Chapter 71 Acer palmatum, Acer japonicum, Acer ginnala

71.1 Japanese, full moon, & Amur maples

Two views of a large Japanese maple in the Kew Gardens Collection. Link to original image 1; link to original image 2.

71.2 Soil: Moist Deciduous Mix

Ratio: 4 parts bark, 1 part grit

Add-ins: dolomitic limestone (~5 lbs/yd), Tree-Tone (15 lbs/yd)

pH: ~6.5 (slightly below neutral)

Moisture: Average

71.3 Moisture

Japanese maples perform best with consistent soil moisture.

71.4 Light

Routine: Dappled shade to part sun

Summer: Mid-day shade will limit leaf burn

71.5 Cultivation Notes

Japanese maples are among the first trees to break buds in early spring. New shoots extend quickly during the cooler days of spring, and only slow down once daytime temperatures reach about 70^oF.

All Japanese maples like soil that is evenly moist, but never boggy, for most of the year. They grow best and reach peak color when they get “open sky” illumination, that is, they are sheltered from direct sunlight, but get ample filtered light from above. They also do well in dappled sunlight. They can tolerate some direct sun, but not during midday.

Most Japanese maples provide a colorful show in autumn without any special care. Allowing them to dry out slightly will intensify their colors, but the key is SLIGHT stress, not serious or extended stress.

71.5.1 Potting

Japanese maples respond well to regular root pruning and repotting. I routinely put mine in fabric bags to develop their feeder roots. Within 3 years, most of the “bagged” trees have a mat of fine roots and are ready to start top styling.

I can take off up to 50% of the root mass of the trees I repot in early spring (mid-March). I remove no more than about a third of the root mass from trees that I repot later in spring (up to mid-April).

I stop repotting and root pruning my Japanese maples after April 15th. I have lost trees I repotted later in the year because they did not have enough roots to survive an early hot spell. Some growers will root prune and repot in autumn, but I have never tried it.

71.5.2 Pruning

Do major branch pruning very early in the season to limit the loss of nutrients caused by removing a branch full of tree sap. Branch cuts may bleed for 1-3 days if the sap is already moving up from the roots.

In the unending debate on whether to seal pruning wounds, I find the best approach on Japanese maples is to leave a small stub, cut off at an angle so water does not accumulate on the end. I seal it with a non-porous sealant like Kyonal or carpenter’s glue. The next spring I cut the stub back further, so it is just below flush with the trunk. This time I do NOT seal the wound, or at most treat the bare wood with iron sulfate. I let the cambium around the new cut start rolling in, wounding it again a couple times over the year to keep it moving. After 2 years, the scar usually is barely visible and covered with new bark. If the cambium does not roll in completely, I carve out the remaining visible wood to form a uro (cavity), and paint the space with a porous preservative. To keep the natural color I use Kaizan Natural Deadwood Preserver, tannic acid followed by iron sulfate to create a medium to darker gray, or diluted India ink followed by Kaizan Tree Gum for nearly black.

Lateral branches emerging from the primary scaffold branches can be pruned up until the first of May in this area. Once the daytime highs go above 80^oF for 3-4 days in a row, growth of Japanese maples starts slowing down, so I stop branch pruning for the year.

New green shoots will extend quickly in the early spring. I remove small shoots that are not placed where I want as soon as I see them. I let shoots that I want to build into new branches extend as much as they can until mid-April. If I want to thicken the new branches and encourage immediate side-branching, I cut them back to 1-2 pairs of leaves; the buds at the bases of the terminal leaves will provide the next pair of extension shoots, either later in the year or the next spring. I find it easier to clip green shoots with heavy scissors rather than pruning clippers or a concave cutter, but that is personal preference only.

Sometimes I do not cut back new green shoots and instead use very fine wire to bend them into branches with movement. This is a good technique for filling gaps in the crown or establishing the secondary branches quickly. These will set by fall, and the wire should not stay on over winter. The next year these 1-year branches will throw off several green shoots of their own. I let the new shoots grow until mid-April to increase the size of the supporting 1-year branch, then cut them back hard to 1-2 pairs of leaves, and start building ramification. If the wired 1-year branch is not thick enough its first year, I leave the new shoots alone for the year, and prune them back hard the following spring (12-14 months after they first emerged.)

Random green shoots will pop out all summer long. Clip them off with scissors at 1-2 pairs of leaves if they are in a place you want to keep them.

