Scientific Name: Acer palmatum
Common Names: Japanese maple, smooth barked Japanese maple, kaede, momiji
Cultivars: There are too many cultivars of Japanese Maples to list here. For a far more complete list, I highly recommend the book, Japanese Maples, by J.D. Vertrees. Listed below is a small selection:
‘Atropurpereum’ – Red maple variety with larger leaves. The name is often used by many nurseries to refer to any Japanese maple with red leaves, and the name has become mostly meaningless. Also known as ‘Blood leaf’.
‘Beni Kawa’ – Green foliage, often tinged red with an upright habit. These are best known for their winter appeal as shoots are a brilliant salmon red.
‘Butterfly’ – a small leafed dissectum (cutleaf) variety with a delicate upright habit. Variable shaped, the variegated foliage is blue-green with white margins. Magenta colour in autumn. Seeds are particularly tiny.
‘Chishio’ – meaning ‘blood’ in Japanese, this cultivar is well known for its stunning crimson foliage in autumn. It also has bright red spring foliage, turning in to a medium green in summer. A slow growing, hardy cultivar.
‘Deshojo’ – Spring foliage is bright orange-red, with light green foliage with reddish bronze edges taking over later, though the spring colour does not hold as long as some other cultivars.
‘Kagiri nishiki’ – Variegated cultivar with deep bluish green with white margins with diffused rose or pink markings that fade during the summer. Also known as ‘Roseo-marginatum’, though confusion occurs, as that name has also been applied to other cultivars as well.
‘Kashima’ – Dwarf cultivar with tiny red edged leaves in spring, green in summer and yellow in autumn. Popular bonsai cultivar.
‘Katsura’ – Bright apricot-yellow foliage with a darker margin in the spring. Summer finds the leaves a bright golden green, with autumn colours yellows and oranges of a deeper and more vibrant hue than the spring foliage. Naturally small leaves and internodes, though not a dwarf cultivar.
‘Kiyohime’ – Emerald green, very small foliage, though slightly larger than other dwarf cultivars. Rarely puts up a central leader in the landscape, preferring a branching shrublike habit.
‘Seigen’ – delicate foliage, deep coral pink foliage in spring, maturing to a mid-green with orange-red edges. Bright crimson in autumn. Foliage very easily damaged by sun and wind, even compared to other Japanese maple cultivars.
‘Seiryu’ – upright dissectum (cutleaf) cultivar. Bright green summer foliage with occasional reddish tones in the margins and yellow to golden autumn foliage. Slightly smaller leaves than most dissectum varieties.
One of the most popular subjects for bonsai, Japanese maples exude a sense of elegance and grace. The five to seven lobed foliage comes in a variety of colours (though usually green) through a myriad variety of cultivars, often exhibiting three different colours over the course of spring, summer and autumn. Japanese maples are generally slow growing, but well worth the time for the incredibly beautiful bonsai they produce.
From a horticultural standpoint, Japanese maple refers to the species Acer palmatum and Acer japonicum and their cultivars. Some references also group all Asiatic maples (those endemic to Central and Eastern Asia) in with this name, but for this guide, we will be referring primarily to A. palmatum. The techniques herein can also be applied to the species A. japonicum, and can be applied to many (though not all) other maple species in bonsai cultivation.
History: A. palmatum has been cultivated in Japan for hundreds of years, with the selection and propagation of specific cultivars going back almost 400 years, to the early 1600’s. Natural variation in wild stands of Japanese maples, and observed mutations and cross pollination in cultivated specimens led to more than 200 cultivars being developed in Japan during the Edo period (1603-1867). Many of those cultivars were lost however during the two world wars, especially in the 1940’s, as Japanese maple stands were cut down for firewood, the land converted to growing food as so desperately needed in the wartime era of the small island nation. Only over the course of the last fifty years (since the 1960’s) has interest been revived in these beautiful trees. We can now count over 250 named cultivars known, with more being created and discovered not only in Japan, but also in Europe and the Americas.
