Smoke tree bonsai

Placement

Cotinus coggygria is a deciduous shrub or small tree native to a broad region stretching from the Mediterranean to central China. Its oval leaves, up to 3 in long, smell like radish when crushed and turn yellow, orange and red in fall. Its tiny flowers go unnoticed; it is the hairy flower stalks that form, in summer, the buff then gray plumes that look like smoke. ‘Royal Purple’ has dark purple foliage and pink plumes, and ‘Golden Spirit’ bright yellow foliage, though the latter lacks vigor in a container.

Grow it outdoors year-round in full sun, with at least six hours of direct sun a day. It tolerates part shade, but in shade the purple forms can revert to green. In the ground, the smoke tree withstands cold of about 5–14 °F, though branches can die back in harsh winters. As a bonsai, the root ball held in a shallow pot is far more exposed than a root system in the ground: protect it once the temperature drops below 23 °F.

Like any temperate tree, it needs the cold of fall to go dormant: wait for the first frosts before sheltering it, and never bring it into a heated room. A cold frame or unheated greenhouse evens out temperature swings and blocks drying winds; vent it on sunny days so the tree stays dormant. You can also bury the pot in the garden, with soil covering the roots, or wrap it in insulation. In spring, opened buds are vulnerable to late frosts.

Soil mix

The smoke tree accepts almost any soil, acidic, neutral or alkaline, as long as it drains well. Its shallow, fibrous roots demand sharp drainage: soggy soil is fatal. In the ground it actually prefers rather lean soil, which suits container culture, where the aim is to restrain vigor. A standard bonsai mix works, for example akadama, pumice and lava rock.

Watering

Water by observation, never on fixed days. When the soil feels barely dry about 0 in down, water thoroughly until water runs from the drainage holes. Rainwater is best; tap water will do otherwise. In the ground the smoke tree handles drought well once established; in a pot it depends entirely on you, especially in hot, windy weather. Some bonsai growers nonetheless avoid pampering it with water and shade: slightly lean culture gives noticeably smaller leaves and plumes. Do not confuse lean with parched. In winter the dormant tree drinks little: water only when the soil dries.

Feed every two weeks from spring until late summer, following the label rate. A higher-nitrogen fertilizer suits spring, and a low-nitrogen one the end of the season. Do not feed during dormancy.

Pruning

The smoke tree regrows well after severe pruning, which makes it possible to rescue a leggy specimen. But beware: hard cutting back triggers vigorous shoots with larger-than-normal leaves, the opposite of what a refined bonsai needs. Keep big cuts for the development stage, in late winter or early spring, before bud break. Use a concave cutter and seal large wounds with cut paste. A healthy tree can lose about a third of its foliage; do not combine structural pruning and repotting in the same year.

During the growing season, simply cut back shoots that extend beyond the outline, several times a season. Light pruning keeps the flower plumes; hard pruning sacrifices them for that year. If part of the crown dies, a new tree can be built from a living lower branch. The resinous sap can cause contact dermatitis, especially in people sensitive to poison ivy: wear gloves.

Wiring

Wire preferably in late winter, when the tree is leafless and the structure is easy to read. Choose wire about one third the thickness of the branch, wrapped at roughly 45° from base to tip. During the growing season branches thicken quickly and wire can scar the bark within one to four months: check it often and cut it at each turn to remove it. Wrap thick branches in damp raffia and use guy wires for those too stiff to bend. Do not wire a weak or freshly repotted tree.

Repotting

Repot every one or two years, in spring, as the buds begin to extend. The fibrous roots handle transplanting well. Remove the old soil with a chopstick, shorten overlong roots without taking off more than about 30% of the root mass, set the tree in fresh mix and water thoroughly. Then keep it out of wind and strong sun until it re-establishes.

The smoke tree suits the informal upright style, with one or several trunks, in medium to extra-large sizes. An old, twisting trunk can also become a literati (bunjin), a slender trunk with a few branches at the top and some deadwood, shown in a shallow round pot. To propagate it, sow seed outdoors in fall, layer in spring or take softwood cuttings in summer; semi-hardwood cuttings also work.

Pests rarely trouble it. The main threat is verticillium wilt. Powdery mildew mostly affects purple forms in dry weather; rust and leaf spot appear now and then, honey fungus rarely. A symptom can have several causes: check your watering too. The smoke tree plant profile describes the shrub grown in the ground.