Sometimes when you try to solve a problem, you create a new one. That’s what happened in the 1950s and 1960s with the massive introduction of Norway maple (Acer platanoides) in eastern North America. At that time, North America’s most popular street tree, the American elm (Ulmus americana), was being decimated by Dutch elm disease, and people were looking for a replacement. Norway maple, although largely untested, quickly became the street tree of choice. It was planted by the hundreds of thousands throughout North America and is still the tree most commonly sold in garden centers in most areas.
It is not an unattractive tree: relatively fast-growing, with dense, dark green foliage (or purple or variegated foliage in some of its many cultivars) and a thick trunk. It is also very tolerant of road conditions: air pollution, compacted soil, road salt, etc. It certainly looked like a good choice…. at first. However, as time went on, it showed more and more weaknesses: Its dense root system lifts and destroys sidewalks, it kills lawns, and it suffers severe fractures during storms, causing millions of dollars in damage. Also, for unknown reasons, it is proving to be relatively short-lived in North America: only about 50 to 60 years compared to up to 250 years in Europe. But who thinks that far ahead when planting a tree?
The main problem, however, is its invasive nature.
Who would have thought? In Central and Eastern Europe, where it is native, Norway maple is not an invasive species. It simply blends into the local forest, one tree among many. In eastern North America, however, its seeds fall everywhere and germinate in a wide variety of conditions. And unlike most other introduced invasive species, which tend to remain in disturbed habitats, Norway maple quickly invades and begins to dominate local forests, displacing native trees and especially its North American counterpart, sugar maple (A. saccharum).
Sugar maple is the dominant tree in much of the deciduous forests of eastern North America, from Lake Superior to Pennsylvania (even farther south in mountainous regions) and from the East Coast to the prairies. It is known as the source of maple sug… and for the brilliant fall color that attracts so many tourists to the area in October.
War of the species
Norway maples produce huge amounts of seed, much more than sugar maples, and their seeds, carried by the wind, tend to move into maple forests where they grow easily. Their seeds are capable of germinating in deep shade, even more so than sugar maples, although the latter are also very shade tolerant. Norway maple seedlings grow quickly and densely and usually shade the sprouting sugar maple seedlings. They are more heat tolerant than young sugar maples and thus better able to survive a climate affected by global warming. As they grow, Norway maples produce more shade and drier soil conditions than native forest species can tolerate, creating forests without understory… except for more Norway maple seedlings! The fear is that if Norway’s maple expansion in the wild continues to be tolerated, a dense monoculture will develop that will wipe out entire North American ecosystems.
The signs of this are already evident. In many neighborhoods, Norway maples are sprouting lushly from hedges, along property lines, and popping up in shady borders. When maintenance is neglected in local parks, they move in and take over any areas not regularly trimmed by lawnmowers. And where urban and suburban meet native woodlands, Norway maple is usually already established and slowly working to become the dominant tree.
In natural forests in Montreal’s Mount Royal Park, for example, more Norway maple seedlings were found in 2003 than sugar maple seedlings, and it was estimated that the species would dominate the park’s forest “within a generation.”
Some agencies have “seen the light” and banned Norway maple. In Massachusetts and New Hampshire, for example, it is illegal to plant them, and many communities have similar laws. One major retail chain, Meijer Garden Centers, has voluntarily removed the plant from its stores. Unfortunately, I am not aware of any other nurseries that have followed suit, and Norway maples remain available in most regions where they continue to be widely planted.
It would be nice if more governments would ban the tree…. but they seem to be turning away from ecological issues these days. And I have to admit to being amazed and puzzled that garden centers continue to produce and promote Norway maples in areas where they could easily invade native forests. The whole problem would be greatly reduced if customers were simply offered other trees instead. Most probably wouldn’t even notice the difference! I don’t think homeowners go to a nursery looking for a Norway maple; they’re looking for a “shade tree,” period, and would happily choose from any species offered. There would be no need for a government ban if garden centers were more considerate of the environment!
Help from the strangest places
Oddly enough, a plant disease could become a major player in the fight against invasive Norway maple.
Tar spot disease (Rhytisma acerinum), a disease-specific to Norway maple and certain other maple species (but not sugar maple*), has been present in North America since at least 2000 and has spread across the eastern part of the continent, especially in the last five years. It causes disfiguring, dark black spots on the leaves of Norway maple and makes the entire tree unsightly from mid-summer through fall. There is no known effective treatment for this disease other than the removal of the infested tree. However, there is not a shred of evidence that picking up and destroying every single infested leaf in the fall (it only takes one to cause an outbreak of the disease the next year! For more information, see Yes, you can compost diseased leaves.
The more gardeners see this eye-catching disease that has turned entire city streets into a scene from a horror movie, the more likely they are to think twice about buying a Norway maple. Unfortunately, new leaves appear healthy, so during the peak season for selling trees, spring, even seriously affected trees show no symptoms. However, as tar spot disease gets more press and information circulates that the only real solution is not to plant Norway maples, this can only help reduce Norway maple sales.
So, spread the word. Tell friends and neighbors not to plant Norway maples, and if you have that much clout, try to get your local government to ban the planting of these trees. Also, express your disapproval to the manager every time you see them at a local garden center. These measures may be small and late, but at least they are a step in the right direction.
How to tell the difference between sugar maples and Norway maples
If you want to eliminate Norway maples in a park or on private property in an area where sugar maples are native, you need to know how to tell the two species apart. And indeed, they are very similar in appearance. However, here are some tips on how to tell them apart.
- Break a leaf stem in half. If the sap is clear, it is a sugar maple; if it is milky, it is a Norway maple.
- Both have leaves with 5 pointed lobes, but if you look closely, the sugar maple’s leaf tip is rounded, while the Norway maple’s is finely pointed. You do have to look very closely!
- Both are easy to tell apart in the fall! The sugar maple turns orange or red relatively early (in October); the Norway maple stays green for a long time and its leaves turn yellow before they fall off.
- In winter, study the buds. They are brown and pointed in the sugar maple, purple-green or purple in the Norway maple, shiny and somewhat rounded.
- On older trees, the bark of the sugar maple is flaking, while that of the Norway maple is finely grooved.
Finally, the samaras (winged seeds) of the two do not look alike at all. Those of the sugar maple are spherical and with narrower wings that are almost perpendicular to the seed, while those of the Norway maple are flattened and the wings are broad and almost in a straight line.