Cotinus smoketree, smokebush

Why you should grow it

Smoketree can be grown for its purple or green foliage or flower clusters. It is easy to grow, grows fast, and is virtually pest-free. C. coggygria grows well in the northern United States.

Why you shouldn’t

Flowers are produced on two-year old wood, so a choice may have to be made between foliage and inflorescences. Some growers have reported that the cut stems have an odd smell and can be difficult to hydrate. The sap can cause allergic reactions.

Species and Cultivars

Cotinus coggygria: Both purple and green leaf forms are grown for their foliage and inflorescences. In general, C. coggygria has showier inflorescences than C. obovatus (Tripp, 1994). The showy part of the inflorescence is actually the large cluster of peduncles as the flowers are small.

• Purple foliage cultivars bear dark pink or reddish-purple inflorescences. Cultivars include ‘Black Velvet’, ‘Cooke’s Purple’, ‘Nordine’, ‘Notcutt’s Variety’, ‘Purple Supreme’, ‘Red Beauty’, ‘Royal Purple’, and ‘Velvet Cloak’. ‘Nordine’ is very cold hardy. ‘Royal Purple’ and ‘Velvet Cloak’ are readily available and generally have the best color that stays true in the dog days of summer. Tripp (1994) states that coppiced plants of ‘Velvet Cloak’ retain their color better than ‘Royal Purple’, while ‘Royal Purple’ has better foliage color if not coppiced.

• Green leaf types include ‘Daydream’, ‘Green Fountain’, ‘Pink Champagne’ and ‘Young ‘Lady’. Green-foliaged cultivars are grown primarily for their rose pink inflorescences, although green foliage is sometimes sold. ‘Daydream’ bears lots of long-lasting inflorescences that bloom later than the species form (Dirr, 1998). ‘Pink Champagne’ performs very well in the Southeast United States (Tripp, 1994). ‘Young Lady’ produces large inflorescences even when young (Dole, 2007).

• There is a yellow or gold leaf type known as Golden Spirit or ‘Ancot’. It may be more susceptible to verticillium wilt than other types.

Cotinus obovatus, the American smoketree, grows slightly larger than C. coggygria. This species is harder to find than C. coggygria, blooms slightly earlier, and is better for higher pH soils (Dirr, 1998). C. obovatus is native to a few areas in the southeastern United States, primarily in limestone-rich soils. Plants are dioecious, with males producing showier flowers than females (Tripp, 1994).

• Cotinus ‘Grace’, a hybrid of C. coggygria and C. obovatus, is a definite improvement over both its parents, with very large, 12 inch (30 cm) long flower panicles and dark purple leaves that hold up well. Spring Meadow Nursery lists it as C. ×dummeri after its developer Peter Dummer.

Production

General growth: Smoketree forms a small tree or large shrub, 10 to 15 feet (3 to 4.5 m) tall. Place in full sun for good color, particularly the purple-leaved forms. Plants are easy to grow but take a year or two to establish themselves. They have medium growth rates but grow faster when pruned annually. Both C. coggygria and C. obovatus are hardy in Zones 5 through 8, although purple forms are slightly less cold hardy.
Flowers mature in early summer. Although the individual flowers are small, they are borne in long panicles along with hairy growths. The entire inflorescence is about 6 to 8 inches (15 to 20 cm) long and flowers during summer. In the North, bloom time lasts longer than in the South.
Cotinus is adaptable to most soils, although it seems to prefer well-drained, infertile soils. Verticillium wilt increases in overly wet soils.

Spacing: As little as 30 inches (75 cm) to as much as 6 feet (2 m), depending on pruning and harvesting expectations. If plants are cut to the ground every year, space them closer together. For flowers, space farther.

Pruning:
 Pruning depends on whether you are harvesting flowers or foliage. If pruning for foliage, best color is produced on new growth, so cut back hard in late winter/early spring, before new growth begins (Brown and Kirkham, 2004; Dean, 1997). Annual pruning also provides larger leaves with darker color but flowers are, of course, sacrificed. Plants respond well to hard pruning. Plants can also be pollarded.
Flowers are produced on 2-year-old wood. The entire plant can be cut to the ground every 2 years, or selectively harvested every year.

Pests and diseases:
 Most pests and diseases do not bother Cotinus, except for Verticillium, a root pathogen that is hard to get rid of. The disease is also called verticillium wilt, because infected plants often have droopy leaves that no amount of watering can cure. Verticillium wilt is not always fatal, but it is easily spread, so use care when harvesting. Healthy, adequately irrigated plants are less susceptible to the disease. High nitrogen levels and improper or very severe pruning can aggravate the situation. Smoketree is also susceptible to fireblight, which can be a problem in areas with large apple and pear orchards.

Harvest and Postharvest

Stage of harvest:
 For foliage, harvest after the stems have hardened off, which occurs in summer and continues through fall. Harvest 20 to 24 inch (50 to 60 cm) stems.

