Norway maple (Acer platanoides)
The Norway maple is a small to medium sized deciduous tree in the maple family. Native to Europe, it was first introduced to North America for cultivation as an ornamental tree. It and many of its cultivars (such as the crimson king maple) have become popular choices for urban tree plantings due to their tolerance of urban stresses and rapid growth. Because of this, it can now be found all across Canada and the US around urban centers.
Norway maples have opposite simple leaves with 5 to 7 lobes. The leaves are dark green and typically wider than long. The leaf stems and twigs contain a milky white sap that exudes from the stem when broken and squeezed.
When mature, the tree can reach heights of 20 – 30 m, and have a broad, rounded crown. The flowers are yellowish green and appear in stalked clusters in mid to late April. The fruit is a double samara (maple key) with wings at a nearly perfect horizontal (180 degree) angle. The seeds mature in September and are dispersed by wind.
Norway maples can be mistaken for native sugar maples, however sugar maple leaves have five lobes only, and are longer than wide. The sugar maple leaf stem sap is clear and the leaf buds are sharp pointed instead of round like the Norway maples.
Norway maples usually invade natural areas in and around cities because of their use in landscaping. Once established, Norway maples form a dense forest canopy that shades out most other species. The seedlings (which are highly shade tolerant) can form a thick mat on the forest floor that will further limit regeneration of other native trees and shrubs. Because few species can grow in the shade of a canopy of Norway maples, forest floor vegetation becomes more scarce, exposing bare soil and leading to increased erosion.
The best strategy to control the spread of Norway maples is to not plant them in the first place. When established, trees are best pulled when young, since they will quickly become too large to remove by hand, and will require cutting and treatment with a herbicide.