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Elm Tree Resistance to Pesticides

Posted on: February 2nd, 2015

Recently Swingle’s Senior Consulting Arborist, Steve Geist, was featured in The Denver Post to discuss elm trees resistance to pesticides.

In the past decade, elms have been hit with heavy infestations of European elm scale.  Elm scale usually doesn’t kill the tree, but is more of a nuisance with sticky secretions coming from the scale.  The insect can be difficult to control, however Swingle’s recent trials revealed that while insect growth regulators do not kill the insect outright, they do interfere with the insects ability to mature.  Swingle will be offering these control applications in 2015.

American elms were widely planted in communities across the high plains – with almost every town boasting an “Elm Street” after the popular tree.  In the early 1900’s, Denver’s Mayor Speer gave away thousands of American elms to residents, with city blocks eventually being monopolized by the tree. American elms plant easily, grow well in poor compacted soils and are tolerant to both drought and flood condition, which made them the perfect choice for Colorado’s climate.  No other tree can compare.

But in 1968, Dutch elm disease arrived in northeast Colorado.  Knowing the importance of American elms to the urban forest, Swingle’s former owner, Dave Dickson, founded the elm preservation program in 1971.  This program provided effective control for Dutch elm disease and other insects (including European elm scale), which are commonly found on elms.

During the discovery of Dutch elm disease, less than 1% of the trees Swingle serviced and protected died from the disease. Sadly many more were left untreated and eventually succumb to the disease. Replacing the American elm were silver maple, honeylocust and green ash trees, which were also commonly planted in the urban forest.  Today, nearly one in five trees in northeast Colorado is an ash.

Many people no longer consider elm trees when planting, as it’s believed they are all susceptible to Dutch elm disease and not a good choice – causing elms not to be grown in nursery trade.  But plant breeders are now hard at work developing a new hybrid elm variety, which has been found to do well in Colorado climates.  New Harmony and Valley Forge elms are Dutch elm disease resistant – however are very susceptible to European elm scale. Accolade and Prospector elm have done very well in Colorado; Frontier elm has an interesting burgundy fall color; Triumph and Emerald sunshine elms are also being offered in the nursery trade and are good to plant.

Though green ash trees are not nearly as majestic as the American elm, they thrive in drought prone soils, which are characteristic of Colorado.  Now with emerald ash borer becoming entrenched in eastern Colorado, again we are looking for tree replacements and the elm might yet again be a popular choice.


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