Ash trees (Fraxinus spp.) seem to be under threat nearly everywhere.
In North America, the culprit is an insect, the emerald ash borer or EAB (Agrilus planipennis), that inevitably kills infested trees. Introduced accidentally from its native Asia and discovered in Michigan in 2002, the EAB is advancing at a rapid rate, having killed over 40 million ash trees in 14 US states and Ontario. It is now well-established in Quebec, having reached Quebec City in 2017.
Europe has not been spared either. Ash dieback or chalara dieback of ash (Hymenoscyphus fraxineus, syn. Chalara fraxinea), a chronic fungal disease that is also thought to have originated in Asia, has been decimating that continent’s ash trees since its discovery in Poland in the early 1990s. 70% of the ash trees in Belgium, for example, have already been infected and there is currently no cure.
In addition, the emerald ash borer has reached Europe as well. Still confined to the extreme east of the continent (Russia), it is nevertheless spreading rapidly. It is expected to reach Central Europe within 15 years.
Treat or Replace?
There is no treatment for ash dieback except to destroy affected trees. While it’s possible to treat ash trees against EAB by injecting them with certain insecticides, notably neem, the cost is prohibitive (several hundred US dollars per treatment), especially when you take into account that will be necessary to repeat the treatment every two years for as long as the tree lives.
Logically, therefore, gardeners will have to learn to get along without ash trees. But with what can we replace them?
Replacement Trees for Dying Ashes
What follows is a list of trees worth considering as replacement trees. None look exactly like ashes, of course, but all are medium to tall shade trees that can, by their size and ability to tolerate conditions in typical garden settings, at least offer a nice green canopy.
My suggestion? If you have ash trees that seem healthy, there’s no need to remove them right away … but it would be wise to plant replacements without delay. Starting early will allow the new trees to put on a few years of growth before the ashes have to go. Thus, when the inevitable occurs, your garden won’t be left treeless, but will still have a good share of decently sized trees.
Note that I have excluded from the following list trees that have their own disease or insect problems, such as the American elm (Ulmus americana, with the exception of Dutch elm disease resistant varieties) and white birches (Betula papyrifera and others), plus species widely considered environmentally harmful because of their invasive nature, such as Norway maple (Acer platanoides) and black locust (Robinia pseudoacacia), and trees whose extensive and invasive roots systems, such as silver maple (Acer saccharinum), most poplars (Populus spp.) and willows (Salix spp.), make them poor choices for suburban lots.
In short, the following trees are all “good solid shade trees,” capable of replacing in that role the endangered ashes.
The trees below are listed in alphabetical order of their botanical name.
- Fir (Abies spp.) – zones 2 to 7, depending on species
- Freeman maple (Acer × freemanii) – zone 3
- Bigleaf maple (Acer macrophyllum) – zone 6
- Painted maple (Acer pictum mono) – zone 6
- Sycamore maple (Acer pseudoplatanus) – zone 5
- Red maple (Acer rubrum) – zone 3
- Sugar maple (Acer saccharum) – zone 4
- Yellow birch (Betula alleghaniensis) – zone 3
- River birch (Betula nigra) – zone 3
- Hornbeam (Carpinus spp.) – zones 3 to 6, depending on species
- Shagbark hickory (Carya ovata) – zone 4
- Catalpa (Catalpa spp.) – zone 5b
- Hackberry (Celtis spp.) – zones 3b to 7, depending on species
- Katsura tree (Cercidiphyllum japonicum) – zone 4b
- Dove tree (Davidia involucrata) – zone 7b
- Ginkgo (Ginkgo biloba) – zone 4
- Honey locust (Gleditsia triacanthos) – zone 4b
- Kentucky coffeetree (Gymnocladus dioicus) – zone 5
- Sweet gum (Liquidambar styraciflua) – zone 6
- Tulip tree (Liriodendron tulipifera) – zone 6
- Cucumber tree (Magnolia acuminata) – zone 4b
- Dawn redwood (Metasequoia glyptostroboides) zone 5b
- American hop hornbeam (Ostrya virginiana) – zone 3
- Amur cork tree (Phellodendron amurense) – zone 3 (male trees only)
- Spruce (Picea spp.) – zones 1 to 7, depending on species
- Pine (Pinus spp.) – zones 2 to 7, depending on species
- Douglas fir (Pseudotsuga menziesii) – zone 5
- Manchurian flowering pear (Pyrus ussuriensis) – zone 3
- White oak (Quercus alba) – zone 4
- Swamp white oak (Quercus bicolor) – zone 4
- Scarlet oak (Quercus coccinea) – zone 4
- Shingle oak (Quercus imbricaria) – zone 5
- Bur oak (Quercus macrocarpa) – zone 3
- Pin oak (Quercus palustris) – zone 4
- English oak (Quercus robur) – zone 4
- Red oak (Quercus rubra) – zone 4
- Japanese pagoda tree (Styphnolobium japonicum, formerly Sophora japonica) – zone 5
- Arborvitae or Eastern white cedar (Thuja occidentalis) – zone 3
- Hybrid elm (Ulmus ×) – zone 4 (varieties resistant to Dutch elm disease only, such as Accolade™ ‘Morton’)
- American elm (Ulmus americana) – zone 3 (varieties resistant to Dutch elm disease only, such as ‘Valley Forge’)
- Japanese elm (Ulmus davidiana japonica) – zone 3
- Zelkova (Zelkova spp.) – zone 6
Disease- and Insect-resistant Ashes?
Another possibility would be to plant ash trees native to Asia where both the emerald ash borer and ash dieback occur naturally. These trees often show natural resistance to these two afflictions.
Manchurian ash (Fraxinus mandshurica, zone 3b), for example, is sometimes offered for this purpose and appears to be very resistant to ash dieback, but its resistance to EAB is variable: some selections appear resistant to EAB, others not. It may be necessary, therefore, to wait a few years for exceptionally resistant varieties to be selected before planting Manchurian ash in areas EAB is expected to reach.
Likewise, other Asian ashes are being tested and some may well prove to be good replacement species for European and North American ashes.
So, gardeners, to your shovels! Plant replacement trees before your ash trees show any sign of decline. In life as in gardening, thinking ahead is the wisest choice!