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RHS Journals

The Garden
May 2006

Demanding your undivided attention

Fern- or dissected-leaf Japanese maples may be small and delicate-looking, but are arrestingly beautiful, versatile and much tougher than they appear, argues Jon Ardle

Images: Tim Sandall

Red numbers in the text refer to the following cultivars of Acer palmatum var. dissectum:
1 ‘Seiryu’
2 ‘Chantilly Lace’
3 ‘Spring Delight’
4 ‘Crimson Queen’
5 ‘Green Mist’
6 ‘Red Filigree Lace’
7 ‘Ellen’
8 ‘Berrima Bridge’
9 ‘Shojo-shidare’
10 ‘Pink Ballerina’
11 ‘Barrie Bergman’
12 ‘Orangeola’
13 ‘Bronzewing’
14 ‘Waterfall’
15 ‘Emerald Lace’
16 ‘Lionheart’
17 ‘Bewley’s Red’
18 ‘Red Feather’
19 ‘Filigree’
20 ‘Felice’

Scale: ‘Lionheart’ is about 10cm across

Acer palmatum var. dissectum cultivars

Red- and purple-leaved selections
‘Barrie Bergman’ 11 Recently introduced. Leaves emerge bright red, ageing to rusty bronze with fine orange-red summer colour.
‘Crimson Queen’ 4 Popular cultivar with deeply divided leaves that retain their deep red spring colour well into summer, when they turn orange-green. Vigorous when young, forming a mound up to 4m. Grows well in pots.
‘Felice’ 20 Unusual cultivar, the leaves are crowded into bunches along the shoots. Two leaf types - one finely divided the other more entire. Both emerge yellow-green with pink edging, but broader ones turn purple-red and more divided ones bronze-green, giving a two-tone effect. Not to everyone’s taste. Height and spread to 3m.
‘Lionheart’ 16 Similarly upright characteristics and vigour to ‘Seiryu’ (to 2.5m in 10 years). Leaf colour like ‘Crimson Queen’ but more bronze in summer. Unusual deep red bark on young shoots. ‘Bronzewing’ 13 has similar colouring to ‘Lionheart’ but is more mound-forming, whereas ‘Bewley’s Red’ 17 has the habit of ‘Lionheart’ but more red-purple leaves.
‘Orangeola’ 12 Emerging foliage bright orange-red, ageing to orange-flushed green. Second flush of orange leaves in midsummer; fiery red-orange in autumn. Taller than wide, to 3m.
‘Pink Ballerina’ 10 Unique variegation: most leaves purple but a scattering of pale pink ones, and some both pink and purple. Recent and rare.
‘Red Filigree Lace’ 6 Exceptionally finely-cut leaves, deep maroon-purple throughout the growing season, turning fiery red in autumn. Slow growing but stands sun well. ‘Red Feather’ 18 is similar but more vigorous.
‘Shojo-shidare’ 9 (and right) Dark maroon foliage retains green stripes down the centre of the leaf lobes. Young branches and twigs are also deep maroon. This beautiful cultivar forms a cascading dome to 3m. It can withstand some sun.

Green-leaved selections
‘Ellen’ 7 Vigorous, wide-spreading cultivar, to only 1m but spreading as wide as 2.5m in 10 years. Leaves, which emerge yellow-green, and turn apple-green in summer, are large and sublobes deeply toothed.
‘Emerald Lace’ 15 Elegant, exceptionally finely divided foliage, emerging yellow-green, darker green in summer. Bright burgundy-red autumn colour. Semi-upright, irregular spreading habit. ‘Chantilly Lace’ 2 and ‘Berrima Bridge’ 8 are newer cultivars of similar vigour and colour but seem more typically dome-shaped.
‘Filigree’ 19 Variegated cultivar with subtle dotting and flecking of pale gold or cream. Twigs and branches striped white like a snake-bark maple. Wider than high at maturity, finely divided leaves. Adapts well to pots, but needs full shade to avoid scorching.
‘Green Mist’ 5 Large-leaved (to 9.5cm long and 12cm wide) cultivar with deeply-divided, feathery sublobes. Vigorous for a dissectum, forming a mound to 3m in 10 years.
‘Seiryu’ 1 Vigorous, distinctive upright-growing cultivar reaching 4-6m tall by 3-4m wide at maturity. Leaves smaller and less divided than many dissectums, much tougher than they look. Spectacular gold and crimson autumn colour.
‘Spring Delight’ 3 Leaves emerge red edged, lasting through spring into early summer. Copes with sun less well than some green cultivars.
‘Waterfall’ 14 Large-leaved, stands full sun well. Sturdy branches, and strongly cascading. Relatively slow-growing; the original tree was 3m high and 4m wide after 50 years.