Expected yields: Flowers are produced on plants as young as 2 years old, depending on growth and cultivar. Plants can be lightly harvested after two years for foliage, or three years for flowers.

Conditioning: Stems of foliage can be hard to hydrate. Cut into hot water or a hydrator and place in a cooler overnight. Devecchi (2005) recommended placing foliage stems in a solution of silver nitrate (25 ppm), aluminum sulfate (50 ppm) and sugar (2.5%).

Storage and shipping:
 Store the foliage in water. Stems can be shipped dry.

Vase life:
 7 to 12 days for foliage.

Propagation

Collect softwood cuttings in early to mid-summer and treat with 8000 ppm IBA (Dirr, 1998) or 10,000 ppm KIBA (Tripp, 1994) and keep under mist until roots appear. Dirr (1998) also recommends holding under 47% shade cloth. Cuttings are slow to establish roots.

Seed will not come true from either green or purple forms. Collect seed in fall, acid scarify for one hour, then plant outdoors (Tripp, 1994) or stratify for 3 months at 41ºF (5ºC).

Marketing

Harvest stems 2 to 3 feet (60 to 90 cm) long and place five stems in a bundle (Dean, 1997). In Italy cut stems with or without inflorescences are sold in 1 pound (0.5 kg) bunches (Unione Cooperativa Floricoltori della Riviera, 1997). 

Some markets can use lots of purple- leaved foliages and others cannot. Flowers are becoming very popular and are scarce, so fine specimens command a good price.

Troubleshooting

Flowering:
 Inflorescences are somewhat wispy. ‘Daydream’, ‘Young Lady’ and ‘Grace’ have premium floral presentations. If growing C. obovatus for flowers, grow males.

Loss of purple color:
 This is typical in warm summer temperatures. ‘Royal Purple’ and ‘Velvet Cloak’ are thought to have the best color, but even these cultivars will not retain their color in the heat and humidity of the southeastern United States.
 
Landscape Plantings

Smoketree makes an attractive shrubby tree and is underused in most of the United States. They flower in early summer, helping to fill the gap between late spring bloomers such as lilac and summer shrubs such as hydrangea.

Green leaf forms are most useful for their flowers, but purple and gold-leaved forms have much longer displays. Use the purple and gold forms beside each other for a knockout combination, or mix them with other purple and gold shrubs such as Himalayan honeysuckle (Leycesteria formosa ‘Golden Lanterns’), red-leaved rose (Rosa glauca), dwarf or weeping redleaf Japanese maples (Acer palmatum), or ninebark (Physocarpus opulifolius ‘Diabolo’ or ‘Dart’s Gold’).

The flowers of American smoketree (Cotinus obovatus) are not as showy, but plants are unrivalled for their incredibly beautiful fall foliage, in shades of yellow, orange, and crimson.

Smoketree is easy to grow in well-drained soil and full sun. Unpruned plants grow 10 or 15 feet (3 to 4.5 m) tall and wide. Pruned shrubs produce long, straight stems in a few months. In the purple cultivars, annually pruned stems have better colored leaves.

Smoketree can be used in groups, but a single dark-leaved specimen is enough to make a statement in a border or informal hedge.

Uses

Foliage makes an excellent filler, with the purple forms particularly useful in fall arrangements, combined with maroon and gold shades. The pink or pinkish-purple inflorescences are large and airy and can be used as line, form or filler. The inflorescences are also used dried.

Other Comments

Maryland grower Mel Heath removed his ‘Velvet Cloak’ plants because they became less productive as they aged. He noted that this cultivar produced longer but fewer stems than ‘Grace’ and ‘Young Lady’.

Photo 1: Smoketree inflorescence.
Photo 2: Purple-leafed smoketree grown for cut foliage.
Photo 3: Continus coggygria ‘Youg Lady’ inflorescence.
 
References

Brown, George E. and Tony Kirkham. 2004. The Pruning of Trees, Shrubs and Conifers, 2nd ed. Timber Press, Portland, Ore.

Dean, Elizabeth. 1997. Wouldn’t you like to add some woodies? Handout at Association of Specialty Cut Flower Growers conference in Portland, Ore. 

Devecchi, M. 2005. Effect of 1-methylcyclopropene on vase life of new cut foliage species: First experimental results. Acta Horticulturae 682:1311-1317.

Dirr, Michael A. 1998. Manual of Woody Landscape Plants. Stipes Publishing, Champaign, Ill.

Dole, John. 2007. 2006 ASCFG national perennial and woody trials. The Cut Flower Quarterly 19(1):28-31.

Tripp, Kim E. 1994. Considering Cotinus. Arnoldia 54(2):21-30.

Unione Cooperativa Floricoltori della Riviera. 1997. Fronde & Foglie, 2nd ed. Sanremo, Italy.