Progressive Plants

Creating an outstanding landscape in the wake of the Emerald Ash Borer

By Progressive Plants

It sounds like a scene from a science fiction novel. Sometime around 1990, a small, green, alien invader entered the U.S. undetected. Quietly, it attached to a host and gnawed the life out of it as it multiplied. Spreading from host to host, its numbers grew exponentially. From its early beginnings in a small area near the Great Lakes, it spread eastward to the coast. From there, it turned south, north, and west, leaving death and destruction in its wake. It has invaded much of North America, and now these emerald green invaders are drawing dangerously close to home.

The trouble is, this isn’t fiction. This alien invader is called the Emerald Ash Borer. It’s so small, you can fit a half-dozen of them on a penny, and yet, researchers have found nothing that can stop it.

Emerald Ash Borer (Agrilus planipennis) on a leaf. 

Photo by Debbie Miller USDA Forest Service.

Emerald Ash Borer (Agrilus planipennis) galleries and size comparison. 

Photo by Eric R Day Virginia Polytechnic Institute and State University.

The Emerald Ash Borer is native to Russia and Northern China. The initial infestation most likely started from a small number of beetles that came to the U.S. via hardwood packing materials. By 2002, many ash trees in southeastern Michigan were dead or dying. By October 2018, it was found in 35 states, and five Canadian provinces and it’s still spreading. Native ash trees in North America have little or no resistance to Emerald Ash Borer, and natural enemies, such as woodpeckers and the Pilose Checkered Beetle, have little effect when borer populations are high. (The Pilose Checkered Beetle only exists east of Minnesota, so they have even fewer predators here in the west.)

So, what can we do?

Often we can prevent borer infestations by maintaining healthy trees, but the Emerald Ash Borer will infect all ash trees. All varieties. Healthy or sick. Forest Service officials are monitoring and trapping invasive insects throughout the state and have not yet found any Emerald Ash Borers in Utah. We want it to stay that way.

The horticultural community is still searching for an effective way to retaliate against this invader, so for now, all we can do is work to slow its spread. This nasty creature has been carried to other locations through the transport of infested trees and wood (particularly firewood). Because we have native Ash trees growing in most of our wilderness areas that would be obliterated by this insect, state officials have closed the door to ash tree sales of all kinds, and they do not allow wood (including fire wood) to come over our border from infested locations.

Emerald Ash Borer Larva have no legs, just protruding nubs along their long, segmented bodies. They bore sshaped galleries in the cambium layer of ash trees that destroy the tree within a very short period of time. 

Photo by Houping Liu, Michigan State University.

Ash trees are beautiful shade trees beloved for their growth habit and outstanding fall color, but if you had plans for an ash tree in your landscape, you’ll need to find other options. They are simply not for sale here. Luckily, there are several amazing trees that will fit beautifully in your landscape in place of an ash. We’ll share some of our favorites with you today.

Zelkova will provide you with shade fairly quickly along with a really nicely shaped tree. Most cultivars have outstanding fall color. 

Photo by Progressive Plants.

Zelkova

We feel like the Zelkova tree is underutilized in Utah landscapes. It’ s a beautifully shaped, low maintenance tree that offers quick shade. Zelkova's leaf structure is similar to an ash tree and their fall color varies from green (or no change) to yellow, bronze, orange, and red, depending on the cultivar. They grow easily in Utah and require little water once they’re established. Plus, from what we’ve observed so far, they’re fairly resistant to pests. There are several varieties of Zelkova trees with varying growth habits to suit your landscape. Most are upright and vase-shaped.

For great fall color, small, persistent, edible fruits, and a variety of sizes and shapes, It’s hard to beat hackberry trees. 

Photo by Progressive Plants.

Hackberry

Hackberry is a hardy shade tree. Its growth habit ranges from a neatly upright oval crown to broad and loose, and there’s even a narrow, columnar option, depending on the cultivar you choose. Hackberry bark develops an interesting, warty appearance with age. It’s highly tolerant of urban conditions and not particular about soil type. Once established, it's drought tolerant and salt tolerant, so it can handle water from Utah Lake. It also does fine with occasional standing water. Its inconspicuous green flowers attract pollinators, and its teardrop shaped leaves become yellow in the fall.

