Non-Native Invasive Species

Norway maple

Acer platanoides

Norway maple

Norway maple
Photo: Jan Samanek, State Phytosanitary Admin, Bugwood.org

Sugar maple may be our signature tree, but in urbanized Vermont woods, sugar maple has increasingly been supplanted by Norway maple. It is such a problem that the state's Invasive Exotic Plant Committee lists it as a one of our most troublesome weeds. New York State too places the species on its primary list of invasive plants.

Norway maple leaves look a little like those of sugar maple from a distance. Up close, however, Norway maple leaves are wider, with blockier lobes. And Norway maple leaf stems exude a milky sap when broken, a feature absent in sugar maple. Fall colors are different too. Norway maple leaves turn yellow (never red or orange), and late in the season, weeks after sugar maple foliage has fallen.

Norway maple

Norway maple
Photo: Jan Samanek, State Phytosanitary Admin, Bugwood.org

Norway maple is an attractive and adaptable tree. It tolerates air pollution, light or heavy soils, and hot and dry conditions. These are desirable characteristics in a tree for urbanized settings, but it is just these traits which enable the species to quickly colonize new sites. Norway maple can create single species stands so shady and dry that few other plants can make a living there. One won't find hepatica or Canada mayflower or snakeroot or other native wildflowers in large numbers in woods dominated by Norway maple.

What to do about this invader? For a start, despite its good points we should all stop purchasing and planting this very popular tree. Norway maple moves from place to place mostly by seed; the fewer sources of seed the smaller the threat. (A note: this caution does not apply to cultivated varieties of the species such as the purple-leaved Crimson King, which do not appear to have invasive qualities. Only the species itself, Acer platanoides, is problematic.)

Norway maple

Norway maple
Photo: Jan Samanek, State Phytosanitary Admin, Bugwood.org

Where woodlots have already been overrun, small trees can be pulled with a tool called a Weed Wrench™, which easily removes woody stems up to two inches or so in diameter. Trees can also be girdled or simply cut down. Sprouts will need to be cut continuously until root resources have been exhausted.