Habit and colouration

Cultivars typically form a mushroom of foliage around 2m high and wide in 10 or 15 years and may reach 4m after many years. The leaves of most emerge brightly coloured in spring, fade to duller green or red/purple in summer and have spectacular autumn shades. Colour also varies with site: right are two leaves of purple-leaved ‘Watnong’, the left grown in sun, the right in shade.

Marvellous maples

Japanese maples are among the finest of foliage plants for small gardens, with a bewildering array of leaf shape, size and colouring; reliable, often spectacular autumn colour; and a delicacy of twiggy form drawing attention even in winter.

Acer palmatum (Japanese mountain maple) is known as momiji in its native land, where it is found in woodland throughout most of the islands of the archipelago, at altitudes up to 1,300m. In cultivation since at least the 1600s, like several Japanese plant species, its varied natural habitat seems to have fostered a natural tendency to genetic variation: Chamaecyparis lawsoniana (Lawson’s cypress) is perhaps the only other woody species to mutate into markedly different forms so readily. Momiji have long been collected in Japan; as early as 1882 there were 202 named cultivars. Today, there are several hundred.

Among the most prized, then as now, have been plants with particularly finely divided leaves. Usually of much smaller stature than the 10m that Acer palmatum is capable of reaching in the wild, such individuals have long been popular in a land where space has always been at a premium. As they occur in the wild, cut- or lace-leaf forms are known as A. palmatum var. dissectum, and a better tree for the small garden is difficult to find.

Dissectum characteristics

What sets dissectums apart is their small stature (most cultivars mature at around 2m, and even the most vigorous rarely top 4m), weeping habit and dense, twiggy growth, producing plants with a domed, mushroom habit often wider than tall. It is the intricacies of their leaves that has given them their name: almost impossibly delicate, the seven or nine lobes of the usual Japanese-maple leaf are, in dissectums, divided right down to the midrib. In most cultivars each lobe, only a few millimetres wide, is subdivided further by teeth along its edge. As a result there is usually more space than tissue to a dissectum leaf - scarcely surprising that they are slower growing than maples with more entire leaves.

Growth is slow but not negligible; some dissectum cultivars, particularly when young, can put on 30-45cm of shoot growth a year. Named cultivars are grafted onto rootstocks of normal Acer palmatum seedlings because they are almost impossible to propagate from cuttings. Leaf colour varies from shades of green to orange, red, bronze and dark purple, and there are several variegated cultivars.

Growing dissectums

Contrary to popular belief, Japanese maples do not need acidic soils to grow well. They will succeed on most garden soils except the extremely alkaline, waterlogged or really dry. These are forest understorey plants, adapted to semi-shade, shallow but humus-rich soils, the shelter of a higher tree canopy and relatively constant moisture levels. Their biggest enemies are winter waterlogging (their fine, fibrous root systems are prone to rots) and late spring frosts. Japanese maples leaf out relatively early in the season and while frost damage to emerging leaves is rarely fatal, it can knock them back severely. Slow-growing dissectums can take time to recover from frost damage, although their small stature does at least make fleecing an entire plant during spring cold snaps possible.

If you garden on chalky, free-draining or heavy clay soils, Japanese maples in general, and dissectums in particular, adapt well to containers, and plants in pots can be brought under cover when frost threatens. A loam-based, John Innes-type compost with up to 20 percent added grit or perlite to improve drainage is ideal. Never allow the compost to dry out entirely; scorched leaves, their edges brown, dry and papery, will result. Pot-on gradually, every two or three years.

Scorch can occur with plants grown in the ground, too. Strong sunlight or winds can strip moisture from the leaves faster than the roots can replace it, and given the amount of edge to the leaves of dissectums they are particularly vulnerable. As with frost damage, scorch looks unsightly but plants usually recover and affected leaves can be cut off.