71.5.3 Feeding

Japanese maples are fairly slow growing and the wood matures slowly, so they cannot process large amounts or manage sudden bursts of nitrogen well. Excess nitrogen produces soft growth that invites disease and dieback in the stems. Maintaining a constant low level of fertility will keep trees healthier.

Fertilize Japanese maples in spring, after new leaves have begun to emerge. Do not fertilize a Japanese maple when repotting. Wait 3-4 weeks. Then feed sparingly til mid-summer.

When feeding a Japanese maple, use slow-release fertilizers that have about three times more nitrogen than phosphorous. Good options:

• Slow release shrub and tree fertilizer, applied at half the recommended rate.
• Organic plant food that must be broken down by soil microbes, such as fish emulsion or Epsoma Tree-Tone.
• Inorganic, water soluble fertilizers at half strength rather than full strength.

For example, feed with 18% N, 6% P, 12% K controlled release granules at half normal dose just after spring leaf-out. In late June or early July, and early August, feed again with 0.5x Miracle-Grow. Maples can be given a 0-10-10 in late September to promote root growth and harden off new growth for winter.

Avoid these fertilizer options:

• Regular 10-10-10 and similar quick-release fertilizers
• Raw manures
• High N lawn fertilizer

Keep in mind also:

• Do not use liquid feed in late fall or early spring. Liquid fertilizers encourage Japanese maples to grow immediately, and this tender growth can be damaged by early freezes in fall and late freezes in spring.
• Controlled release granules on the top of the soil may not maintain a constant moisture level inside the pellet, resulting in sporadic feeding.

A Japanese maple in full autumn colors. Link to original image.

71.5.4 Pests & Problems

Occasionally I have had a tree die suddenly from root rot, but in my hands, Japanese maples have been very pest-resistant overall.

71.5.5 Winter Storage

Store in mulch outdoors. Try to place in a shaded area so buds do not break in a mid-winter warm spell. No other special precautions are needed.

71.5.5.1 Managing Dwarf Cultivars

A lot of what I just wrote about managing Japanese maples does NOT apply to dwarf cultivars. I have lost quite a few dwarf trees by trying to manage them the same way as full-size trees. It took some time to understand why, but I THINK this is what is happening.

Dwarf cultivars originated as “witch’s broom” mutations in full size trees. The trigger can be scale or aphids, viral infection, chemicals, or just random errors. The effect is basically the same: a dramatic change in branching patterns and leaf size. Also, most dwarf cultivars will not grow on their own roots and must be grafted onto a non-dwarf root stock to thrive.

Here is the speculative part; I looked and could not find any research that supports or refutes this idea. Many genes regulate the shape of leaves, others affect branching, and still others affect root growth. What ties all of these systems together is that they are regulated by just a few plant hormones. I personally think that dwarf cultivars are not caused by mutations in the genes for branches or leaves, but by mutations in hormone signaling and growth regulation. If this is true, we should not expect dwarf trees to respond to pruning in the same ways as their parent species.

After some trial and error, I have worked out a way to manage dwarf cultivars that seems to work for me. If you are handling dwarf cultivars, my general advice is to go much slower and be more conservative at every stage. I would give the same advice for slow-growing cultivars like ‘Shishigashira.’

1. Since the roots are not from the dwarf cultivar, potting and root pruning is the same as for a non-dwarf cultivar. The one adjustment I make is that I grow my dwarfs in deeper containers than what the caliper of the trunk dictates.
2. Remove sprouts from below the graft line immediately. A shoot from the rootstock will be a full size tree, and pulls water and nutrients away from the dwarf top-graft.
3. Dwarf cultivars may not back bud as well. I try never to remove more than half of the length or foliage mass from any existing branches.
   
4. Branches are more brittle on dwarf cultivars, so I limit wiring.
5. Similarly the crotch angles between branches tend to be smaller in dwarf cultivars, so I avoid trying to separate or bend out two branches on either side of a narrow crotch.
6. Don’t expect long green extension growth. Dwarfs rarely put on more than one inch of new growth per year.
7. Dwarfs tend to ramify on their own without much pruning. Think about pruning in terms of managing where the ramification goes, not as a way to create ramification. Don’t try to force ramification either; dwarfs will not back bud nearly as much as non-dwarfs.
8. Work with the natural branching patterns. Treat the crown more like a topiary to be managed, not as branches to be sculpted.
9. Provide protection from the mid-day summer sun. Dwarfs burn quickly and badly.
10. Fertilize the same way as for other Japanese maples.
11. Dwarfs need more insulation and protection from wind in winter than full-size cultivars.
12. Some dwarf cultivars are prone to tip and apical die-back, both in summer and winter. Clip off dead branches immediately to provide space for other branches to fill in.

