Ash trees are dying throughout North America and Europe. Here emerald ash borer is killing a forest in Ontario. Photo: Michael hunter, Wikimedia Commons
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.
You’ll probably never see an emerald ash borer, but you will see the damage it causes. Photo: U.S. Department of Agriculture
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
There are plenty of good shade trees that can replace ashes in our parks and gardens. Photo: pxhere
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.
American hornbeam (Carpinus caroliniana) is just one example of a good ash replacement tree. Photo: Daderot, Wikipedia Commons
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). Male cultivars are recommended, as fertile female trees can lead to a problem of invasiveness . Photo: Bruce Martin
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). Photo: Shelmerdine Garden Center
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!
An important element of residential landscaping seems to be going the way of the dodo in modern cities. Tall trees — the big, majestic ones that gave the landscape its character — are increasingly being left out in favor of smaller trees or even shrubs grafted on short trunks.
This is a fairly recent trend. If you look suburban developments over 60 years old, tall tree species dominate. Big maples and huge majestic oaks are everywhere. They helped create an atmosphere of tranquility and well-being. Take a walk in a neighborhood 40 years old or less, though, and you mostly see green lawns and shrubs, maybe a flower bed or two, and a few smaller trees you couldn’t even fit a lawn chair under, but not many larger ones. True enough, a somewhat sparse landscape can be still attractive, but it tends be so in a cold and impersonal way. Such neighborhoods aren’t really inviting. It’s as if the residents planned all along to leave their dreary home landscape on weekends for a break at the cottage … surrounded by tall trees, of course!
Just Don’t Think About It, Plant Them!
Why do larger trees deserve a place in suburban and urban lots?
First, for the shade they offer. Gardeners may complain you can’t grow anything under big trees (actually, you most certainly can: I’ve written an entire book about shade gardening, so I know it’s far from impossible!), but the fact is we’re attracted by shade. During the dog days of summer, a neighborhood well decorated with broad, shade-producing trees is livable; one denuded of any tall vegetation simply is not.
Also, human beings, by their very nature, seem to need trees in their surroundings. Is this a reminder of the long distant past when our ancestors took refuge in trees when they were attacked by predators? No one knows. Still, the feeling of peace and security that emerges from a big tree seems very real. In fact, it can be seen in different cultures all over the world: when people are given a choice of where they would like to live, they inevitably prefer not a forest, but a landscape dotted with mature, tall trees.
Of course, maybe you do feel you have trees on your property. But can you really call a small weeping tree barely 10 feet (3 m) high a tree? Or a flowering crabapple or a Japanese lilac or one of the many other “small trees” so heavily planted these days? They may be trees by definition, but you can’t walk under them without bumping your head, you can’t fit a lawn chair underneath them without your legs sticking out in the hot sun and they don’t create the atmosphere of permanence and security that large trees can provide.
How to Use Trees Wisely
Ideally, to recreate the sense of peace and permanence you want, you’d need at least one large tree per yard. Preferably two, in fact, on in the front and one out back. Obviously, the larger the lot, the more trees it needs.
In addition to the atmosphere they create, big trees offer other advantages:
Reduced cooling costs in the summer;
Reduced heating costs in winter;
Increased land value;
An environment healthier for human physical and mental well-being;
An inviting landscape for our feathered friends;
And much more.
Trees have certain disadvantages, of course, but these are generally easy to overcome.
Planting them, for example, is fairly arduous … but at least you only have to do it once!
The shade they produce will reduce the choice of plants that will grow underneath, but there is still a good choice of shade plants. Where it’s too shady for a dense lawn, for example, there are dozens of equally dense, maintenance-free groundcovers.
Some trees do produce seeds or fruits that can be briefly annoying when they fall, but there are many cultivars that are either sterile or male (male trees produce no seeds).
Finally, there will be leaves to rake up each fall — yes, even so-called evergreens tend to lose leaves at that season —, but fortunately that’s only a once-a-year thing … much less work than maintaining a lawn, which usually requires weekly mowing.
Plant Tall Trees Where They Can Reach Their Full Size
When planning to buy a tree, ask about its maximum height and spread and use that info to find a suitable location. For example, don’t plant it where it can interfere with overhanging wires, too close to the house, or directly in front of a window. Nor should it reach over into a neighbor’s lot, otherwise there is a serious possibility of conflict.
Lots of choice!
What follows are some suggestions of tall trees (over 30 feet/9 m) that can decorate your property. All are low- to no-maintenance trees that will enhance your property’s value.
Note that the trees shown here were chosen for a cold climate region. In areas with more temperate or even warm climate, you’ll have an even greater choice. Measurements are averages reached under normal growing conditions.