In the past (and still) there has been much confusion in the naming of cultivars of Japanese maples. Between nomenclatures in different languages (Japanese, Dutch and English common), different names for the same cultivars, regional differences and simple miscommunication amoung other things, pinning down exactly which cultivar of Japanese maple you might be looking at can be difficult (and some nurseries are more or less helpful than others with this). While the problem is slowly being rectified, confusion can still occur. The book, Japanese Maples (as mentioned above) by J.D. Vertrees is an indispensable guide to the many cultivars available.
Anatomy: Acer palmatum, native to Japan, is part of the large family of Aceraceae, and share common characteristics with most other maple species world wide. Both the scientific name palmatum, and the common Japanese names (kaede and momiji) refer to the appearance of the foliage, and its resemblance to a hand (human or otherwise, see the trivia section). Different varieties exhibit a huge range in foliage colour, bark quality, preferred growing conditions in the landscape and general natural habitat. These trees seem to have a natural propensity to genetic drift, as new cultivars and random mutation in even standard A. palmatum seedlings is so prolific.
Japanese maples can typically be found growing best in altitudes from 330-4300ft (100-1300m) above sea level. While they are native to Japan (with a few regional to Korea and China), they are now grown across most of the temperate areas of the world as popular garden and landscape trees.
Deciduous trees or large shrubs, Japanese maples may have single, or multiple trunks in the landscape. Immature trees tend toward an upside down pyramid appearance, while older, more mature trees have rounded domes of upper branches and foliage. These trees range in size from full sized trees of around 30ft (10m) to the smallest dwarf cultivars whose mature height may be as little as 3ft (1m). While there are some notably larger exceptions in the landscape, most Japanese maples are not towering giants.
Foliage: Leaves vary greatly by cultivar, though most range from 1 ½-5in (3-12cm) tall and wide. All are palmate with five to seven lobes, though the depth of the lobes also vary greatly. Margins are frequently toothed, though some more obviously and deeply than others. Japanese maples are frequently divided in to groups by their foliage, both shape and colour: red leafed, green leafed for colour, and then broken up by how deeply divided the palmate lobes are. Some references refer to five groups based on leaf divisions, those listed below taken from J.D. Ventrees:
Amoenum – Lobes are shallowly to moderately divided; no more than two thirds of the way to the leaf base.
Palmatum – Lobes are moderately to deeply divided; two thirds to three quarters of the way to the leaf base.
Matsumurae – Lobes are deeply divided; more than three quarters of the way to the leaf base.
Linearilobum – Leaf lobes are narrow, straplike and divided to the leaf base.
Dissectum – Lobes are deeply divided and deeply dissected in to sublobes.
Not all cultivars can be so easily split up however, and the general groups above have been used for convenience and as an identification tool, not as hard and fast rules. These leaf shape groupings should also not be confused with the subspecies they are named after, as not all of the cultivars that fall in to each group actually resemble the subspecies all that much.
Stem and Trunk: Both smooth and rough barked varieties exist, with bark colours ranging from greys and browns to brilliant reds.
Flowers/ Fruit/Seeds: Small, inconspicuous flowers grow in cymes. Individual flowers have coloured sepals, ranging from cream coloured to yellow-green and red, protecting five small whitish petals appear in spring in May or June depending on the local climate. Small winged fruit (called ‘samara’ or ‘keys’) develop and ripen in September or October. Samaras of Japanese Maples are some of the smallest in the Acer genus.
Hardiness: Zone 5 or 6 in landscape. Winter protection is necessary, as though it belongs outdoors, it cannot survive without protection in areas where it generally dips below freezing for the winter. Root damage to Japanese maples occurs at 14F(-10C) in the landscape (below this point, total root destruction can occur), while trees in the small pots of bonsai cultivation can be damaged even easier. Keep in a frost-free shed or garage after leaf fall for protection.
Light: Dappled sun to light shade. The dappled light is important in the summer, to protect the delicate leaves from sun-scorch. Some cultivars are more tolerant of strong sun than others, but almost all benefit from at least some shade on hot summer afternoons. Red leafed varieties, while more prone to leaf scorch than green leafed varieties, tend to have less vibrant foliage when grown in too much shade, and will actually revert to green from red if not given enough light. A balance must be found between too little light (such that the red leaves change to green) and too much (where the leaf margins scorch). Also protect from excessive wind.
