A Familiar and Easily Overlooked Intruder

Contact: Bud Reaves, Anne Arundel County | IPREAV00@aacounty.org

Norway Maple Foliage
Photo: Paul Wray, Iowa State University, Bugwood.org
Norway maple leaves excrete an indicative milky-sap from the base of petiole. Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

Norway maple, Acer platanoides L., was first introduced to the eastern United States in the mid to late 1700’s as an ornamental. and has become one of the most widely planted shade and street trees. The tree has been found here for so long most folks believe it is a native. Norway maple is a species of maple native to eastern and central Europe and southwest Asia. The wood is hard with no heartwood and is used for furniture and other wood products. A handsome tree, Norway maple grows to about 40-70 feet tall and has large leaves and distinctive bark. The leaves on Norway maple are very similar to those of the native sugar maple, Acer saccharum. They can be distinguished from sugar maples leaves by the fine hair like bristles at the end of the lobes and the milky sap that is exuded when the petiole is detached. The fruit is a samara that is 3-4 inches long. The two sides of the samara come together at a 180 degree angle; sugar maple samaras have about a 60 degree angle and resemble a horse shoe. The seed is very flat on Norway maple whereas native maples have thick seeds. The bark on older trees is distinctly furrowed with interlacing ridges, often turning a sooty black, whereas sugar maple has platy bark. Norway maple leaves turn a bright yellow in the fall, after most trees have lost their leaves. There are numerous cultivars including the very popular ‘Crimson King', 'Schwedleri', which has dark purple leaves and ‘Drummondi’ which has variegated leaves.

Cold weather appears to be the limiting factor for Norway maple’s growth or spread. Norway maple can potentially be found in the United States in the hardiness zones 4- 7. These zones stretch from New Brunswick and Cape Breton Island in Canada west to Minnesota and south to Tennessee and North Carolina. Maryland lies completely within these zones. Not all states are reporting Norway maple as invasive; however this covers the area in which Norway maple can potentially invade. In the West it is found in British Columbia, Washington, Idaho, and western Montana.

Fruit
Paul Wray, Iowa State University 
Bugwood.org

Norway maple is a very shade tolerant tree and prolific seeder that will rapidly invade roadsides and waste places, hedgerows and roadside thickets and will spread into adjacent forests. The seeds are persistent in the leaf layer of the forest and can germinate over several years after the parent trees are removed. Once established, Norway maple has demonstrated growth rates 2-3 times that of native trees growing in the same place. Although sometimes cited as alleopathic, research has shown it is not. However, Norway maple is very effective at keeping competing vegetation in check with dense shade, thereby reducing the growth of other species’ seedlings and changing wildlife habitat. In the native forest, sugar maple and American beech are the only hardwoods as tolerant of shade.

Mature Bark
Bud Reaves, AA Co. Forestry Board

The bad news about Norway maple is it has been planted extensively as an ornamental and can be found throughout Maryland. The good news about Norway maple is it is relatively easy to control if found invading forestland. The seedlings can be easily pulled by hand when the soil is moist. Saplings can be pulled if a weed wrench or similar device is used. If the ornamental value of an individual tree is substantial, the tree can be effectively managed if seedlings are diligently removed. Large Norway maples have a tendency to sprout vigorously when cut down so an appropriate herbicide is recommended if removal is required. Talk to a local forester or extension agent for recommendations, and as always follow the label directions and use appropriate cautions when using any pesticide. Norway maple has few disease problems but is fed on by many defoliating caterpillars. The Asian Longhorn Beetle (see the July 2009 Invader of the Month) is a serious pest to Norway maple, however it is a serious pest to many other desirable trees too, so no help from those herbivores is welcome!

Of course, the easiest way to prevent this seemingly benign and therefore overlooked invader from spreading is to not plant the tree in the first place. There are many native alternatives such as red maple, sugar maple, black gum, American beech, sourwood and the many oaks that make attractive shade trees. Do your local forests a favor and turn to native canopy trees for your shade.

For more information about this and other Invasive Species of Concern, visit www.mdinvasives.org

photos available electronically on request.