Red maple (A. rubrum): Long neglected by arborists, this North American native is becoming more popular, especially in colder climates. Its bark, smooth and pale gray in its youth, becomes rough over time. Its three-lobed leaves turn bright red in autumn. Prefers moist growing conditions. Height: 60 feet (18 m). Spread: 50 feet (15 m). Hardiness zone: 3b. There are also smaller, more symmetrical selections, such as ‘Morgan’ (50 x 50 feet/15 x 15 m) and ‘Red Sunset’ (30 feet x 20 feet/9 x 6 m). ‘Autumn Flame’ (35 x 20 feet/11 x 9 m) is the best choice for colder climates (zone 3).
Freeman Maple (A. x freemanii): This hybrid maple results from a cross between red maple and silver maple (A. saccharinum), but is closer to red maple in overall habit. It’s perhaps even superior to red maple as a city tree and is even a bit hardier: zone 3. There are several cultivars, including Autumn Blaze (‘Jeffersred’), 50 x 30 feet/15 x 9 m), which turns fiery red in the fall.
Sugar maple (Acer saccharum): It used to be that arborists shunned this North American native, considering it unsuited to urban areas, and recommending Norway maple (A. platanoides) instead. Nowadays, attitudes have changed and that there are few situations where a sugar maple wouldn’t be considered a better choice than its Norwegian relative. Planted in isolation, it takes on a beautiful speading, rounded shape quite unlike its fairly scrawny appearance in forested areas. With its excellent orange-red fall color, it’s also much more colorful than Norway maple and less subject to winter damage. Plus its smaller, rapidly decomposing leaves have less tendency to choke out grass. Finally, it isn’t subject to tar spot disease, this disease which turns the leaves of Norway maple into an unsightly mess. Both, however, do have dense and shallow roots: there’s no denying that maintaining a perfect lawn under either maple is a challenge, though. Height: 60 ft (18 m). Spread: 40 feet (12 m). Hardiness zone: Zone 4. There are several horticultural selections, including ‘Green Mountain’ and ‘Legacy’, which offer a more regular habit on a somewhat smaller tree than seed-grown sugar maples.
Common Hackberry (Celtis occidentalis): This North American tree looks a bit like an elm but with a more rounded crown. Corklike bark. Yellow color in autumn. Height: 65 feet (20 m). Spread: 50 feet (15 m). Hardiness zone: 4.
Ginkgo (Ginkgo biloba): Very slow growing, but totally resistant to insects and diseases. Attractive yellow foliage in the fall. Always ask for a male specimen: the females drop messy, stinky fruits. Height: 45 feet (14 m). Spread: up to 40 feet (12 m), but much narrower than wide for the first 60 years or so. Hardiness zone: 4.
Honey Locust (Gleditsia triacanthos inermis): Tree with an open, often irregular crown that lets sun through. Rough bark. The compound leaves are composed of leaflets so small that they decompose quickly: you don’t even have to rake them up in the fall! The extremities of the branches often freeze during the winter in colder climates, but that doesn’t really affect its appearance. Look for the cultivars ‘Moraine’ and ‘Skyline’, as several honey locusts, like ‘Sunburst’, are too small to make good shade trees. Height: 65 feet (20 m). Spread: 35-55 feet/10-17 m. Hardiness zone: 4b. ‘Northern Acclaim’ is an extra-hardy variety: zone 3.
Amur Cork Tree (Phellodendron amurense): Little known, but very attractive and virtually without cultural problems. The bark on mature specimens is very corklike. Height: 40 feet (12 m). Spread: 40 feet (12 m). Hardiness zone: 3.
Siberian Pear (Pyrus ussuriensis): A large tree that dwarfs its smaller fruit tree cousins: apples, plums, cherries, etc. It’s also essentially immune to most of the diseases and insects afflicting fruit trees. It blooms abundantly in spring, covering itself with white blossoms, but its tiny fruits are of no interest to humans, although they do attract birds and small mammals. It will only bear fruit if there are two different clones in the area, since cross-pollination is obligatory for this species. Height: 40 feet (12 m). Spread: 33 feet (10 m). Hardiness zone: 3.
Oaks (Quercus spp.): A large group of trees, most tall and spreading, generally with toothed leaves. They’re considered among the most majestic of trees, but their growth is fairly slow, at least after the first 10 years or so. Height: 65 feet (20 m). Spread: 50 ft (15 m). Hardiness zone: generally, zone 4. Red oaks (Q. rubra) and scarlet oaks (Q. coccinea) are particularly interesting for their massive shape and fall color. Where space is limited, consider columnar English oak (Q. robur ‘Fastigiata’) which reaches the same height as the other oaks but rarely exceeds 13 feet (4 m) in diameter. The best oak for cold climates is bur oak (Q. macrocarpa), hardier than the others: zone 3. Shingle oak (Quercus imbricaria) is unusual among hardy oaks in that it has narrow untoothed leaves.