Soil: Well draining, slight acidic. Some organic matter (conifer bark, chopped sphagnum moss- not peat moss), but not enough to become waterlogged. Pure akadama can be used if careful attention is paid to watering and fertilizing. Chopped sphagnum moss is a good choice as a soil additive, as it is slightly acidic.
Watering: Japanese maples require slightly acidic environments, so use rainwater instead of tap water if possible, as tap water tends to more frequently be slightly alkaline. Keep soil moist at all times, as maples do not tolerate drought, but keep from being waterlogged or boggy. Whether watering heavily or watering lightly (to encourage growth or to restrict growth respectively), ensure a consistent amount of water. Uniformity of watering, rather than quantity, matters most to these trees. Water burning the leaves during the heat of the day is a myth, and you don’t need to worry about it, but still consider watering before the heat of the day. Not because you might burn the leaves, but so the tree has enough water to make it through without drying out.
Fertilizer: Feed every week to two weeks with a high nitrogen fertilizer as leaf buds open in spring. Every other week during the spring and summer after leaves have hardened off with a diluted, well balanced fertilizer, easing up on either strength or frequency during the hottest part of the summer, as growth slows. Switch to a nitrogen free formula in late summer/autumn, tapering off before winter. Lower fertilization can result in shorter internodes on more developed, mature trees, while too much fertilizer can result in leggy growth. Keep the age and maturity level of your bonsai in mind when deciding on fertilizer strength and frequency. Do not fertilize right after repotting. Wait until you see new growth, and then start light, moving in to stronger summer feeding.
Maples can occasionally be dosed with an acidic fertilizer (like that for rhododendrons) or alkaline fertilizer (like that for roses), especially if there is yellowing of the leaves (chlorosis), as alkaline or overly acidic situations can lead to a binding of nutrients in the soil. Test the pH of your soil if this is occurring, and apply as needed, but this is not necessary if the tree is happy and healthy in other respects.
Some Japanese maples may ‘resent’ the use of ammonium sources of nitrogen in fertilizer. A fertilizer with a non-ammonium source of nitrogen may be a consideration if you are noticing a problem or have overly acidic soil composition, though this is more of a problem with Japanese maples in the landscape than in bonsai cultivation.
Keep in mind that variegated cultivars may lose their key characteristics if over fertilized. Atypical leaf shapes may revert to normal leaf shapes, while distinct colours and patterns may fade. If you notice this occurring, reduce the amount of fertilizer and/or the frequency with which you are feeding.
Pests and Diseases: Healthy Japanese maples are not prone to infection, but that doesn’t mean they shouldn’t be watched, as infestations and infections, when they do occur, can rapidly spiral out of control. They can be particularly prone to aphids and scale when stressed. Aphids can infest young shoots in spring, leading to distorted leaves. Spider mites, weevils, and bark beetles (especially if the tree is already unhealthy) can also attack. Also watch for local caterpillars, as they can defoliate a tree swiftly. Verticillium wilt can be fatal in maple bonsai. Watch for anthracnose, powdery mildew, and cankers. They can also be prone to fungal infections from overwatering which will lead to root rot.
Leaf scorch is a common problem with Japanese Maples, though not a disease. It is caused when the leaves lose water faster than they can take it up. While typically attributed to too much hot sun, it can also be caused by late/early frosts, drought (did you water your bonsai today?), drying winds, and even a build up of minerals or salts in the soil, due to an imbalanced pH or over fertilizing.
Repotting: Repot early in spring before the buds open. Every year or two for trees under ten years old. Two to three years for more mature trees. Do not remove more than half of the root mass (unless it is rotted or damaged) from young trees during repotting, and less than that for older, established trees. Japanese maples naturally have very shallow root systems and easily take to shallow bonsai dishes, though care needs to be taken to never too radically prune back the roots due to impatience to have the tree in the ‘perfect’ container.