71.6 Propagation Notes

Japanese maples prefer a moderately acid to neutral soil ranging from 6.0 to 7.0 on the pH scale. Once you know the soil pH, if necessary, you can adjust the pH to meet the needs of your Japanese maple.

Both Japanese maple species can be grown from seed. Some cultivars have been so heavily selected over time that their seedlings resemble the parent tree. These seedlings may be sold under the same name as the cultivars, despite not actually being that genetic type. More often though, seedlings will vary some from their parent.

Not all Japanese maples produce viable seed. I have collected seed from dozens of different trees in several states, and on average I estimate that less than 25% of the trees I have sampled have provided viable seed. Some trees produce viable seed reliably year after year. Others produce only a few viable seed, or only produce sterile seed. Some trees never form seed at all.

All maple seeds require cold, wet stratification in order to germinate.

To ensure their form and color remain true, nearly all cultivars are propagated asexually by grafting or budding onto seedling stock. Grafting is performed using 2-3 year old seedlings as the rootstock. Scion wood from the desired cultivar is added just before bud break using a saddle or whip graft. Alternatively, buds from the desired cultivar are inserted beneath the bark of the rootstock in late summer.

71.7 Artistic & Styling Notes

Smaller Japanese maples styled as a sakei. Link to original image.

Full size Japanese maples in the wild do not have tight, congested foliage. Instead they have a naturally open, airy feel to the crown. Styling should accentuate this natural feature rather trying to force the tree to form thick, tight foliage pads.

Final show pots should be glazed in a color that complements the colors of the foliage or bark in the season when the tree is its most attractive. For example, an Osakazuki cultivar will have brilliant red fall leaves. These will clash with a red-brown glazed pot, but would be offset nicely by a darker blue or blue-gray glazed container.

71.8 Additional Information

71.8.1 Natural History

Japanese maples still grow as wild trees, but are quite different from what we might imagine. The highest concentrations are in sub-boreal forests on the northern Japanese island of Hokkaido and the Korean peninsula. These are cool, temperate forests with regular rainfall, similar to what we see at higher elevations in central Appalachia, and across much of western Massachusetts and New York state. Like the sub-boreal forests here, the dominant Asian species tend to be oaks and elms, with ash, alder, hornbeams, and some fir or spruce mixed in.

Native Japanese maples are understory trees that inhabit a niche similar to our dogwoods, hackberry, wild cherry and hornbeam. Wild Japanese maple seeds tend to germinate and grow quickly for a few years, then growth slows to a crawl. Like other understory trees they wait for an older, larger tree to die and fall. When an opening appears in the forest canopy, they rapidly grow up and fill the space. If larger areas are opened up by fire or other natural disturbance, the oak overstory trees may be replaced entirely by opportunistic understory maples. The heavy shade under these dense maple groves prevents oak acorns from germinating, letting maples become the dominant species. These groves can persist for a century or more, and trunk calipers can reach 4-5 inches.

Wild trees display considerable genetic variation. Seedlings from the same parent tree can have very different leaf size, shape, and color. So far I have been unable to find a biological reason why they are so variable. It may be buried in the literature, or simply not yet known.

Botanical records describe true Japanese maples as deciduous shrubs or small trees that can reach heights of 20-30 feet. A tree may have a single leader, or multiple trunks joining close to the ground. The most common form for the crown is an upside-down pyramid (especially when younger) or a dome shape, especially when mature. Leaves can have five, seven, or nine pointed lobes. Like all maples they produce small winged seeds called samaras.

71.8.2 Cultivation & Cultivars

Botanists recognize 23 species in the genus Acer as endemic to Japan. A. palmatum and A. japonicum are the main species in the nursery trade. Technically, Japanese maples are trees grown from seeds of either of these species, plus all cultivars derived from them. Unfortunately the nursery trade has gone overboard and put the name “Japanese maple” on cultivars of A. shirasawanum (full moon maple), A. truncatum (Shantung maple), A. pseudosieboldianum (Korean maple), and sometimes A. buergerianum (trident maple). The last two seem particularly silly, because they are not even natives of Japan!

