For Love of Maples

Credit: Ian Adams

These days there is a tendency among garden writers to champion smaller trees. I admit I’ve advocated on behalf of magnolias, three-needle pines and rowans for residential gardens, but had to qualify the recommendations because, given time, some of those trees aren’t really that small. Perhaps because large – really large – trees such as Douglas fir and western red cedar are still a common sight around Vancouver, I think of small trees as anything less than 25 metres tall. To absolve myself of past arboreal advice, I hereby pledge to recommend only the truly smallish.

For me, the best trees for residential gardens must have good architecture. Whether fully flushed out or leafless, a tree that is viewed often and at close range should be appealing and restful; the eye soon tires of novelty and extravagance. I’m unabashedly partial to deciduous trees – the seasons are so eloquently expressed by them.

High on my list of suitable trees is the Japanese maple (Acer palmatum), a species that populates mountainsides throughout Japan, usually growing no taller than eight or nine metres and forming a broad, rounded crown in the open. Under shadier conditions, trees are more sparsely branched and naturally drawn up. Most Japanese maples exhibit a similar form in gardens, the exception being the cut-leaf types, which are generally even smaller – essentially mounded shrubs.

Because it naturally makes a small tree, the scale of a Japanese maple is appropriate for smaller gardens. It is easily sited and can be allowed to grow to its potential without any special training or pruning. In fact, pruning (other than the removal of dead wood) is nearly always the fastest way to ruin the elegance of a Japanese maple. Unregulated, its stems are slender and barely arching – never stiff and formal – and its narrow branching delicate. The leaves of Japanese maples are relatively small and always pleasing to look at, even on the ground, where they should be left to decompose atop the roots.

What really makes Acer palmatum popular is the enormous range of foliage colours and types available (more than 200 cultivars). One of my favourites is ‘Osakazuki,’ a strong-growing, full-size cultivar noted for its bold, fawn-green leaves that turn spectacular flame colours in the fall. For truly small spaces, dwarf cultivars can be used, but I prefer containers instead. Restricting root growth by planting in containers is an effective way to keep plants relatively small, and luckily A. palmatum is supremely adapted to this treatment. Two important considerations: It is vital to keep planters and containers well watered, as evaporation may be significantly higher than from open ground; also, plants in containers should be lifted and the soil replenished every few years to ensure adequate nutrition and root health.

A wedding gift ‘Osakazuki’ grew happily in a 90-litre pot for many years; that is, until we got tired of carting it around from postage-stamp patios to second-floor apartment balconies. It now resides at a friend’s house, planted in the front yard, where it has gained considerable weight and looks smashing. (See below for a list of good A. palmatum cultivars and short descriptions.)

In the ground, most Japanese maples can survive winter temperatures of -25°C, but are poorly adapted to hot or dry conditions. Because containers don’t enjoy the insulating qualities of the open ground, potted plants are all the more sensitive to environmental fluctuations. With their roots more or less exposed to ambient temperatures, as they are in pots or small planters, plants may not survive much below -12°C. Mulching for weed suppression or to provide insulation for roots, either in containers or on the ground, must be done carefully. Maples are shallow-rooted and require an aerated root zone; if mulching is overzealous (much over about eight centimetres deep), roots can easily drown.

Generally tough and long-lived in gardens, maples occasionally suffer from disease. Verticillium wilt causes sudden dieback of whole branches in warm weather and is most common where plants are under stress – where drainage is poor, for example. Under crowded conditions where branches regularly lash each other when it rains, or when pruning is carried out in wet weather, bacterial blight may be a problem, although this disease typically disfigures only the ends of branches.

Many maples are grafted, as cultivars don’t always make good roots on their own, and seedlings may not exactly resemble the parent tree. Grafting is a specialized nursery technique, so adds considerably to price. The cheaper palmatums are usually “understock,” that is, seed-grown plants upon which a desired cultivar is grafted, and where the graft has failed. These plants may or may not be exemplary, and the worst usually find their way onto commercial landscape sites, instead of the burning pile where they belong.

Most maple species require considerable moisture, particularly when in leaf, but this must be coupled with good drainage, as plants are disease-prone in waterlogged conditions. While Japanese maple and its relatives dislike stagnant air, they are woodland species, by and large, and are also unhappy being battered by strong winds.

