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2003 Annual and Perennial Trial Results

September 2005
Plant of the Month

Cotinus obovatus
American Smoketree or Chittamwood
zone: 4
by Stan Hokanson, University of Minnesota

The American smoketree or Chittamwood is a semi-shrub/tree that can grow 15 to 30 feet in height with equal width in its native habitat.  The tree is a North American native, existing in small, scattered populations from Georgia and Alabama in the east, north to Kentucky and Tennessee, and west through Missouri and Arkansas to the Edwards plateau region of Texas. 
A member of the Anacardiaceae family, American smoketree is closely related to the Common smoketree or Smokebush, Cotinus coggygria, which is a southern European, Asian native and the most likely of the two species to be encountered in Minnesota landscapes.  Superficially, the two species appear similar.  However the American smoketree assumes a more upright tree-like form while the Common smoketree has a looser, wider, shrub-like form.  The two species differ considerably in regards to the flower display, with the Common smoketree being the showier of the two.
 The flower structure merits some explanation.  The true flower for both species is an inconspicuous greenish, pale yellow, five petaled structure.  The flowers are arranged into a panicle that can measure 6 to 8 inches long.  The real ¡®flower¡¯ show is due to the filamentous hairs occurring on the flower stalks (pedicels) and stem of the panicle (peduncle).  The Common smoketree has longer, more densely packed hairs which can vary in color leading to a showier appearance.  Cultivars have been selected with a range of hair colors ranging from yellowish to dark purple, leading to their popularity in the landscape. While the American smoketree inflorescence is less showy, the species is dioecious (meaning there are separate male and female trees) with the male specimens having a larger, showier inflorescence than the females.
 The American smoketree has several additional characteristics worthy of note from a landscape perspective.  In its native habitat American smoketree exists in limestone glades, limestone outcrops, and rocky balds.  The capacity to grow in these habitats, where low soil moisture and elevated soil pH are common, suggests that American smoketree might perform well in managed landscapes. 
American smoketree has alternately arranged, simple, entire, 2-5¡± long leaves with an obovate to elliptic-obovate shape.  Summer leaf color is dark green with a bluish cast on the top, and a lighter silvery cast below.  In the fall, American smoketree stands out among trees, consistently displaying brilliant fall color, ranging from yellow through the orange/red spectrum to near purple.  The color appears to vary from tree to tree, suggesting selections exhibiting superior fall color could be made.  
The gray to gray/brown bark becomes scaly in age, exposing a brown underbark color which provides additional landscape interest.  Interestingly, in the 19th century, the species was harvested nearly to the point of extinction.  The extremely durable wood was utilized for fencing materials and could be boiled to produce a water soluble yellow/orange dye in the Civil War era.
 Surprisingly, American smoketree, native to locations in USDA Plant Hardiness Zones  6a to 8a, with minimum winter temperatures ranging from -10¢ª to 10¢ªF can be hardy in Minnesota.  Since 1996 the Woody Landscape Plant Breeding program at the University of Minnesota has been evaluating the hardiness of American smoketree seedlings purchased from commercial sources.  Trees in field trials at the Horticultural Research Center in Excelsior, Minn. have survived with little or no damage since 1999, which is remarkable given that in the winters of 2000 and 2004 low temperatures reached -24¢ªF and -25¢ªF respectively.  Several selections made from these original trials were tested in laboratory controlled freezer tests in midwinter, 2003.  Three of the trees proved to be wood hardy to -40¢ªF.  The hardiest of the selections were established in replicated plantings at the North Central Research and Outreach Center in Grand Rapids, Minn.  In the winter of 2004 the trees experienced an official low temperature of -47¢ªF.  Although all of the selections experienced some cold injury and several were completely killed, the fact that a number of the selections survived under such extreme conditions suggests there is potential to at least identify genotypes suitable for USDA Plant Hardiness Zone 4a locations.
 Unlike so many landscape plants, American smoketree is relatively problem free in the landscape.  A review of the literature reveals only one mention of susceptibility to leaf spot, rust, and wilt in the landscape, rendering it a great candidate for low maintenance landscape situations.  We have seen no disease or insect problems on trees growing at the HRC or on the St. Paul campus of the University of Minnesota.
 American smoketree will thrive on a range of soil types, including dry, rocky, heavy clay, acidic, and elevated pH soils.  Best growth will be achieved on a well-drained loamy soil.  While the tree will grow in shady situations, it will develop the best fall leaf color in an exposed sunny location.  The tree will not grow well in saturated wet soils. The tree would find its best use in small groupings, in shrub orders, or possibly in island plantings or street side plantings where moisture and rooting space are limiting.
 American smoketree is propagated vegetatively as softwood cuttings taken just before the current seasons growth hardens off.  We dip cuttings in 8,000 ppm of IBA suspended in alcohol.  Cuttings are rooted under shade cloth with a 15 second mist every 15 minutes.   As soon as the cuttings root, the mist should be tapered off and discontinued.  The cuttings will rot if given too much water.  The species is rarely propagated from seed as seed production is sporadic.
Interestingly, no named cultivars of American smoketree currently exist.  The small size and upright mounded form of the tree make it a perfect option to serve as a small-stature tree for increasingly small, modern residential landscapes.  Given its great fall color, soil adaptations, and cold hardiness, without a doubt, named cultivars will arrive on the market in the not-so-distant future.








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