Pruning and Styling: Pinch out the tips of new shoots when one or two pairs of leaves have formed completely to retain shape or encourage ramification. In Japan, the centers of new shoots are removed with tweezers and a magnifying glass just as the shoot is opening up for maximum internode reduction, though this should only be used on mature trees that are focusing on ramification, rather than immature trees still focused on growth. Pruning in late spring to midsummer will lead to masses of new shoots from the internodes. Prune out new, unwanted growth with long internodes. Heavy pruning or trunk chopping can be done at almost any time of year, though there are several opinions on when it is best to prune heavy branches. Some say spring, when the tree is growing the strongest and will respond best and heal quickly. Some say late summer to early fall, so the tree still has time to heal before winter dormancy, but will not throw out as much new growth as it will in the spring (though this runs the risk of new growth not having enough time to harded off before the first frost). And some say after leaf fall in autumn or in late winter when the tree is in dormancy, to reduce the amount of stress placed on the tree and because you can better see the tree’s structure. There are equal disagreements about using wound sealant or not, as some claim it protects the tree while others say that it only traps bacteria already present.
Maples will back bud reliably, throwing of shots from old and new wood alike. But keep in mind that they back bud primarily from internodes. Cutting a branch back too far, back beyond the first node, can run the risk of losing the branch. When trunk chopping, while they will frequently backbud from dormant buds, keep an eye out for visible nodes on the trunk below your chop point. These will be clear bands in the bark on younger trees that still have immature bark. If you have a choice between stock with clear nodes below the point where you want to chop, and none, choose the stock with the nodal bands if your aim is smaller bonsai. On smaller bonsai especially, these nodes will mark where the branches will grow, and are very important in future planning.
Keep in mind that many cultivars of Japanese maples are grafted onto root stock of a different, hardier cultivar or a standard A. palmatum. Never trunk chop below the graft point, unless the specific cultivar is of no interest to you.
Wire: Wiring can be done almost any time of year, though early spring, before bud break is ideal, as the structure of the tree is clear and visible. The tree is also getting ready to put on new growth, and the branches will set better. Watch carefully if wiring this time of year, as due to the quicker growth, the wire can quickly damage the bark and branch. Be careful also of dislodging leaf buds, as they are easily damaged at this point. Wiring after summer defoliation is another option, as is wiring after autumn leaf drop. If wiring in autumn, provide additional protection as the branches will not heal until spring. Do not wire in winter, as the branches become brittle and break easily. Even if not defoliating, wiring in midsummer is a good choice, as the growth will have slowed slightly from the earlier part of the spring, but the branches will still have a chance to set. Wrap branches with raffia to protect the delicate bark, and rewire in six months (or earlier if you notice the wire starting to bite).
Leaf Reduction: Defoliate healthy trees in early to midsummer to reduced leaf size and increased ramifications on healthy trees. Full or partial defoliation can be used (though with partial, remove leaves randomly from all over the tree, rather than from specific branches, to avoid confusing the tree), leaving the petioles (leaf stalks) on the branches. Avoid defoliating and repotting in the same year, and do not defoliate two years in a row.
Jin/Shari: No. Though wood can be preserved with wood hardeners, it is generally not recommended as the wood rots easily.
Styles and Forms: Suited to many forms, including: broom, informal upright, group plantings, raft, twin trunk, clump and weeping. Cascade and semi-cascade are possible, and even beautiful, but can look contrived with this species with the exception of some dissectum cultivars.
Seeds: Growing Japanese maples from seed can be frustrating. Germination rates vary, and even viable seeds can take up to five years to germinate! So do not throw out your Japanese maple seed trays if they don’t germinate the first year. Leave them out for several years, though the highest germination rates will occur in the first three or so years, with sporadic germination following that. If you have purchased seeds, and they arrive dry however, other preparations should be done. Soak seeds in warm water for 24-48 hours. Treat with fungicide and place in polyethylene bags filled with equal parts sharp sand and peat. Store bags in the refrigerator (no warmer than 40F (5C) for at least 60 days, but no more than 120 days. Plant in seedbeds or trays outside (sometime around March, though take local climate in to account, though light frosts are not known to damage germinating seeds). Planting mediums usually contain peat and sand or perlite, percentages depending on how damp your local climate is. A slow release, balanced, weak fertilizer can be mixed in with the planting medium. Plant seeds at a depth of about twice the diameter of the seed.