Acer palmatum belongs to a group of maples that includes species centred primarily in Asia and especially southern China, but that also includes both our native vine maple, A. circinatum, and the downy Japanese maple, A. japonicum, which also goes by the name full moon maple. These closely related species are more robust than A. palmatum and somewhat coarser, with larger leaves, buds and shoots. Both are also better adapted to shade and more tolerant of summer drought. Seed-grown vine maples should be sought out, rather than collected, as wild collections tend to be fragile and not adaptable. Besides, it’s nearly impossible to know whether plants have been removed from the forest responsibly. Of full moon maples, A. japonicum ‘Vitifolium’ has exceptionally broad leaves, and ‘Aconitifolium’ is a superb cut-leaf cultivar. All have spectacular autumn colour and are hardy to about -20°C. (Acer japonicum ‘Vitifolium’ was one of the Great Plant Picks of the Pacific Northwest selections for 2001.)

Another maple of note from Japan is A. shirasawanum ‘Aureum,’ the so-called golden full moon maple. This is a stunning diminutive tree with small chartreuse leaves, stiffly pleated and nine- to eleven-lobed. The tips are often red-stained. For many years this much sought-after cultivar was attributed to A. japonicum, and is sometimes still sold under that name, but it is a shrubbier tree (to only five metres) and much finer in texture than full moon maple. It can be treated as for A. palmatum, but should be protected from the full force of the sun, as its delicate foliage is easily scorched. A number of attractive green-leaved versions of the species are worth looking for, including ‘Palmatifolium’ and var. tenuifolium, both of which are sometimes offered by specialists.

Somewhat larger, but no less attractive, is butterfly maple, Acer tschonoskii (pronounced chon-osk-ee-eye), a hardy species native to Japan, Korea and China (Manchuria) that has only recently become available commercially. This species grows to perhaps seven metres in cultivation and exhibits neat, dark-green, serrated leaves that turn fiery orange-red in autumn. Subspecies koreanum is the type in commercial production, and displays brilliant red twigs that are a significant winter feature. Some publications list butterfly maple as hardy to at least -30°C. Although of similar stature to Japanese maple, A. tschonoskii belongs to a different maple group – the snake-barks. While this group is known for elegant striped stems (e.g., Acer davidii), stripes are only visible on the youngest stems of butterfly maple.

A small snake-bark maple that does show white-striped stems is the hawthorn maple, A. crataegifolium. This is a lovely maple, again from Japan, ultimately smaller than A. palmatum, with unlobed, birch-like leaves and purplish young stems. ‘Veitchii’, a novel form with white-splashed leaves is sometimes available, but A. crataegifolium itself is not commonly seen.

Every year, more small maples are being brought into commerce, many through plant exploration in Asia. In the Botanical Garden at the University of British Columbia, there are a number of Pacific Rim maples showing promise as good garden plants. One is a form of A. crataegifolium that is both compact and elegant, showing blue-green leaves that turn deep scarlet and orange in the fall. It is one of my favourites, and I think deserves wider cultivation.

Small gardens are intimate spaces that cry out for elegant, low-maintenance plants. Early on, many larger tree species will suit a small space, and, all too often, well-meaning gardeners imagine they can control tree size. The results are generally disastrous: trees trussed up and limbs lopped off, neighbours offended and property values reduced. Instead of relaxation and contemplation in the garden, there is chaos, guilt and unease. But perhaps I’m projecting here. Pruning – even when expertly done – is seldom an effective solution, and that moment of enlightenment usually comes too late.

The key is to start with plants that are naturally programmed to grow to an appropriate size, and to give them encouraging conditions. Sturdy and elegant, the smaller maples from the Pacific Rim are among the best-adapted trees for our gardens. Not that Magnolia dawsoniana isn’t also staggeringly good-looking, but for smaller gardens at least, the maples won’t come back to haunt me.

The following plants are hardy up to the zone number indicated:
• Acer circinatum – zone 5
• A. crataegifolium – zone 6
• A. japonicum – zone 5
• A. palmatum – zone 5
• A. shirasawanum – zone 6
• A. shirasawanum var. tenuifolium – zone 6
• A. tschonoskii ssp. koreanum – zone 5

Douglas Justice is the UBC Botanical Garden’s Associate Director and Curator of Collections. He is a founding member and vice-president of the North American Branch of the Maple Society and is actively involved with a number of other local, national and international botanical and horticulture organizations.