Documents: Hot Horticulture Issues:

ASH TREE ALTERNATIVES
by Leonard Perry
by Leonard Perry

email: lpperry@uvm.edu

In extension I serve as an advisor and consultant to the greenhouse and nursery industry, primarily in Vermont but throughout the region and beyond as well.

I give presentations on my research to the industry, and to home groups. In Research, my focus is "herbaceous perennial production systems".

His website is at http://www.uvm.edu/~pass/perry/index.html  Leonards zone of gardening: home with my trials, generally USDA 4a. Campus in Burlington is 5.


April 21, 2019

Some of the better native trees in our home and city landscape plantings are ashes, available in several species and cultivars (cultivated varieties).

Most common are the white ash (Fraxinus americana), rounded in habit and up to 60 feet tall when mature, and the green ash (Fraxinus pennsylvanica), oval to upright reaching 60 feet tall. Both have a moderate to fast growth rate, with yellowish fall leaves.

These ornamental trees also are valuable for their timber which, being tough, is used for tool handles and sports equipment. Unfortunately, a new invasive and exotic pest—the emerald ash borer— threatens these in the Midwest and mid-Atlantic states, and is slowly spreading throughout New England. It now has been found in Vermont, too, and some experts predict that within 10 years there will be few ash trees left there. The purple rectangles you may see hanging along roads from trees are monitoring traps for this pest. Since trees affected with this borer lose half their leaves in 2 years and often die within 4 years, it may be wise to plant trees other than ash. Or, if you have ash trees, you may want to plant others growing near them as future replacements.

This introduced pest was first spotted in southeastern Michigan near Detroit in 2002, likely coming into our country from Asia on wooden packing materials. A study was begun in 2003 by Dr. Bert Cregg and others at Michigan State University, near the epicenter of the original outbreak, on suitable alternative trees to the ash.

Recently, several cities and towns in Vermont have begun planting replacement trees near ash, so they’ll be established when the ash trees die. If you’re planting more than one tree, it’s best to plant a diversity of species. This is better for wildlife and, if another pest or disease comes along, it will most likely not affect all the trees. A research geneticist with the U.S.D.A. in 1990 (Dr. Frank Santamour) is credited with popularizing the 10-20-30 rule for planting, to avoid catastrophic losses from tree pests. This rule says to plant no more than 30 percent of the same family (such as the maple family—now reclassified by botanists as part of the soapberry family), no more than 20 percent of a genus (such as maples), and no more than 10 percent of a species (such as sugar maple).

So, to start with maples, there are several that you might consider instead of ashes, including the sugar (Acer saccharum), red (A. rubrum), Freeman (A. x freemanii)—a hybrid of red and silver maples, and Miyabe (A. miyabei). All these are hardy to at least USDA zone 4 (average winter minimum temperatures of -20 to -30, F). The Miyabe can reach 25 feet in 10 years. 'Morton' (also seen as State Street) is a Miyabe cultivar (cultivated variety) from the Morton Arboretum near Chicago, having a dense crown and dark green leaves.

Even though Norway maples (A. platanoides) have been widely planted in the past, they are no longer recommended as they can be quite seed invasive. Make sure to not confuse the ‘Crimson King’ Norway maple with its dark red leaves, with the red maple species.

There are several Freeman maple cultivars available, including the broadly oval Autumn Blaze, the upright Scarlet Sentinel, and the narrowly upright ‘Armstrong’. Freeman maples are generally orange to red in the fall, 40 to 60 feet tall, and can tolerate drought or wet soils when established. Celebration is a particularly drought-tolerant cultivar, with yellow fall foliage.

There are many red maples to choose from, fall colors varying with cultivar, but generally with a red effect in spring from the seeds and emerging leaves. Most are dense and oval in shape, 40 to 60 feet tall.

Some tolerate wet soils, and many are moderate to fast growing. A few examples include Autumn Flame which, as its name indicates, has bright red fall color; Brandywine has a deep red fall color; Red Sunset is bright red to orange in fall; Karpick is orange to yellow in fall, with a narrower habit; and October Glory turns orange to red early in fall.

Sugar maples grow 40 to 80 feet tall, generally with a rounded shape. Although they grow relatively slow, they generally are long-lived. Most know the yellow to orange gorgeous fall colors, and the leaf shape from the symbol of Canada. Of course, it is the Vermont state tree. Examples of this species include ‘Green Mountain’, which is orange to scarlet in fall, broadly pyramidal, and somewhat faster growing; similar is the newer Fall Fiesta; ‘Goldspire’ is narrower in shape, turning orange-yellow in fall.

All three linden or basswood cultivars (Tilia) in the Michigan trials have proved outstanding. After 10 years of growth, 'Redmond' (T. americana) was 20 feet tall, 'Greenspire' (T. cordata) was 22 feet, and American Sentry (T. americana) was 23 feet. All are pyramidal with dark green leaves. 'Greenspire' is hardy to zone 4, the other two even colder to zone 3.

Several oaks (Quercus) have proven good ash alternatives, although they may grow more slowly. The northern pin oak (Q. ellipsoidalis) may reach just over 10 feet high in 10 years, is hardy to at least zone 4, is native to the Midwest, and doesn't get the yellowed chlorotic leaves you may find on the standard pin oak (Q. palustris). A couple other of the hardier oaks to consider are the Bur (Q. macrocarpa) and the Swamp white (Q. bicolor). Other good oaks, hardy to the warmer zone 5 found in much of central New England, include the shingle (Q. imbricaria), chinquapin (Q. muehlenbergii), and the sawtooth (Q. acutissima).

It's ironic that some of the American elm replacements, bred to resist the Dutch elm disease, now are recommended to replace ashes as these are killed by the emerald ash borer. Accolade elms are hardy to zone 4, and fast growing-- reaching 27 feet after 10 years. Triumph elm, also developed at the Morton Arboretum as was Accolade, has a similar growth rate but is rated hardy to the warmer zone 5 (average annual minimum of -10 to -20 degrees, F).

The maindenhair tree (Ginkgo biloba) has a similar yellow fall color to ash, with a pyramidal shape when young and wide-spreading when older. It is a tree that has been around for 150 million years, and has a unique triangular leaf shape. It tolerates most soils and tough conditions. A native tree tolerant of tough soils and conditions, the Kentucky coffeetree (Gymnocladus dioica) has fruit (beans) that were used by native Americans to brew coffee. It can be a slow grower, produces an irregular open habit, and has yellow fall color.

Amur corktree (Phellodendron amurense) is hardy to zone 3, rounded at 40 feet eventually, with yellow to bronze-yellow fall color, a moderate to fast growth rate, and tolerates drought and pollution.

Hackberry (Celtis occidentalis) is native to much of eastern North America and tolerates dry, compacted and alkaline soils. When mature, it reaches 50 or more feet tall, with a vase shape and arching branches. Fall leaves are yellowish. The native yellow buckeye (Aesculus flava)— is hardy to zone 4 and can reach about 20 feet in 10 years, forming a rounded habit. For a lower tree, consider the American hophornbeam (Ostrya virginiana) with its flowers resembling hops. It is hardy to zone 4, but rather slow growing, reaching 14 feet in 10 years.

A publication from Dr. Cregg in the MSU horticulture department (www.hrt.msu.edu, search for “ash tree alternatives”) lists many more ash alternatives. You can get to know more about the emerald ash borer, its damage, location, how to spot it, and controls, from many websites, including the U.S.D.A. (www.aphis.usda.gov/aphis, search for “emerald ash borer”).

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