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The Invasive Tree that Threatens Our Forests

Updated: Mar 20, 2023

In the case of Norway Maples, taking down these trees actually helps the forest thrive.

By Liz Thomas, Native Plant Assistant

Last year, after hiring a local landscaper to plant several maple trees in her yard, my mom was horrified to come home to a row of “red maples” mulched into the lawn. No, she wasn’t referring to our native red maple but to the purple-leaved ‘Crimson King,’ a cultivar of the invasive Norway maple.

Norway maples have become ubiquitous throughout the Western New York landscape, where you’re likely to meet them flanking city streets, dotting suburban lawns, and crowning rural homesteads. Unfortunately, you’re also likely to find them in our forests and natural areas, where they invade native plant communities and eventually use the resources required by our local wildlife.

A Eurasian species, the Norway maple was first brought to the United States in the 1750s, but didn’t gain popularity until the mid- to late-19th century, and remains one of the most popular street trees in the Northeast and Midwest. This popularity has come with a cost: as with so many other plants brought into this country from abroad, it escapes our domestic landscapes to proliferate in our natural areas. In the case of the Norway maple, its wind-dispersed seeds travel into neighboring woodlands where they may shade out native species and continuously reproduce, spreading throughout the woods and destroying the diversity and integrity of the ecosystem.

While the Norway might not look very different from our other maples, it has some key differences which contribute to its invasive nature: faster growth, a denser and longer-persisting canopy (with an earlier leaf out time in the spring, and later leaf drop in the fall), and a shallower root system. These characteristics lead to a one-two punch of over-shading and nutrient/water competition, creating intolerable conditions for our native spring ephemerals, perennials, shrubs, and tree saplings, which all evolved to cohabitate with red, sugar, or silver maples instead. Of course, these adverse conditions don’t discourage its own more tolerant seedlings and saplings from thriving, effectively creating a positive feedback loop of invasion and degradation.

But wait, you ask, if they’re so invasive, why don’t I find purple-leafed maple trees in the woods? A good question, with an easy answer: the offspring of ‘Crimson King,’ Norway maples do not retain the purple leaf color of their parent tree (every ‘Crimson King’ tree is a clone from one original tree), and instead exhibit green leaves. Many trees that people see in the landscape and take for granted as sugar maples are actually regular green-leafed Norway maples, which are also still widely sold and planted.

To the untrained eye, the leaves of Norway maple might be mistaken for sugar maple, as they have a very similar shape and lobing pattern. But there are a few key differences you can look for to distinguish between the two species. See the graphic below.

At the Western New York Land Conservancy, we have been working hard to eradicate invasive plant species from our properties, including Norway maple. This past December, we hired a tree service to fell over forty Norway maples that had established themselves in the woodland around the office building at our Kenneglenn property. Many of these trees had trunks up to two feet in diameter! An even more dramatic example can be found in our restoration project in the Niagara Gorge, where we have been working with the State to reduce 10 acres(!) of Norway maple that had taken over in this special, protected habitat.

If these trees are so invasive, why are they still available for purchase? Sadly, even plants that are classified as “regulated invasive species” by the state are allowed to be sold, as long as they meet certain labeling requirements. If you already have a Norway maple planted in your yard, we don’t expect you to run out and chop it down, but you can still help by choosing not to buy one in the future. If you’re working with a landscaper to choose trees, you should request only native trees. Norway maples are chosen by landscapers because they are readily available and can tolerate harsh urban conditions, but there are plenty of other trees that are accessible and can thrive just as well, without the environmental repercussions.

If you’re not in a tough urban spot, consider one of our native red or sugar maples, which have the added benefit of a much better fall color than their Norway cousins. For urban areas, the native American elm and swamp white oak are attractive shade trees that can tolerate harsh planting sites. American elm cultivars such as ‘Princeton’ and ‘Valley Forge’ are highly resistant to Dutch elm disease, giving us the chance to reclaim this once-quintessential American street tree.

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