The Japanese have two names for Japanese maples: momiji (lit. ‘reaching baby hand’ or ‘turning red’) and kaede (‘frog hand’); the usage varies, and a lot of cultural context is lost in translation. Some references use both terms to refer to Japanese maples in general. Other sources use the two words to refer to distinct leaf shapes, or to the species versus the cultivars.

Japanese nurseries have been selecting and propagating cultivars of their eponymous maples since the early 1600s. They were first introduced in England in 1840, then in the U.S. in 1862 by Dr. George Hall, through a nursery in Flushing, N.Y. To date over 1,000 named cultivars have been described. Some of these have never been cultivated in the West or have been lost, but new cultivars are introduced regularly.

We tend to think of these mainly as foliage trees, whose greatest assets are their summer and autumn colors. Indeed some cultivars are sturdy trees that can be even larger and more hardy or vigorous than is typical of the seed-grown species. Many traditional Japanese gardens have an iromomiji, a mid-size upright maple cultivar positioned to send lateral branches up and over the viewer’s head. In this manner the delicate green leaves of the maple float above and soften the mood of the entire garden space. The maple reflects the changing seasons, with tender green leaves in the spring and brilliant displays of color in the fall. Its purpose is not to overwhelm the viewer’s senses, but to create a harmonious connection between the tree and other elements within the garden.

Unlike Westerners, the Japanese have developed a much more complex aesthetic relationship with their maples, and cultivars have been selected for distinctive features that serve many different roles in the landscape or garden. Some have spring foliage that unfurls from colorful buds, much like flowers. They leaf out unevenly, and so provide a colorful display for 1-2 months. Dissected forms provide textural interest, color, or both, while their usually weeping form echoes the movement of water. Dwarf forms can provide focal interest and rarely reach 2 feet tall. A few very delicate cultivars rarely reach more than 1 foot, and are grown only in pots.

There are several books and online resources that list many of the known cultivars. Most separate them into cultivar groups (also called varieties) based on shared traits, but different sources use different criteria for the groups; there is not a single uniform standard. These are some the more commonly recognized cultivar groups (some cultivars belong to more than one group):

‘Atropurpureum’ includes all of the Japanese maple cultivars with dark red leaves. The archetype for this group is ‘Bloodgood.’ Clones of ‘Bloodgood’ are robust trees that can reach 20 feet tall, but they do not stay red all year. By mid-summer the leaves have turned green tinged with red, but in fall they return to red. Other cultivars in this group may keep their color all season. Despite their color similarity, not all members of the group are genetically related.

Dissectum, filigree, or lace-leaf maples are upright or weeping trees with leaves that are finely cut and incised, with 7-11 lobes. The fine-textured leaves can appear threadlike.

Bamboo cultivars have leaf lobes split even more deeply than the dissectums, all the way down to the leaf base. At first the leaves appear to be the ends of bamboo shoots. These cultivars are not widely available outside of Japan, and usually are only available from specialty growers.

Variegated cultivars have leaves in color combinations of gray, green, white, and/or pink.

Bark cultivars are prized for their brightly colored bark on younger branches. As secondary bark forms and thickens on the trunk and older branches, it fades to gray. ‘Sango Kaku’ or coral bark cultivar is the best known. Textural cultivars like Arakawa (rough-bark or cork-bark) are available too.

Dwarf cultivars are self-explanatory. Many of these cultivars originated as cuttings from a witch’s broom, which is a terminal branch that does not end in well–defined leaves. Instead it has a cluster of a dozen or more stunted, twisted twigs, which may or may not have abnormally small leaves. Many things can cause witch’s brooms, but only brooms caused by an inheritable genetic mutation in the growing tip can be propagated as a stable cultivar.

Most cultivars coming out of Japan retain their Japanese names. Unfortunately because English and Japanese don’t share a common alphabet, it can be a matter of opinion how Japanese words should be spelled in English. Also, names sometimes get run together or inappropriately divided. For example, is it ‘Kihachijoh,’ ‘Kihachijo,’ or ‘Ki Hachijo?’ All three names refer to the same cultivar; this is something to watch for when buying.

The best reference book on this group is Japanese Maples: The Complete Guide to Selection and Cultivation (4th ed., 2009) by J. D. Vertrees and Peter Gregory, from Timber Press. It has over 600 easy-to-follow descriptions for identifying cultivars and 700 color photographs.

Online, a good source for information and trees both is Pacific Coast Maples (http://www.pacificcoastmaples.com/). While they definitely fall into the category of nurseries who lump too many species in this group, they have a great selection of varieties.