The following trees may be useful in some cases … but have problems that can seriously reduce their usefulness under certain circumstances. It’s up to you to decide whether they are worth growing under your conditions!
Horse Chestnuts, Hickories, Walnuts (Aesculus spp,. Carya spp., Juglans spp.): They make beautiful trees, but their large fruits can be an annoyance, especially near roads. In addition, walnut trees are allelopathic (toxic to plants that grow at their base).
Birch (Betula spp.): Most make very nice trees, with attractive bark, but they’re often short-lived (especially silver birch [Betula pendula] and its varieties) and rarely make it to their full size. In addition, they are susceptible to a wide range of diseases and insects and that can mean a lot of spraying … under some circumstances. Ask a local arborist their opinion, as the problems tend to vary greatly, from minor to major, depending on local conditions. One exception is river birch (B. nigra), especially the cultivar Heritage (‘Cully’), with bark that exfoliates gracefully: it’s long-lived and disease- and insect-free under most conditions. Height: 50 feet (15 m). Spread: 35 feet (10 m). Hardiness zone: 3.
Catalpa (Catalpa spp.): Although catalpas survive in cold regions and thus some nurserymen rate them as zone 4 trees, in fact, they tend to suffer severe winter damage in zones 4 and 5, at least periodically, and, as a result, grow very irregularly. With their abundant white to lavender blooms, they make an excellent choice in areas 6 and up, though. Height: 50 feet (15 m). Spread: 30 feet (9 m).
Linden (Tilia spp.): Tree with a strong trunk and heart-shaped leaves, plus highly scented flowers. The little-leaved linden (T. cordata) is very popular and offers many interesting cultivars, however … this genus is not a good choice in regions infested with Japanese beetles, as they literally defoliate the tree every summer. Height: 100 feet (30 m). Spread: 80 feet (25 m). Hardiness zone: 3.
Elm (Ulmus spp.): American elm (U. americana) almost inevitably falls victim to Dutch elm disease, which is difficult and expensive to fight. There are, however, several elms, including hybrid varieties, which share the American elm’s majestic upright spreading habit while showing good resistance to the disease. Before buying an elm, always ask if it’s resistant to Dutch elm disease. The Siberian elm (U. pumila) is resistant to Dutch elm disease, but is a weak-wooded tree with a poor growth habit and susceptibility to other diseases. Plus it self-seeds excessively and is considered an invasive species in many areas.
Trees to Avoid at All Costs
Ash (Fraxinus spp.): The arrival of the emerald ash borer (Agrilus planipennis), a deadly tree-piercing insect, in North America—and even now in Europe—has killed pretty much any interest in this formerly popular street tree. You might want to maintain the ones you have, but it’s probably wise to avoid planting new specimens.
Poplars, Willows, Silver Maple (Populus spp., Salix spp., Acer saccarhinum): The roots of these fast-growing trees are extremely invasive and often cause damage to domestic and municipal water and sewage pipes. They often also sucker extensively or self-sow and so become very invasive. It’s illegal to plant these trees in most municipalities.
There you go: a list of big and beautiful trees you might want to consider growing. And don’t delay, as it will take a few years before you can savor their full effect on your property!
A pheromone trap is an insect trap that gives off pheromones (chemical substances similar to hormones) used to attract the insect being controlled. Pheromones can mimic the smell of an insect’s favorite food (fruit, flower, etc.), but more often the pheromones used in traps are a bit more libidinous than that: they imitate insect sex pheromones, normally that of the female pest. Thus, the males travel from afar, attracted by the smell of what they think is an attractive young virgin. Once they enter the trap, though, the males can’t get out and therefore can’t impregnate any females, leading to, at least in theory, a drop in the local insect population.
To make the trap even more effective, it is usually colored yellow, blue, green, or purple, depending on the favorite color of the insect.
A Trap for Each Pest
Pheromone traps are very specific: each is designed to attract a particular type of pest. There is therefore no danger they will trap beneficial insects. You therefore have to purchase a different trap for each insect you’re trying to control.
The possibilities for pheromone traps are almost limitless, but for the moment, only one is widely available in most areas in North America: the Japanese beetle trap. It is, of course, designed to repress Japanese beetles (Popillia japonica). It actually contains two pheromones: a sex pheromone that mimics the smell of the female Japanese beetle, which therefore attracts male beetles, and another that gives off a floral scent that attracts both sexes.
Because of the floral pheromone, the Japanese beetle trap can also be used to catch rose chafers (Macrodactylus subspinosus), a Japanese beetle relative. However, there is also a specific trap for rose chafers that only gives off the floral scent. It seems to be more difficult to find in local stores.
