Norway Maple: the Great Invader!

Sometimes when you try to solve one problem, you create another. That’s what happened in the 1950s and 1960s with the massive introduction of the 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 substitute. The Norway maple, although largely untested, quickly became the street tree of choice. It was planted by the hundreds of thousands across North America and remains, in most areas, the tree most commonly sold in garden centers.

It’s not an unattractive tree: relatively fast-growing, with dense, dark green foliage (or purple or variegated foliage in the case of some of its many cultivars) and a thick trunk. Also, it’s very tolerant of street conditions: air pollution, compacted soil, road salt, etc. It certainly looked like a good choice… at first. Over time, it showed more flaws: its dense root system uplifts and destroys sidewalks, it kills lawns and it suffers severe breakage during storms, causing untold millions of dollars of damage. Also, it turns out, for reasons unknown, 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?

But the main problem turns out to be its invasive nature.

Who knew? In Central and Eastern Europe, where it is native, Norway maple is not an invasive species. It simply melds into the local forest, one tree among many. In Eastern North America, though, its seeds fall everywhere and germinate under a wide variety of conditions. And, unlike most other invasive introduced species, which tend mostly to stick to disturbed habitats, Norway maple quickly moves into and starts to dominate local forests, outcompeting native trees and especially its North American counterpart, the sugar maple (A. saccharum).

The sugar maple is the dominant tree in much of eastern North America’s climax hardwood forest, from Lake Superior to Pennsylvania (even further south in mountainous regions) and from the Eastern seaboard to the Prairies. It is well known as the source of maple sugar… and of the brilliant fall coloration that attracts so many tourists to the area in October.

 War of the Species

Norway maples produce huge seed crops, much greater than sugar maples, and their seeds, carried by the wind, tend move into maple forests where they grow readily. Their seeds are capable of germinating in deep shade, even more so than those of the sugar maple, although it too is highly shade tolerant. Norway maple seedlings grow quickly and densely and generally shade out any sugar maple seedlings that do sprout. They are more heat-tolerant that young sugar maples and thus better able to survive a climate influenced by global warming. As they grow, Norway maples create greater shade and drier soil conditions than native woodland species can tolerate, creating woodlands barren of undergrowth… other than more Norway maple seedlings! It is feared that, if the spread of Norway maples into the wild continues to be tolerated, they will create a dense monoculture, eliminating entire North American ecosystems.

You can see the signs already. In many neighborhoods, Norway maples sprout abundantly from hedges, along property lines, and pop up in shade borders. When the care in local parks is neglected, they move in and take over any areas that are not regularly clipped by lawn movers. And where city and suburb meet native woodland, the Norway maple is usually already established, slowly working on becoming the dominant tree.

In natural forests in Montreal’s Mount Royal Park, for example, Norway maple seedlings were found to outnumber sugar maple seedlings 4 to 1 in 2003 and it was estimated the species would dominate the park’s forest “within a generation”.

Some authorities have “seen the light” and banned Norway maples. It is illegal to plant them in Massachusetts and New Hampshire, for example, and many municipalities have similar laws. One major chain store, Meijer Garden Centers, has voluntarily removed the plant from its stores. Unfortunately, I know of no other nurseries that have followed suit and Norway maples remain widely available in most regions where they continue to be planted on a large scale.

It would be nice if more governments banned the tree… but they seem to be turning away from ecological issues these days. And I must admit I’m amazed and puzzled that garden centers continue to produce and promote Norway maples in areas where it could easily escape into native forests. The whole problem would be reduced considerably if shoppers were simply offered other trees instead. Most probably wouldn’t even notice the difference! I don’t think home owners set off to a nursery looking for a Norway maple; they’re looking for a “shade tree”, period, and would happily choose among whatever species are offered. No government ban would be needed if garden centers showed more concern about the environment!

Help from the Strangest Place

Oddly, a plant disease may become a major player in the fight against the invasive Norway maple.

Tar spot disease (Rhytisma acerinum), a disease specific to Norway maples and certain other maples (but not the sugar maple*), has been in North America since at least the year 2000 and has spread throughout most of eastern part of the continent, especially over the last 5 years. It causes disfiguring dark black spots on Norway maple leaves, making the entire tree unattractive from midsummer until fall. No effective treatment for this disease is known other than removing the affected tree. Municipalities have been spouting nonsense about raking up and destroying leaves to reduce the effect of tar spot, but there is not one iota of proof that striving to pick up and destroy every single diseased leaf in the fall (it takes only one to cause the following year’s outbreak!) does anything other than annoy gardeners. For more information, see Yes, You Can Compost Diseased Leaves.

*There are other tar spot diseases that target the sugar maple, but they are not very common and tend to appear only sporadically and on a much more limited basis.

The more gardeners see this striking disease, which has turned entire city streets into something like 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 prime season for selling trees, spring, even seriously contaminated trees show no symptoms. Still, as tar spot disease gets more and more press and the information circulates that the only real solution is avoid planting Norway maples, this can’t but help reduce Norway maple sales.

So, spread the word. Tell friends and neighbors not to plant Norway maples and, if you have that kind of influence, try to get your local government to ban planting them. Also, express your disapproval to the manager each time you see them in a local garden center. These measures may be little and late, but at least they’re a step in the right direction.

How to Distinguish Between Sugar Maples
and Norway Maples

If you want to eliminate Norway maples from a park or private lot in an area where the sugar maple is native, you have to know how to distinguish between the two species. And indeed, they are very similar in appearance. However, here are some tips on how to tell them apart..

1. Snap a petiole (leaf stem) in half. If the sap is clear, it’s a sugar maple; if the sap is milky, it’s a Norway maple.
2. Both have leaves with 5 pointed lobes, but if you look closely the very tip of the leaves, that of the sugar maple is rounded while that of the Norway maple is finely pointed. You have to look very closely!
3. Both are easy to distinguish in the autumn! The sugar maple turns orange or red relatively early (in October); the Norway maple remains green for a long time and its leaves turn yellow before falling.
4. In winter, study the buds. They are brown and pointed on sugar maples, purple green or purple, shiny and somewhat rounded on Norway maples.
5. On older trees, sugar maple bark is exfoliates while that of the Norway maple is finely grooved.

6. 

   Finally, the samaras (winged seeds) of the two do not look alike at all. Those of the sugar maple are globular and with narrower wings placed almost at right angles to the seed, while those of Norway maple are flattened and its wings are wide and almost in a straight line.