DR. LEONARD PERRY
University of Vermont
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 two years, and often die within four 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.
Plant diversity and maples
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.
Dr. Frank Santamour, a research geneticist with the USDA in 1990, 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).
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, with average winter minimum (extreme) temperatures of 20 to 30 degrees below zero.
The Miyabe can reach 25 feet in 10 years. “Morton” is a Miyabe cultivar 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 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 slowly, 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.
Even more options
Three linden or basswood cultivars (Tilia) in the Michigan trials have proved outstanding. “Redmond,” “Greenspire” and “American Sentry” are all pyramidal with dark green leaves.
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 and the Swamp white.
It’s ironic that some of the American elm replacements, bred to resist the Dutch elm disease, now are recommended to replace ashes as they are killed by the emerald ash borer. Accolade elms are hardy to zone 4, and fast growing, reaching 27 feet after 10 years.
The maidenhair 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 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.
For more ash alternatives and information, search for “ash tree alternatives” at hrt.msu.edu.
Dr. Leonard Perry is a horticulture professor emeritus at the University of Vermont.