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2003 Annual and Perennial Trial Results

September 2004
Plant of the Month

Cornus alternifolia
Pagoda or Alternateleaf Dogwood
tree
zone: Zone 3
by Stan Hokanson, University of Minnesota

Cornus alternifolia – Minnesota’s native tree dogwood


Pagoda or Alternateleaf dogwood (Cornus alternifolia L.) is a native tree that is worthy of increased consideration from a landscape and research perspective.  A relative of the highly admired Flowering (Cornus florida L.) and Kousa (C. kousa (Buerger ex Miq.) Hance) dogwoods, Pagoda dogwood occurs naturally in all states east of the Mississippi River and Minnesota, Iowa, Missouri and Arkansas.  In Minnesota, populations are mainly found in the eastern half of the state, but are reported as far west as Polk and Clay counties and north to Lake of the Woods, Koochiching, St. Louis and Lake counties.  It is worth noting that populations existing in St. Louis county would contain plants hardy to USDA Plant Hardiness Zone 2B (-45 to -40˚F).  This is in sharp contrast to the showy relatives that are marginally hardy to USDA Zone 5 at best.


Pagoda dogwood is generally a 15-25’ high tree, which can spread to one and half times its height.  Unlike other members of the genus, C. alternifolia has alternately arranged leaves (hence its species name).  The leaves are medium to dark green, generally 2-5” long and 1 to 2.5” wide, with a characteristic ‘bowed’ or curved (arcurate) venation.  While the species is not noted for dramatic fall color displays, the leaves can develop a reddish/purple color, although this can vary by the tree and is dependent on local site conditions.  Trees situated in heavy shade will be less likely to show fall color than those exposed to at least a half day of sun.


Cornus alternifolia flowers, clustered in 1.5-2.5” cymes, emerge from terminally borne, reddish/purple valvate buds in mid-May to early June.  While not as dramatic as their showy cousins the Flowering and Kousa dogwoods, the flowers of the Pagoda dogwood progress from light yellow to cream-white in color, providing a garden-worthy show for 7-10 days each spring.  Moreover, the Pagoda dogwood flowers are richly fragrant, in contrast to the cousins, which actually posses nearly inconspicuous flowers with no fragrance (the showy parts of the Flowering and Kousa dogwood inflorescence are actually four bracts or modified leaves).  Pagoda dogwood fruits progress from green to blue to blue/black, generally ripening in August into early September.  When ripe, the fruits are borne on pinkish-red pedicles and are quite showy.  However, birds relish the fruits and make quick work of them.


The spreading, horizontal or ‘planier’ habit of the tree, which is similar to that of the Flowering and Kousa dogwoods, is it’s most attractive garden attribute.  The tiered, horizontal nature of the tree serves to break strong vertical elements in the landscape and provide good contrast for more vertically inclined plant materials.


This tree is not problem free.  Unfortunately, the tree rarely attains a trunk diameter greater than 3-4” before it succumbs to a devastating trunk canker.  Although the tree will often survive the initial infection, it is subsequently reduced to a small, multi-branched shrub.  While a recently published compendium on woody landscape plant diseases makes no reference to canker diseases in Pagoda dogwood, a literature search reveals the disease was first reported to be isolated from C. alternifolia in 1874.  The disease, now known as Cryptodiaporthe canker is incited by the fungus Cryptodiaporthe corni (Wehm.) Petrak.  Like many plant diseases, Cryptodiaporthe canker is more likely to occur in trees that have been subjected to environmental stresses such as prolonged drought, freeze damage, sun scald, and/or mechanical damages.


Proper siting and cultural conditions will help prevent disease occurrence and promote longer life for the tree.  Given the species naturally occurs in moist to wet woods margins in slightly acidic soils, preferred planting sites would provide morning or filtered afternoon sun and moist, well-drained, slightly acidic soils.  It would be best suited to a semi-shady naturalized woodland garden in company with ferns, shade tolerant perennials, broad leafed rhododendrons and/or low lying flowering annuals.  This small statured tree also works well when used as a contrast to strong vertical elements such as the corners of buildings or tall overstory trees.


Cornus alternifolia is generally propagated from seed which requires a warm stratification at 20-30˚ C for 60 days followed by cold stratification at 5˚ C for 60 days.  Dirr reports the species propagates from softwood cuttings with the caveat that cuttings should be allowed to overwinter and break dormancy before being repotted.


Relatively few named cultivars of the species exist.  Cultivars include ‘Argentea’ a white variegated type introduced before 1900 in the U.S. and ‘Wstackman’ a yellow variegated form that was developed in Chicago and patented in 2000.  According to the University of Minnesota Landscape Arboretum’s online plant and seed source locator; http://plantinfo.umn.edu/arboretum/sources/default.asp (subscription required) a number of local retail and wholesale nurseries stock seedling Pagoda dogwood.  Only ‘Argentea’ and ‘Variegata’ are available as cultivars from out-of-state mail order sources.


In the Woody Landscape Plant Breeding and Genetics program at the University of Minnesota, we are developing a program to screen seedlings for resistance to Cryptodiaporthe canker.  The program will require a couple of components. First, we need to verify the presence of Cryptodiaporthe canker in diseased trees in Minnesota.  We are in the process of collecting canker samples from around the state.  Culturing the isolated fungus will allow us to artificially inoculate seedlings to screen for resistance.  The second key component of the project involves collecting seed from populations in USDA Zone 2, 3, and 4 locations in Minnesota.  Using these seedlings should ensure the selection of reliably cold hardy cultivars.  In addition to selecting for disease resistance and cold hardiness, we also hope to identify improved fall color, richer/varied flower color, and larger flower size.  

 

 

 

 

 

 

 


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