Smoke tree

Identification and origin

Cotinus coggygria belongs to the family Anacardiaceae, the cashew and poison ivy family; it was long classed among the sumacs under the name Rhus cotinus. Its native range is vast: from southern Europe, from France to the Balkans and Crete, it extends into central Europe and Ukraine, the Caucasus, Turkey, Iran, and Central Asia, as far as the Himalayas and central China.

It is an upright, loosely spreading deciduous shrub with several trunks, and it can also be grown as a small tree. It typically reaches 10–15 ft tall and as wide, sometimes more with age, and grows quickly. Its alternate, simple, oval to rounded leaves with smooth margins are 2–4 in long; blue-green in summer, they have prominent veins and smell like radish when crushed. In fall they turn yellow, orange, or reddish purple, with wide variation from one plant to the next. The bark is smooth and light gray on young stems, becoming rough and flaky with age.

Its flowers are not what makes it famous: tiny and yellowish, they appear in late spring in branched clusters 6–8 in long. After bloom, the stalks of the sterile flowers lengthen and become covered in long, fine hairs that turn smoky pink to purplish pink: the plant then looks wrapped in smoke, or crowned with a wig, all summer long. The fruit is a small kidney-shaped drupe.

The genus name comes from the Greek kotinos, “olive,” and the epithet coggygria is the Greek name for the smoke tree. Common cultivars include ‘Royal Purple’, with nearly black purple foliage and reddish-purple smoke; ‘Velvet Cloak’, dark purple; ‘Nordine’, one of the hardiest purple forms; ‘Ancot’ (Golden Spirit), with golden-yellow foliage and a more compact habit; and ‘Daydream’, green-leaved and heavy blooming.

Light

Plant it in full sun, with at least six hours of direct sun a day. That is where it blooms best and develops a well-balanced crown. In part shade it grows lopsided, leaning toward the light, and purple cultivars tend to revert to green.

Watering

Once well established, it is remarkably drought tolerant and usually gets by on rainfall. It even thrives in dry, rocky soil that is never irrigated. Until it is well rooted, though, it does not yet have that resilience: keep an eye on the soil during dry spells after planting.

What it fears most is excess water: its shallow, fibrous roots do poorly in soil that stays waterlogged.

Potting mix

It adapts to most soils, whether clay, loam, sand, chalk, or rocky ground, and to acidic or alkaline pH, on one condition: perfect drainage. It even prefers rather lean soils; in very rich ground it grows fast but is often shorter-lived.

It tolerates clay as long as water does not stand: just avoid heavy, wet, poorly drained soils, which suffocate its shallow roots.

Temperature

It is a hardy shrub, grown in USDA zones 4a to 8b. In most situations it withstands winters of 5–14 °F. In very cold winters, though, the branch tips can freeze and die back: remove them in spring.

Repotting

Its fibrous root system makes it easy to transplant. Plant it without enriching the soil, since overly rich ground shortens its life, and water well to settle the roots.

It can also be grown in a large container or planter, as long as the container has drainage holes and the mix drains very freely. In the ground, allow a space 6–12 ft wide for a mature plant.

Pruning

It does not need regular pruning: simply remove dead, damaged, or badly placed wood.

• To enjoy the plumes, prune very lightly in early spring, only as needed, because the smoke forms on the current season’s growth.
• For striking foliage, especially on purple or golden cultivars, cut the stems back hard in late winter to 6–8 in above the ground, or to a low framework. You get vigorous shoots with larger, more intensely colored leaves, but no flowers that year.
• Middle ground: cut back only two-thirds of the old stems and leave the remaining third to keep some of the plumes.

It responds very well to renewal pruning on an aging plant. Wear gloves: its sap can irritate the skin.

Propagation

Cultivars are propagated vegetatively. Take semi-hardwood cuttings, though they root unevenly.

Layering is often more reliable. In fall or spring, choose a young, flexible stem on the outside of the shrub and bend it down to the ground. About 12 in from the tip, make a 1–2 in cut along the stem through a bud, prop it open with a small sliver of wood, and apply rooting hormone. Bury this section in a trench 4–6 in deep, pin it down with a loop of heavy wire, and tie the tip to a stake so it grows upright. Roots form in about a year; then sever the layer from the parent and plant it.

On a vigorous shrub cut back hard in spring, French layering yields several plants: the following spring, peg the new stems down like the spokes of a wheel around the base and mound soil over the side shoots that grow upward.

Common problems

• Whole branches wilting and dying: verticillium wilt, the most serious disease; remove affected branches and water during dry spells.
• Spots on the leaves: leaf spot diseases or rust, rarely serious.
• White powdery coating on the leaves: powdery mildew, especially in dry weather.
• Rolled, mined leaves: leaf roller caterpillars; pick them off by hand.
• Small scale insects on the twigs: San Jose scale.
• Purple cultivar turning green, few plumes: too little sun, or recent hard pruning.

Less often, honey fungus can attack the roots. The same symptom can have several causes: also consider exposure, drainage, and recent weather before drawing conclusions.

Toxicity

Smoke tree belongs to the same family as poison ivy: contact with its resinous sap can cause contact dermatitis, especially in people sensitive to poison ivy. Wear gloves and long sleeves when pruning. Veterinary references report no toxicity to dogs, cats, or horses; if in doubt after a pet eats some, contact a veterinarian.