Chapter 27 Tropical Bonsai

An enormous Bougainvillea in the collection of the Brooklyn Botanical Garden. This tree falls squarely in the Imperial size class for bonsai. Link to original image

Tropical species occupy a distinguished place in bonsai history, in addition to having stylistic variations all their own. How do tropical bonsai differ from traditional Japanese styles?

27.1 Defining “Tropical” Bonsai

The terms tropical and indoor bonsai often are used interchangeably, especially in commercial trade. This is not accurate because many species can be grown indoors that do not have tropical origins. The definition I am using for tropicals is evergreen or semi–evergreen species are native to the tropical or subtropical regions of the world, that are likely to be injured or die when temperatures fall to or below 50^oF. Some species that are grown or styled as tropicals (jades and Mediterranean olive for example) do not strictly fit this definition, but we have to start somewhere.

Figs (Ficus sp.) capture most of the attention but there many other widely used tropical species. They include schefflera (which grows much like a ficus), various bamboos (Phyllostachys, Arundinaria) Fukien tea, fuschia, orange jessamine (Murraya), snow rose (Serissa), bougainvillea, jaboticaba, and various citrus. Other species are used too, but these are seen most often.

27.2 How are Tropical Bonsai Different?

Generally, tropicals are styled more freely, and lean more towards Chinese penjing rather than Japanese bonsai. These differences make sense if you know a bit about their origins. Japanese styles developed in areas where temperate species (pines and maples) were easily obtained and used most often. Consequently, Japanese rules of bonsai work best on temperate species. Conversely, several of the tropical species were first cultivated in Southern China, where the Lingnan School of penjing dominated. In Lingnan style, foliage is not arranged in clusters. Trees often feature an erect majestic posture and exuberant growth. Trunks have an unrestrained natural flow, with a lean and hard look like twisted iron. Shaping is not achieved by wiring, but through repeated pruning and extension growth; in fact, the Lingnan School originated our modern clip and grow techniques. Lingnan artists in the Guangdong province routinely used orange jessamine, Fukien tea, and banyan fig as subjects, because they adapted well to the school’s pruning–based styling techniques. The thin, brittle, easily scarred bark of these species was not a stylistic obstacle, because wire was not used. It also turns out that Lingnan penjing’s general approach to styling tends to work quite well for thin–barked tropical natives from other continents.

Styling tropical species less formally also makes aesthetic sense. Remember that the goal of a specimen is to evoke a feeling or memory of nature. Close your eyes for a moment and imagine a tropical forest. Think about the trees there. I imagine thick twisted masses of roots anchoring enormous trunks crowned with lush dense canopies that hide the forest floor. A few stones from a long–abandoned temple are scattered on the ground, nearly hidden under clasping roots. Lianas and vines hang in the shade, while exotic orchids, bromeliads, and flowers peek unexpectedly from every crevice. The entire scene feels wild and disarrayed, the antithesis of order. Now think about the minimalist lines of classical Japanese styles; none capture the feelings my mental image evoked by. We need to change our styling expectations somewhat if we are going to evoke that feeling on a small scale.

27.3 What are the Best Styles for Tropicals?

No single style works with all tropicals, and some suit them better than others. Remember, the style should work with the natural tendencies of that particular species. Also some styles better suggest a tropical locale by their very nature than others. Styles to avoid include formal upright, literati, and broom style, which all work against the uncontrived nature of tropicals. Root over rock, windswept, and driftwood styles are not appropriate because these conditions rarely exist in tropical areas.

27.3.1 Banyan

A Chinese Banyan (Ficus microcarpa ‘Retusa’) bonsai on display at the National Bonsai & Penjing Museum. According to the tree’s display placard, it has been in training since 1971. Link to original image.

This is the only style that is unique to tropicals. Multiple aerial roots drop from branches to form prop trunks for overlying lateral branches. In larger specimens the roots may enlarge to the point of fusing to each other and the main trunk, forming a single massive undulating base.

Unfortunately, many banyans are groomed like a Japanese pine. Main branches are bent too far down, are placed according to Japanese rules, and have tight pine–style foliage pads created by closely trimming the twig tips. There are several disadvantages to grooming banyans to resemble pines. First, aggressively growing and grooming the dense foliage pads demands a lot of the stylist’s time and the tree’s energy. Second, abnormally tight foliage is very prone to insects and disease. Third, the tree does not look as it would in nature. Wild banyans have looser, more open crowns, and main branches do not become horizontal unless they are supported by a string of aerial buttress roots.

Ficus and schefflera form banyans naturally, but they are hard to create using other species. Banyans require formation of numerous aerial roots, either by packing the trunk and branches in moist moss, or by maintaining continuously high humidity, then selecting only certain roots to grow on. They also can be created by grafting multiple cuttings together, or smaller trees onto larger base trees.

27.3.2 Exposed Roots, Strangling Roots, Root Over Rock

Ficus natalensis grown as a root over rock style. This specimen is in the Italian National Bonsai Museum collection. Link to original image.

