There is more to consider when selecting new trees for the yard, acreage or public green space than just choosing attractive and regionally hardy non-ash trees. Infestations of Emerald Ash Borer have been confirmed in Omaha and Minneapolis metros, but have not yet been confirmed in South Dakota. That’s according to Dr. John Ball, forester at South Dakota State University, in his online tree publication, The Update. He addressed tree selection considerations with South Dakota Master Gardeners at their annual conference in Aberdeen this fall.
When replacing damaged trees, or adding new trees, avoiding the emerald ash borer susceptible genus and species for ash trees and its cultivars makes sense.
Ash trees are popular for their beauty and adaptability to this region. They became a common replacement choice as elm trees died from Dutch elm disease. American elm trees became common here, as were American chestnut trees in the Northeast two hundred years ago, before the chestnut blight fungus.
Trees may live fifty years or more in their setting and fight various pests. A genus such as poplar, for example, may become resistant to the pests over generations. But these pests may be moved to a new region where poplars have not built up defense to them. Defenseless poplars are vulnerable. The practice of choosing replacement trees like those we see commonly growing around us, appears to favor pests over trees.
Nearly thirty years ago, Dr. Frank Santamour, a research geneticist at U. S. National Arboretum urged for diversity when selecting trees. He proposed that no more than 10 percent of trees in a community be of one species, no more than 20 percent be of one genus, and 30 percent within a tree family.
Dr. Ball points out that if a community has 10 percent green ash trees and 10 percent white ash trees, the community is still quite vulnerable to the emerald ash borer.
Ball proposes that when we replace trees, we aim for only 5 percent of a community’s trees to be of one genus. He said that this may not reduce the chance of an exotic pest but tends to reduce its impact because there are fewer trees of a kind for its food supply.
Tree cultivars are popular to provide leaf color variation and other desired features. Freeman maple cultivars offer much variety but each 5 percent cultivar fills the 5 percent Acer goal for a community.
Dr. Ball urges special care in choosing trees within ash, elm, linden, oak, maple, pine, poplar and spruce genera. These are trees with many species on three northern continents — Asia, Europe and North America. Oak pests and oaks on one continent may be quite adapted to each other but the oaks on the other continents may not have been exposed to those exotic oak pests. Sometimes the wide-ranging genus can be host to a disease of other plants without itself showing symptoms.
Conversely, if a tree genus is found only on one continent or has few species of that genus, the chance is greater that its pests are few. Examples of such trees include hoptrees, Kentucky coffeetree, Yellowwood, Catalpa, Homestead buckeye, Siberian larch, Meyer’s spruce and cork trees. See other examples in Dr. Ball’s Trees: Fruit, Nut, Ornamental, Shade and Windbreak Trees for the Northern Plains.
For finding a list of replacement trees of general interest, the online Arbor Day organization has a tree finder by zip code on its website. With a regional source such as Trees, one may concentrate on a short list of personally attractive trees that are hardy to the region, and then find out more about their ranges and multiple species. This added step in tree selection may help the trees selected to live long and well in this region.
More on this topic may be found in the article “The 5 Percent Rule” by Dr. Ball, found online in the January 2015 American Nurseryman publication. For South Dakota tree information including location of confirmed cases of Emerald Ash Borer in the region, see http://sdda.sd.gov/conservation-forestry/forest-health/tree-pest-alerts/PDF/2017/11-01-2017.pdf