Small, dark purple fruit is persistent into the winter months, attracting birds to your landscape almost year round. Hackberries have a thin, very sweet purple skin surrounding a crunchy shell with a tiny nut inside. All hackberries are edible and highly nutritious. They have been consumed by humans for millennia and are, actually, one of the first known foods that humans have eaten and stored.

Maackia MaacNificent has a little bit of everything spring flowers, deep green foliage, a nice size, and interesting structure. Leaves don’t have a notable fall color, but mature trees will develop exfoliating bark. 

Photo by J Frank Schmidt.

Maackia amurensis

This is one of the few trees that doesn’t have a common name. It’s actually related to the pea family, similar to honeylocust and Kentucky Coffee Tree.

Maackia MaacNificent is the most vigorous Maackia we have seen so far, with upright branching that forms a beautiful, symmetrical vase shape. Spike-like racemes of white flowers decorate the tree in early summer and its green foliage has a silvery glow in spring. It’s a slower-growing tree that will eventually reach around 30 feet high in your landscape. Prune it in the summer after flowering. Pruning in winter causes excessive bleeding and the wounds heal more slowly. Maackia are resistant to pests and diseases.

Kindred Spirit, Crimson Spire, and Chinkapin Oak trees all have leaves that are quite different. 

Photo by Progressive Plants.

Oak trees

Yes, oak trees are slow-growing and certain varieties hold on to their brown leaves throughout the winter. But that's also what makes them unique. They provide shade and a focal point for your landscape with year-round interest. They also attract birds and other wildlife to your yard by offering food and shelter. There are so many varieties of oak trees that you’re sure to find one perfectly suited for your yard. Oak leaves vary greatly from cultivar to cultivar, so you can have a lot of variety simply by switching up the oaks in your yard. Some of our favorites include Columnar

English Oak, Crimson Spire, Chinkapin Oak, and Kindred Spirit, but all of them will do well in the right situation.

Accolade Elm, shown here, is a fantastic tree for Utah landscapes. 

Photo by Progressive Plants.

Elm Trees

The Siberian Elm gives elm trees a bad name, but not all elm trees are invasive. In fact, many elms are stunning landscape trees. The Frontier Elm is a deciduous upright, pyramidal tree with proven superior resistance to Dutch elm disease. It features smooth, gray-green bark and remarkable red to purple fall foliage. Accolade Elm is a vase-shaped, medium to large, deciduous tree with glossy dark green foliage and strong yellow color in the fall. Another favorite is the Allee Elm. This lacebark Elm is very resistant to Dutch Elm disease and elm leaf beetle. Its upright, spreading shape is similar to the American Elm, but Allee features ornamental exfoliating bark for yearround interest. It’s a good urban street tree with glossy leaves that turn orange to rust red in autumn.

Honeyocusts, like the Skyline Honeylocust shown here, have a beautiful shape and bring dappled shade to your yard. 

Photo by Progressive Plants.

Honeylocust

Honeylocust trees are common in Utah for a variety of reasons. They grow well in our soil, don’t require a lot of water, and tolerate irrigation water from any source, even Utah Lake water with its nasty dissolved solids. Their compound leaves are green from spring until fall when they turn bright golden yellow. They’re a great shade tree for filtered shade. Older cultivars of this tree sport some impressive thorns, but most nursery varieties nowadays are thornless. You can even find honeylocust trees with ruby-tinted leaves!

Hornbeam trees are a fantastic option if you’re looking for a tree that will be happy in an area with partial shade. 

Photo by Progressive Plants.

Hornbeam

This beautiful tree is a vigorous grower that’s easy to care for. It has a well-defined shape with a strong central leader. The serrated edged green leaves turn yellow, orange, or red in the fall, depending on the cultivar. It’s an excellent small shade tree that prefers full sun to part shade. Most cultivars (but not all) prefer a little shade, especially in the afternoon.

So, even though we’re fighting a serious battle to save our ash trees from the Emerald Ash Borer, we can be grateful that this invader is host-specific. We can still enjoy a fantastic landscape by loading our yards with trees they don’t like to eat.

The Emerald Ash Borer is now considered the most destructive forest pest ever seen in North America. The scope of this problem has reached billions of dollars. To learn more about it, visit the USDA Animal and Plant Health Inspection Service. If you believe you have sighted Emerald Ash Borer in your area, contact your county extension office or the nearest Department of Agriculture office. You may also contact the USDA Emerald Ash Borer Hotline toll-free at (866) 322-4512.