In areas of high rainfall, soil often washes away from a tree’s roots, leaving it standing on stilts. At other times, roots from an epiphytic fig will descend on all sides of a larger tree, eventually strangling and killing it. The original supporting host rots, leaving a twisted hollow mass of roots and branches standing in the air. Root over rock styles form in the same way, but the support does not decay. All three styles are created the same way. Roots of a tropical tree are draped over the rock or wood support and tied or wired in place. The assembly is potted up in a deep container. After 1–2 years soil is slowly washed out (over several months) to expose the clasping roots. When roots have hardened, the supporting structure can be removed to leave the stilt roots. Strangling root style can also be created by grafting. This style works particularly well with ficus and schefflera. Not suited to bamboos, jades, or Fukien tea.

27.3.3 Raft, Clump and Multi–trunk Styles

A jade grove planting. The wide basal tapers indicate the jades in this grouping are quite old. Link to original image.

Pithecolobium flexicaule (Texas ebony) grown in a multi-trunk style. Link to original image.

In a tropical zone, no source of nutrients is ever wasted. When larger trees die and topple, they become nurse–logs for the next generation of canopy trees. Live trees that topple in wet soils produce groves of saplings from undamaged upright branches. Works well with most tropical species, but is harder to achieve with olive, bougainvillea, and jades.

27.3.4 Informal Upright

A Fukien tea (Carmona microphylla) styled as an informal upright. In training since 1956. Link to original image.

Given the crowded canopy of most tropical areas, this style is not very common in nature, and it is hard to achieve without wiring. Consider using citrus, olive, Serissa, or other trees with harder bark. Jades work well as informal uprights, but should be shaped by pruning only, as they do not tolerate wiring.

27.3.5 Twisted Trunk (Bankan)

This procumbens juniper is styled as a multi-trunk raft, but the three largest stems have been trained in bankan twisted style. Link to original image.

The trunk of a sapling is bent into a spiral around a stake, tied, and allowed to grow for several years until it sets. After the stake is removed, side buds are selected and allowed to grow out as branches. The image this style evokes is of a sapling that has grown around a support which has subsequently rotted away. A nice blending of artistic and natural that works with many different species. Often used with Fukien tea. Can be done with bamboo, but tends to look contrived and tacky.

As shown in the figure above, bankan can be combined with other styles.

27.3.6 Slanting or Cascade Styles

A silverberry (Eleagnus commutata) grown as a semi-cascade. This particular species is not tropical, but rather native to the northern tier of the US and boreal Canada. Link to original image.

This more Japanese form works in certain situations. Consider it for fuschia, schefflera, or bougainvillea, but not for apically dominant species like ficus or jades.

27.4 Miscellaneous Tips on Training and Care

Tropical species prefer soil that is 60–80% organic matter. The exceptions are jades and olive, which do best in 70% grit, 30% organic. Ironically, over–watering is a common problem; most prefer soil that is routinely moist, but not waterlogged. What they DO like is high humidity. Indoors, mist trees every day or two. Also, place a shallow tray of pebbles beneath the pots, and add water until the pebbles are barely covered. Evaporation will keep humidity high. Another option is a room humidifier, turned on for 1–2 hours a day.

Like maples and pines, tropicals need a rest period each year. It is triggered by cooler night temperatures, and allowing the soil to stay slightly drier. If the rest period is timed so it coincides with moving trees indoors for winter, they will drop fewer leaves and show less signs of stress. In autumn, reduce watering and stop fertilizing. Leave trees outside for 2–3 nights when the temperature is predicted to be in the low 50’s.

In the wild only the outermost leaves are exposed to full sun; the majority of leaves are shaded by their own canopy. So outdoors, most tropicals like part shade in the afternoon. Indoors, tropicals like high light levels. Most will need supplemental light to look their best. Cool white fluorescent lights are the best choice; the pink–colored plant lights are designed to encourage flowering and produce less light overall. Arrange the lights so they are less than 6 inches from the trees, and turn trees weekly.

Tropicals have thin bark, and branches may not set for a year or longer. Rather than wire an entire branch, pull branches into place with tie–downs anchored to the pot. Always put a pad between the wire and the branch; a scrap of landscape fabric or folded nylon panty-hose works well and does not trap excessive moisture. After a branch has been pulled into place, slit the bark on the underside with a clean razor, parallel to the branch. The callused wound will help set the bent branch.

When the aerial roots of a banyan each the soil, they rapidly thicken and begin pumping nutrients to their parent branch. This causes swelling of the branch beyond the root origin. Do not try to prevent this. It cannot be avoided, and is a normal, expected feature of this style.

Many tropical species take extremely well to grafting. Young cuttings can be bridge–grafted over scars where large branches were removed, improving the callus cover. Multiple trunks can be fused together, additional aerial or nebari roots added to existing trees, and new branches created. The process is not complicated, but details are important. Grafting is described in more detail here.