Japanese Maples 101
(Editor's Note: This article was originally published on January 6, 2008.)
What is a Japanese Maple?
To most people, Japanese maples are the various forms and selections of Acer palmatum, a species endemic to Japan. In a broader sense, some nurseries will classify Japanese maples as including those species that are closely related and/or look similar to A. palmatum. These include A. japonicum, A. sieboldianum, A. pseudosieboldianum , A circinatum and A. shirasawanum. Technically, this is a false classification as A. pseudosieboldianum is native to Korea, not Japan. Meanwhile, A. circinatum, the vine maple, is native to the Pacific Northwest and is simply an American ‘Japanese maple’ wannabe! To plant taxonomists, Japanese maples include all those species native to Japan, which surprisingly includes 23 species! For the purpose of this article we will classify Japanese maples as selections and forms of A. palmatum.
Few maples exhibit as much variation in plant form, leaf form, size or colour as Japanese maples. The Japanese have been breeding and selecting this maple for well over 300 years. More recently, new cultivars are being released from Europe (in particular The Netherlands), United States, Australia and New Zealand. Today there are literally hundreds of named selections. Leaves range from fine and lacy to quite coarse; foliage may range from yellow, orange, red, burgundy, green or variegated; some have spectacular spring colour; most have excellent fall colour; plants may be nearly prostrate, mounding, umbrella-shaped to fastigiate. Heights can vary from under 1 m (3 feet) to over 12 m (40 feet). Thus, depending on your space and requirements, there will be at least one suitable Japanese maple for your garden. The only truly limiting factor is hardiness zone. The hardiest forms generally cannot survive colder than USDA zone 5. Some selections are even damaged during winters in zone 6.
Japanese maples are surprisingly adaptable. They are successfully cultivated from the wet Pacific Northwest to dry and sunny California. You will find them throughout the Midwest, southern Ontario and Quebec and along the Atlantic seaboard from Atlantic Canada south to the SE USA. They thrive throughout much of Europe, southern Australia and New Zealand and of course, eastern Asia.
The best light conditions would be morning sun but shade during the hottest afternoon hours. Many green-leaved selections can tolerate full sun but variegated and golden-leaved forms will often burn if exposed to hot afternoon sun. The red-leaved types certainly need at least morning sun to properly develop their characteristic colour.
They need reasonably moist soil; too dry and the leaf tips will burn, especially on the lacy-leaved types. The old adage "one inch of water once a week" works well. To help maintain adequate soil mositure, dig in plenty of organic material such as peat, leaf mould and compost. Japanese maples prefer acidic soil lending themselves as companion plants for Rhododendrons and other ericaceous plants. On the other hand, they can suffer from chlorosis if the soil is too alkaline. Use soil acidifying fertilizers in this latter situation.
Examples of Japanese maples interplanted with other acid-loving plants: left to right are 'Omure yama', 'Red Dragon', 'Sumini gashi' and 'Bloodgood'
In the wild, Japanese maples grow as understory trees thus are rarely exposed to strong winds. In the garden, they resent breezy locations. This is especially so in drier climates where dessication can cause leaf-tip burn in summer and in colder regions where dry, cold northwest winds in winter can cause stem die-back.
Japanese maples have relatively fine, fibrous roots which are easily damaged. Unless very young (under 4 years old), they should never be moved in a bare-root state. They also do far better when transplanted in spring rather than fall. Most Japanese maples are grafted but unlike roses, they do not need to be planted at or above the graft union. Rather, plant them at the same depth as they are growing in their pots. Mulching after planting will help keep the soil cool and maintain better soil mositure. Newly planted maples can be fertilized with any balanced fertilizer recommended for shrubs. If your soil is reasonably fertile, established plants will not require extra fertilizer.
Japanese maples are excellent subjects for container planting. The more dwarf types can tolerate very restricted root zones. This will slow their growth rates which, when growing plants in containers, is a good thing! Pots can be left aboveground in areas where the winter temperatures do not fall below -10 EC (14 EF). In colder areas, sink the pots in the ground over the winter months or use insulated containers.
Examples of container-grown Japanese maples: left to right are 'Koto-no-Ito', 'Ukigumo' and 'Red Pygmy'
Leaf shape classification
Details on the various types of Japanese maple selections that exist are topics for future articles. However, at this point it may be prudent to at least discuss their main groups. Cultivars are placed into one of six groups, based primarily on the leaf shape or in the case of the dwarf group, the ultimate height of the plant. These groups include:
Amoenum Group: these selections have the coarsest or fullest leaves. The leaf lobes are shallowly to moderately divided: up to 2/3 of the way to the leaf base. eg. Osakazuki
Palmatum Group: these have leaves typical of the wild species. Their lobes are moderately to deeply divided: 2/3 to 3/4 of the way to the leaf base. eg. Aka Shigatatsu Sawa
Matsumurae Group: these have deeply divided lobes: more than 3/4 of the way to the leaf base. These are often considered the most elegant group. eg. Sumini gashi
Linearilobum Group: these have the most divided lobes, essentially being divided to the base, but in addition, the lobes are narrow and strap-like. eg. Red Pygmy
Dissectum Group: these have deeply dividied lobes but in addition, the lobes are also deeply dissected. This results in a very lacy effect. eg. Red Dragon
Dwarf Group: these may have variable leaf forms but the mature heights are generally under 2 m (6 feet). eg. Sharp’s Pygmy
The above pictures illustrate the various groups. The examples shown are those cultivars noted above