The Norway maple (Acer platanoides) is a large, hardy, beautiful shade tree. Unfortunately, it is also a non-native, invasive tree that is a real threat to our native forests. Some states ban Norway maples and it is classified as invasive in many others, including Pennsylvania.
The Norway maple is indigenous to Europe where it usually occurs as an understory tree. It was introduced to this country by known other than John Bartram, the 18th c. naturalist and botanist, and soon was planted in farms and towns because of its shade and its ability to grow in adverse conditions such as compacted and poor soil. Urban pollution does not bother this tree. The ability to withstand adverse conditions is one factor contributing to its invasiveness. Other factors include the huge number of seeds it produces and the high germination rate of those seeds. Saplings of native trees cannot grow under the heavy shade produced by the Norway maple. In addition to the shade, the roots of this tree grow close to the surface and tend to soak up most of the moisture near it. This results in very few plants that can survive underneath a stand of Norway maples, including grass and some weeds. Two plants that can are English ivy (non-native) and lesser celandine (non-native, invasive). The latter is in the buttercup family but its petals are more pointed than our native buttercup.
Norway maple infestation of our forests results in monotypic forests. It displaces our native trees and understory plants and eliminates animal habitats. This can be seen very plainly in our Park. Off the beaten track, there exist two groves of trees. This spring, on one side, I saw a stand of diverse native trees with a rich understory of plants including native wildflowers such as May apples and Trout Lillies. On the other side was a grove of only one type of tree, the Norway maple. The only plant growing under the Norway maple tree canopy was the lesser celandine, a non-native, invasive plant in the buttercup family. The lesser celandine had already jumped the path and invaded the other side. It is only a matter of time until the Norway maples do likewise.
However, I saw that several Norway maples along the perimeter of the Norway maple grove were girdled. Girdling is a method of slowly killing standing trees by cutting away a wide area of the wood and bark completely encircling the tree. Girdling interrupts the flow of nutrients between the roots and the crown. As the trees die, more sunlight will reach the ground and saplings of other trees will be able to flourish. After the trees die, the next group of trees closest to the dead ones can be girdled and so on. The environment in the vicinity will change slowly allowing plants and wildlife to adapt. Chopping the whole grove of Norway maples down would be very disruptive. Girdling is how the early settlers slowly cleared their fields of gigantic trees. One downside is the danger of falling dead branches and even the toppling over of the whole tree.
Landscapers still sell Norway maples and the seeds from these trees can travel to places where they do no good, like our Park or other forested areas. Don’t buy them. You may ask how to recognize a Norway maple. You can do it by looking at the leaves, the fruit and seeds, the color of the sap, or the bark of a mature tree. The leaves have very pointy tips on all the lobes and at the very end is a fine hair – you have to look very closely to see it.
The fruit is a paired samara. A samara is a type of fruit in which a papery wing develops from the wall of the ovary. Like other maples, the seed is on one side and the wing on the other. Other names for the samara are key, wingnut, and helicopter. The samara of the Norway maple differs from that of the native sugar maple by having the wings at almost a 180 degree angle and having flattened seeds. The sugar maple samara looks like a letter U and has rounded seeds.
If you break a leaf petiole (stem) off where it joins the twig of a Norway maple, you will see milky sap leaking out of the petiole. In a sugar maple, the sap is clear.
The bark of a mature Norway maple has intertwining grooves, while that of a mature sugar maple is somewhat shaggy.
Now that you can identify the Norway maple, you should be able to find one when you are out for a walk in your neighborhood or in the Park! There are some nice mature specimens in the Norris City Cemetery adjacent to the Park. Enter the stone gate across from the right angle turn in Stanbridge St (heading toward Norris City Avenue). Walk up the path to the top of the hill. On your way up on the left side, you will see a huge old sugar maple with its shaggy bark. At the top of the hill, look to your left and you will see a grove of Norway maples with their vertical neatly-grooved bark.