Most purple-leaved dissectums colour best in good light, but given the potential for scorch, this does not mean full sun, which is risky for all cultivars. Variegated cultivars are most prone to scorch in sun, so shade is crucial. Like all Japanese maples, dissectums have shallow root systems, so an organic surface mulch both conserves moisture and feeds the plant (more on Japanese maple cultivation). Be sparing of feritiliser - too much tends to produce lots of soft growth that is vulnerable to sun, wind, drought and early autumn frosts.

None of this should suggest dissectums are difficult to grow, however, for they are not - give them moisture at the right time and site them correctly in sheltered locations with some shade and they will repay handsomely with cascades of peaceful foliage that often changes colour with the seasons.

Placing dissectum maples

Dissectums should not be given vigorous neighbours, whether herbaceous or woody. Their delicate appearance suggests equally fine- or small-leaved companions. Selections with different leaf colours contrast effectively with each other, and they also look beautiful placed in front of more vigorous, upright Japanese maples such as Acer palmatum ‘Osakazuki’, A. palmatum ‘Sango-kaku’ (coral-bark maple) or exquisite A. shirasawanum ‘Aureum’ (golden full moon maple). Keep them far enough in front of the rootball of the larger plant, however, to avoid competition. Dissectums also mix well with other favourites of the Japanese garden such as evergreen azaleas, camellias and pieris, and even feathery grasses such as Stipa tennuissima or Deschampsia cespitosa.

On a smaller scale, including pot groupings, team dissectums with other foliage plants using complementary or contrasting colours and shapes. Red and purple dissectums go well with blue hostas, Ophiopogon planiscapus (black-leaved ‘Nigrescens’ or plain green), or the strappy, arching leaves of broad-leaved sedges such as Carex siderosticha or C. glauca, and plain or variegated Luzula (woodrush). The finely-divided leaves of smaller ferns such as Athyrium niponicum var. pictum (painted lady fern), Cyrtomium falcatum (Japanese holly fern) or cultivars of Polystichum setiferum Plumosum or Divisilobum Groups echo the dissectum leaf beautifully. Mounded green dissectums could be matched in shape with refined grass Hakonechloa macra ‘Aureola’ or the intense blue cushions of Festuca glauca ‘Elija Blue’, or contrasted with the bold, blood-red uprights of Imperata cylindrica ‘Rubra’ (blood grass) or the green, black-banded stems of Equisetum hyemale (scouring rush).

It is not just in Japanese or foliage gardens that dissectums can shine, however: their low mounds of foliage are equally at home in modern spaces, or potted on a contemporary deck. Green-leaved selections can blend demurely into the background, while purples and reds used as specimen plants demand attention. All are irresistibly tactile. Dark blue or reddish glazed pots complement rusty red and purple cultivars well, and a pair of potted dissectums would frame a doorway beautifully.

All colours look wonderful in splendid isolation close to water, and fare well at the edges of streams or ponds (if the soil is free-draining enough), where their foliage can luxuriate in the high humidity.

The less-vigorous dissectum cultivars are small and slow growing enough to be used in large rock gardens, as they are at RHS Garden Wisley. The delicate tracery of their bare, twiggy outline makes an excellent foil for early spring bulbs such as snowdrops, Chionodoxa, Leucojum and dwarf daffodils, while their mounded form makes an elegant backdrop to cushion-forming alpines such as saxifrages and sedums.

Pruning

Although dissectum maples do not need pruning, they form such a congested dome that the curved or downright serpentine beauty of their trunk and branches is hidden. Selective pruning to open up this internal structure to sight not only makes them look better aesthetically, it allows light and air into the canopy, reducing potential problems with fungal diseases and pests lurking inside. Dissectums are best pruned in summer.
In Japan, dissectums are often grown in pots raised well off the ground to bring their fine structure closer to eye level, and are prized as much for the filigree beauty of their winter branches as for their foliage.
Fern-leaf or dissectum variety Japanese maples are among the most beautiful of all foliage plants. They may not flower conspicuously but certainly mark the changes of season: the shy emergence of their vividly-coloured spring leaves declares the true end of winter, subtly shifting into a more demure but luxuriant midsummer dress of lush, feathery, cascading foliage before taking on dazzling autumnal robes. Even when the last leaves have fallen, the elegant scaffolding that is revealed continues to have impact.

These are plants that command attention in all seasons, with a presence far larger than their diminutive size suggests.

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Jon Ardle is Features Editor for The Garden

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