break to June with half strength Miracid. Once in August and again in september give a fertiliser with NPK of 0-10-10 at a tablespoon to eight pints of water. Feed with trace ­elements in spring.


Watering: Keep maples damp. Dappled shade helps immeasurably in keeping the tree moist it must never dry out. Trees that are well-watered bud back strongly and are a delight to grow, but ventilate well. Lift plants so that there is no standing water beneath the pots. Aeration is vital.

STYLES

Acer palmatum ‘Deshojo’ Janina Kreimer (CH)

PESTS & DISEASES

Windburn

Symptoms: Leaves resemble potato crisps with brown brittle edges, or all the leaves are shrivelled. Red maples in particular Seigen display blackening of the leaves and shoots.
Treatment: Move the affected plant to a well ventilated, shaded structure such as a shaded greenhouse or polytunnel and maintain soil watering. Unless it has been recently re-potted, the tree may be fed lightly with half strength Miracid. The increased humidity and light feeding will soon repair the damage.

Desiccation

Symptoms: Fissured wrinkling running in parallel lines along the limb. This usually indicates a root problem often associated with the plant being root-bound.
Treatment: If the season permits, wash off the roots and cut away any damaged or rotted areas. Soak the tree in a solution of Vitamin B1. Prepare a large container with a very open sandy soil mixture and raise it on blocks to create a good airflow. Plant the tree. Moisten the soil with Vitamin B1 solution. If the tree is in full leaf, simply pot it on into the container without root disturbance and water in with Vitamin B1 solution. If the treatment is successful, the tree will send out new leaves in about a month.

Extensive Branch Dieback

Symptoms:
Branch terminals die back to inner buds. When neglected for a year or so, the branches show up white against the live foliage usually as a result of drought.
Treatment:
As for windburn. Clean off dead branches and prime back to live wood.

Tar spot


Symptoms: The appearance of large black spots with yellow zones. Tar spot can occur with warm spring weather, or late in summer when temperatures and humidity are high, and the leaves ill ventilated.
Treatment: Spray with copper fungicide and reduce leaf bulk by pruning to admit more air. Elevate the tree to encourage air circulation and refrain from watering the leaves. Check that the soil is not soggy.

Mildew

Symptoms: Whitish grey areas appear on leaves and the tree has a tired and wilted appearance. There are wet and dry forms of mildew.
Treatment: Remove damaged leaves and burn them. Improve ventilation. Elevate the tree and if necessary move its location. Spray with a fungicide. Also spray the adjacent growing bench.

Coral spot

Symptoms: Masses of pink dots like pinhead sized pods, appear on dead timber. The danger is that live tissue can become infected, leading to dieback. The fungus often invades via dead timber killed by frost. Live branches wilt as the fungus chokes their water vessels.
Treatment: Clean out dead areas and prune back to sound wood, spray with fungicide and seal all cuts with a good quality wound paint

Acer palmatum – John Pitt (UK)

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