Reconsidering an old standby: Norway Maple

By Susannah Wood

Norway maple (Acer platanoides) has been growing in the United States since John Bartram brought it to Philadelphia in the mid-18th century. It grows all across Europe from France to Russia and south to Iran. Today, it grows all across the United States as well. The Norway maple has characteristics which make it an ideal urban tree. It tolerates a wide range of soils from acidic to alkaline, poor, compacted ground to fertile and loamy. The toxic cocktail of car exhaust doesn’t seem to bother it either, and, as elms disappeared off Main Streets across America, Norway maples seemed the perfect replacement. Unfortunately, a tree that has worked so well in cities poses real threats to native forests, where the Norway maple outcompetes our native maples. Norway maple flowers appear earlier in spring than those of native trees, giving it an advantage in early pollination. It leafs out earlier and hangs on to its leaves longer than native maples, producing such a dense shade that native wildflowers are suppressed along with other seedlings. Its shallow root system draws nutrients and water away from surrounding plants. It produces thousands of seeds, which fly on the wind and germinate far from the parent tree. The seeds, more abundant than that of sugar maples and red maples, remain viable for a number of years in the leaf litter, and the saplings are shade-tolerant, allowing them to dominate the forest understory. While native maples support a wide range of native insects and wildlife, the Norway maple contributes much less to wildlife biodiversity. To identify a Norway maple, look at its leaves and bark. While the leaves resemble those of native sugar maples, they are easily distinguished close up. Break open the stem of a Norway maple leaf and you’ll see a milky sap ooze out. Sugar maple sap is clear. The bark of a mature sugar maple roughens into shaggy, flat plates. The Norway maple bark remains smoother, more evenly grooved. This time of year, while sugar maples shift from gold to orange and red, Norway maples are mostly a pale yellow. You can control Norway maple by pulling out small seedlings as they appear, by mowing and brush-hogging, and by girdling larger trees in spring (making sure to cut through the outer cambium layer). You can also apply a foliar herbicide containing glyphosphate (like Round-Up) in early spring or later fall; or by painting a cut stump with triclopyr (Brush-B-Gone) or a solution of Round-Up (1:1), which is best done from fall to spring. Note that Round-Up comes in different strengths depending on the job to be done. Most sources seem to agree that these two chemical herbicides (glyphosphate and triclopyr) are relatively safe for the environment as they break down within several months. You should always wear eye protection, gloves and long sleeves when applying these chemicals. In replacing a Norway maple with something else, you have many choices: sugar and red maples, American basswood, black gum, oaks, and tulip tree to name a few. The following sites provide further information: http://www.fs.fed.us/na/durham/coopforest/stewardship/text/invasivess.shtml http://www.hort.uconn.edu/cipwg/art_pubs/GUIDE/guideframe.htm