Also on the market are apple maggot traps, usually shaped like a red ball that resembles a mature apple. Some models include contain a pheromone: a fruit essence that attracts the apple maggot (Rhagoletis pomonella).
Trap for emerald ash beetles.
While the home gardener has only a limited choice of traps, farmers and foresters have access to a wide range of pheromone traps for just as wide a range of crop pests. You may, for example notice, traps placed in ash trees (Fraxinus spp.) In a park in your municipality. With this trap, authorities try to determine whether the dreaded emerald ash borer (Agrilus planipennis) is lurking in the area.
The Downside of Pheromone Traps
When the trap is too full, the insects can’t enter.
But there is one major flaw with pheromone traps. While they do indeed attract insect pests to the area, but the latter don’t all enter the trap! Sometimes they simply miss the trap because the scent is carried elsewhere by the wind. Or the trap may already be full.
Whatever the reason, the result is that pheromone traps often actually don’t reduce insect damage. The insects that didn’t enter the trap, now starving, flock to the nearest available food plant and start to chow down. This is sadly the situation with Japanese beetle traps: they do catch beetles and lots of them, but they also draw more beetles into the sector, so instead of the damage being reduced, it is often worse.
The joke usually proffered is to buy traps and offer them to your neighbors so the beetles will go to their garden instead of yours! That really would work, but I suspect your neighbours would be a bit upset when they find out!
Effective Use of a Japanese Beetle Trap
You can however use the trap effectively if you follow three simple rules:
Place Japanese beetle traps well away from the plants they eat (at least 50 feet/15 m). For example, on a pole in the middle of a lawn.
Empty the traps regularly. Sometimes you have do it every day, otherwise they fill up and new insects can’t get in. Just dump the pests into a bucket of soapy water.
Collect beetles daily from nearby vegetation, preferably early in the morning when they are not very active, using a hand vacuum (empty the vacuum afterwards over a bucket of soapy water). Children, especially, seem to find collecting beetles with a vacuum a lot of fun.
If you start using this combined method of insect control at the beginning of the season, you can make serious inroads into reducing the infestation.
It keeps appearing in the press and is spreading on the Internet: the rumor that our lilacs could be devastated by the emerald ash borer (Agrilus planipennis). But it simply isn’t true. I’ll explain why in a second, but first a short history.
Ashes (genus Fraxinus) are the main hosts of the emerald ash borer, a piercing insect accidentally imported from Asia and Eastern Russia and first discovered in Michigan in 2002. It will attack all true ashes, but rarely kills Eurasian ashes. It will not attack mountain ashes (Sorbus spp.) either, as they are not true ash trees and are not even related to ash trees.
North American ashes are the most susceptible to this insect and inevitably die from the infestation unless very expensive treatments are applied. The pest has eliminated tens of millions of ash trees so far and is spreading across North America, so tens of millions more are expected to die of the infestation. So owners of North American ash trees should indeed be concerned.
But where does the rumor the pest can also attack lilacs (Syringa spp.) come from? From a supposition that went a bit too far.
You see, lilacs and ashes are in the same plant family, the Oleaceae, as are, by the way, the olive tree (Olea europaea), privets (Ligustrum spp.), forsythias (Forsythia spp.) and jasmines (Jasiminum spp.), among others. And sometimes an insect specific to a certain plant can make the jump to a close relative.
White fringe tree (Chionanthus virginicus)
This has happened in the case of the emerald ash borer. It has been found on the white fringe tree (Chionanthus virginicus) and a few specimens have died from the attack. However, it has shown no sign of a general move to fringe trees and indeed, in general larvae that do move to healthy fringe trees quickly die before doing any serious damage. They only seem to cause damage to weak or dying fringe trees.
Common lilac (Syringa vulgaris)
The fringe tree is sometimes grown as an ornamental, but not nearly to the extent of lilacs. The latter are among the most popular garden shrubs. And the emerald ash borer has been tested on lilacs (as well as on other ash relatives, such as forsythia) and it simply cannot survive on them. Even when given no other choice of food species, borers quickly die when placed on lilacs.
And if they did attack lilacs, we certainly ought to know by now, as many lilac species are grown in Asia and Russia, where the emerald ash borer comes from, and in fact quite a few are native there. The fact that the emerald ash borer has never been a problem on lilacs growing inside its native range should already be a sign that this “problem” is in fact a tempest in a teapot.
So let’s quash that rumor once and for all: the emerald ash borer is a disaster for North American ashes, may sometimes attack weakened fringe trees, but it will not harm lilacs. So lilac lovers can rest in peace: their precious shrubs are in no danger whatsoever from this new pest.