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The leaves are opposite and palmately lobed with five coarsely toothed lobes; petioles are 8–20 cm long. The tree grows 20–30 m tall with a trunk up to 1.5 m diameter. The yellow to yellow-green flowers, with 5 petals and 5 petal-like sepals, are each 3–4 mm long and occur in corymbs of 15–30 together. The flower clusters, which appear in early spring before leaves have emerged, are larger and more conspicuous than in many North American maples. The fruit, produced in large amounts, is generally a pair of widely diverging samaras (hard nutlets with winged seeds); clusters of three samaras sometimes occur. Maples were classified in their own family, Aceraceae, but are now generally included in Sapindaceae (Stevens 2001)—see Systematics.
In the past 20–30 years, Norway maple has naturalized widely in North America, particularly in urban woodlots and forest edges (Barnes and Wager 2004, Cincotta et al. 2008), and can become dominant in mesic (moist) soils where Acer saccharum (sugar maple) would otherwise grow. Norway maple is classified as invasive in 20 Northeastern U.S. states (USFS 2011) and eastern Canada; it is banned from sale or planting in Massachusetts and New Hampshire. The species is, however, still available at nurseries in other states and continues to be widely planted.
Adverse ecological effects include inhibition of understory growth (including tree saplings) due to its densely shading canopy and its release of allelopathic (defensive) chemicals, so it tends to create bare, muddy run-off conditions immediately under the tree (Galbraith-Kent and Handel 2008; Swearingen et al. 2010). It suffers less herbivory and fewer fungal diseases than sugar maple, which may give it a competitive advantage over sugar maple (Cincotta et al. 2008).
A. platanoides is sometimes confused with A. saccharum (sugar maple) in North America, but they can be distinguished in most characters. Norway maple leaves are usually broader than they are long, while sugar maple leaves are generally longer than wide (or with length=width). Norway maple seeds are flattened and its samara’s wings are widely spread (to 180 degrees); sugar maple seeds are globose, with wings diverging at 45 to 90 degrees. Norway maple terminal buds are large, rounded, and blunt, with only 2–3 pairs of scales; sugar maple has long, sharply pointed buds with many scales. Bark of mature Norway maples has small, often criss-crossing grooves (reminiscent of Fraxinus americana, white ash), while sugar maple bark occurs in broader flat or shaggy plates. Norway maple autumn leaf color is usually yellow to yellow-orange, rather than the brilliant oranges and reds of sugar maple. Finally, Norway maple tends to leaf out earlier in the spring and hold its leaves later in the autumn than sugar maple.