Heavenly bamboo bonsai

Placement

Nandina domestica is an evergreen shrub native to China and Japan, the only species in its genus. Its upright canes rise from the base and carry large compound leaves, divided two or three times into pointed leaflets. Young leaves are red or bronze in spring, blue-green in summer, then purple to red in fall and winter. In late spring, panicles of white flowers appear, followed by red berries that persist all winter.

As a bonsai, give it full sun. It tolerates light shade, but its fall and winter colors are far more vivid in sun, and they are the reason to grow it. Choose a spot sheltered from wind. In the ground, nandina is hardy, but in a pot its roots dislike frost: protect the root ball once the temperature drops below 28 °F, by grouping pots together, wrapping them in an old blanket, or moving them into a greenhouse, onto a porch or to the foot of a sheltered wall. Growth only really resumes above about 45 °F. Nandina can also be grown indoors, right against a sunny window.

All its parts, berries included, contain cyanogenic glycosides, toxic to dogs, cats, horses, birds and people. Keep the bonsai out of reach of pets and children.

Soil mix

A standard bonsai mix suits it. It should hold enough water to keep the root ball moist while letting the excess drain away immediately. A mix of equal parts akadama, pumice and lava rock meets both needs: add a little more akadama if you cannot check on watering often, and a little more lava rock in a humid climate. Nandina accepts acidic, neutral or alkaline soil. Sift the components to remove dust, and avoid garden soil, which compacts and suffocates the roots.

Watering

Keep the mix moist, without excess. Check every day by pushing a finger in about 0 in: if the mix feels slightly dry, water thoroughly, until water runs out of the drainage holes, then repeat a few minutes later to soak the whole root ball. If it is still damp, wait. Never water on fixed days: needs vary with sun, wind, pot size and season, and drop sharply in winter. Never let the root ball dry out completely, and never leave it soggy either. Rainwater is ideal, but tap water is fine.

Fertilizer: feed every two weeks throughout the growing season, following the rates on the package. A tree wintering indoors that keeps growing gets one feeding a month. Do not feed a freshly repotted or weakened tree.

Pruning

Nandina is shaped mainly by playing with the height of its canes. It takes hard cutting back very well: a cane cut low sprouts again and leafs out in about six weeks. Make major cutbacks before new growth starts in spring, just above a node or bud, at different heights from one cane to the next: new shoots emerge below the cut and the foliage spreads over several levels. Do not always cut at the same point, because repeated cuts in one spot form an unsightly bulge on the trunk.

With age, the species loses its lower leaves and its canes become bare. Then cut back a few canes, or remove one or two of the oldest, thickest canes at the base: light reaches further in and new canes arise from the underground stems. Keep in mind that next year’s flowers and berries form on new growth. For thick cuts, use a concave cutter and cover the wound with cut paste.

Wiring

Nandina’s outline depends more on pruning than on wire, but light wiring lets you steer a young cane or give movement to a trunk. Use aluminum wire about one third the thickness of the cane, wound at about 45°, and bend gradually, pressing with your thumbs on the inside of the curve. During growth the canes thicken quickly: check the wire regularly and remove it before it bites in, cutting it at every turn rather than unwinding it.

The best-suited styles are the clump, which mirrors the shrub’s natural habit, the twin trunk and the single trunk. Dwarf cultivars such as ‘Nana’, whose leaves have only three to nine leaflets, or ‘Firepower’, with yellow and red foliage, make more compact bonsai and bear few or no berries.

Repotting

Repot every other year, in spring, in a standard bonsai mix. Tease out roots circling the root ball with a chopstick and do not cut back more than about one third of the roots. Take the opportunity to separate suckers that have formed around the base. After repotting, keep the tree out of wind and harsh sun, and wait until it recovers before feeding or pruning hard.

Propagation: take cuttings in summer, divide suckers when repotting, or sow the seeds, two per berry. Nandina has few enemies. Watch mainly for cottony cushion scale. Some dwarf cultivars, including ‘Firepower’ and ‘Harbour Dwarf’, carry a nonlethal virus that makes leaves cup, crinkle and twist, and slows growth.