Tuesday, November 7, 2023

Kousa Versus American Dogwood

Cornus is a genus of more than 30 woody plant species commonly known as "dogwoods." 

Cornus florida fruit
Cornus florida, often called "American dogwood," is native to eastern North America. In forests, this is an understructure tree, reaching heights of 15-30 feet and propsering in shade. Lifespan typically 40-60 years. Varieties were developed as ornamental trees for yards and parks. In the spring, these have inconspicuous flowers surrounded by four large petal-like leaves called "bracts" that range in color from white to pink to red. 

In the fall, the tress display small clusters of hard red fruit. The fruit are an important food source for dozens of speices of birds, when then distribute the seeds in their feces. The leaves are a food source for several moth varieties. The fruit is astringent and ecxtremely bitter, and thus is not considered human-edible. The wood is close-grained and dense, used for making of tool handles.

Wild populations of American dogwood were severely afflicted by Discula destructiva (great name!), a fungus that first appeared in North America in the late 1970s, origin and means of reaching North America unknown. The diseased condition is called dogwood anthracnose. Millions of trees died. Trees with the disease usually show medium‑large, purple‑bordered leaf spots and scorched tan blotches that may enlarge to kill the entire leaf. Die-back spreads to the twigs. As the disease progresses, lower branches die, cankers (detected as dark brown discoloration under the bark) form on the limbs, and sprout development (small twigs and leaves appearing on treet trunks and mature branches) increases.

Dogwood anthracnose is favored by wet, rainy weather, and slow foliage drying. A sunny exposure from the east to dry the tree early in the day is most helpful. Trees should be watered during drought. New dogwood cultivars of the ‘Appalachian’ series are anthracnose-resistant.

Kousa dogwood in bloom. Note pointed ends 
of the white bracts. Native trees have bracts
with rounded ends.
Cornus kousa, also known as Kousa dogwood and Chinese, Koran or Japanese dogwood, is widely cultivated as an ornamental tree throughout New England, and in some places has naturalized. It is not currently designated as an invasive species, but has potential. 

When in bloom, in late spring, inconspicuous flowers are at the center of four white bracts, similar in appearance to native dogwood, but pointed at ends rather than rounded. The fruit is soft , pale red, round, and slightly smaller than a ping-pong ball.  Kousa dogwood fruit is not consumed by American birds or other animals. The leaves are not consumed by American insects. Mature trees are about 20 feet tall and often multi-trunked. Lifespan can be 50-150 years.  

Kousa fruit
Kousa dogwood is resistant to the fungal disease that afflicts American dogwood, and so has been widely planted as a replacement. Given the large volume of fruit production and lack of consuption by birds or mammals, fallen fruit can accumulate on sidewalks and driveways, creating a squishy mess. 

Commercial varieties include dwarf, weeping, single trunk, etc. "Milky Way" (pictured above) is profusely covered in white bracts in late spring. 

There has been some success in breeding a florida/kousa hybrid with disease resistance. Several named varieties are available from plant nurseries. The hybrids are sterile.  

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