Tuesday, April 18, 2017 By: Ask A Master Gardener

Planting Disease Resistant Elm Trees

Master Gardener: Elm hybrids are good choice for shade trees

 Brian Jervis: Master Gardener

 Tuesday, April 18, 2017

Q: I am looking to plant a large shade tree and wonder what my choices are. Is it too risky to plant elm or ash trees due to their diseases? J.C., Tulsa
A: Several species of our great shade trees in the U.S. have suffered from the impact of imported pests and disease, and the process is still ongoing.
Most of our stately chestnuts that were so common in the past were killed due to chestnuts bark fungus, imported from Asia in 1904. In the following 40 years, more than 30 million acres of chestnuts were destroyed in the eastern U.S. However, some hybrids are now available that have excellent resistance to the fungus.
Ash trees are being decimated by the Emerald Ash Borer, an insect introduced into the country in 2002. Until recently, it was found only in states surrounding Oklahoma, but now has been reported infecting ash trees in the northeast part of Oklahoma. It is now found in most every state from Colorado eastward. To date, it has killed many millions of ash trees, no control is developed and will likely kill most of the 8 billion-plus ash trees in the U.S.
The majestic American elm tree, which had covered many of the streets in the U.S. with beautiful canopies, have mostly been destroyed by Dutch elm disease (DED), a fungus introduced in the U.S. around 1930. Elms were used so much in the past that most towns and cities still have an “Elm Street.” Of the 77 million elms in North America, is was estimated that 75 percent had been lost to the disease by 1989. The fungus is spread by the Elm Bark Beetle, and all of the native U.S. elm species are susceptible to the infection.
Since the disease began, horticulturists have been involved with selective breeding of elm survivors of the disease. Now there are several hybrids that have shown good to excellent DED resistance and are a great source of high-canopy shade trees. All are hardy into the USDA zone 4, which may have temperatures as low as -30 degrees.
The best of these hybrids are listed below.
Princeton: Selected years ago for its resistance, fast growth rate and a canopy similar to the original American elm.
Valley Forge: A USDA-recommended cultivar that has excellent disease resistance and form. It also is a rapid grower.
New Harmony: Another USDA selection of an American elm hybrid that appears to have superior form when compared to Princeton and Valley Forge.
St. Croix: Selected from a massive elm in Minnesota. It has good disease resistance.
Prairie Expedition (“Lewis and Clark”): A recent selection from North Dakota State University. Classic vase-shaped American elm form and growth rate, outstanding autumn gold color.
These trees may be difficult to find, but some, such as Princeton, may be found at local nurseries. Most others are available online. You will find no better tree to plant for shade and an aesthetic addition to your landscape.
Garden tips
·       Don’t spray insecticides during fruit tree bloom or pollinators may be killed. Disease sprays can continue according to schedule and labeled directions.
·       Mowing of warm-season lawns is beginning now. Cutting height for bermudagrass and zoysia grass should be 1 to 1½ inches high, and buffalograss 1½ to 3 inches high.
·       Harden off transplants outside in partial protection from sun and wind prior to planting.
·       Hummingbirds arrive in Oklahoma in early April. Get your bird feeders ready using 1 part sugar to 4 parts water. Do not use red food coloring.


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