Ripe seeds that have not been allowed to dry out can also follow the above method, though soaking in warm water is neither needed, nor recommended. Sow ripe seeds as soon as they ripen outdoors, which will allow the winter for cold stratification.
Maple seeds are ripe when the wings are brown and have dried out, but the seed still retains its original colour and has not dried out. Do not remove the wings, as it is not only a boat load of unnecessary work, but also causes the seeds to dry out more as well as opens things for fungus and disease.
Keep in mind that many cultivars do not breed true to seed, and while you will get a Japanese Maple seedling, it may not match the parent plant. Spontaneous mutation is more common with some forms of Japanese maple than others, so watch for interesting flukes in your seed bed!
Cuttings: Softwood cuttings, 4-6 inches (10-15cm) long, in early to midsummer, just as new growth is beginning to harden off. Many cultivars do not root easily and the failure rate can be extremely high on these. Remove all but the top pair of leaves. Bottom heat has been shown to increase the rate of successful cuttings. Keep shaded.
Grafting: Grafting can be a long and complicated process, especially as some Japanese maple cultivars have very specific grafting requirements. In general, grafting is one of the few ways to consistently propagate many cultivars, using standard A. palmatum understock and the cultivar as scion.
Other: Airlayering and ground layering are good options to preserve a particular cultivar, some time in mid to late spring as new growth hardens off. Some cultivars are not as strong on their own root stock as they will be on grafted rootstock.
Other Facts and Trivia:
Brent Waltson at Evergreen Garden Works has a great article on chosing Japanese Cultivars for bonsai which I highly recommend: http://www.evergreengardenworks.com/japmaple.htm
When buying A. palmatum stock: Watch for poor graft sites in cultivars of Japanese Maples. Few nurseries, especially those that do not focus in stock for bonsai, take the time to do a nice graft, as it matters less for landscape trees. If the cultivar you are looking for must be a graft, look for low, smooth graft sights. Any ugliness in the graft will only get worse with age, not better, so choose carefully.
Display notes: Display maples indoors for a few days at most when in leaf, or for an hour or two when dormant. Bringing it inside for longer during dormancy risks confusing the tree and starting the spring awakening too soon.
In Japanese, maples are known as kaede and momiji. There is little distinct separation between the uses, though kaede is more commonly used in everyday speech to refer to maples in general, while momiji is most often applied to maples with deeply serrated leaves (like many cultivars of A. palmatum). Linguistically, kaede stems from the ancient word kaerude, where kaeru = frog, and de = hand, as the leaves of maples reminded people of a frog’s webbed hands. Momiji can be translated two ways, one as “baby’s hands”, though this does not apply directly to the translation, acting more as a simile (“Little baby extends its hands which are like the leaves of momiji.” Quote from Japanese Maples, by J.D. Vertrees). It is also linguistically related to an ancient verb, momizu = becomes crimson leaved.
For a full view of Japanese Maples in all of their glory, visit the Japanese Maple Gallery at the Art of Bonsai Project: http://www.artofbonsai.org/galleries/maples.php
Japanese Maples, Third Edition, J.D. Vertrees, 2001
Bonsai Survival Manual, Colin Lewis, 1996
Trees and Shrubs, Ernie Wasson, Tony Rodd, 2004
Growing Shrubs and Small Trees in Cold Climates, Nancy Rose, Don Selinger, and John Whitman, 2001
Evergreen Garden Works Japanese Maples – http://www.evergreengardenworks.com/japmaple.htm
BCI Japanese Maple Entry – http://www.bonsai-bci.com/species/japanese-maple
A. palmatum Wiki entry: http://en.wikipedia.org/wiki/Acer_palmatum