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Norway maple: ornamental but an invader

October 20, 2009
By HILARY SMITH

As autumn wanes and winter arrives, the seasonal change provides landowners and managers with some reprieve from invading plants and animals. Ice and snow pack may limit biological activity but deep in the water invasive aquatic plants over-winter until ice-out. In the ground the roots of terrestrial perennials lay in waiting for the spring thaw. Already burrowed into their living hibernacula, forest pests remain still until the flush of spring and then begin their exit strategy.

At least one invasive, however, is still on the scene for a few more weeks. While many native trees have dropped their leaves, invasive trees, such as Norway maple (Acer platanoides), tend to sprout earlier than their native counterparts and hang on to their leaves later. Norway maple leaves are the last to change color in the fall; they remain green until October then turn bright yellow. Late fall is the best time to survey for Norway maples as they are very conspicuous at that time.

Norway maple is a large, deciduous tree with a broad, rounded crown and bark that is smooth and gray-brown. The tree can reach heights of 90 feet with a trunk diameter of as much as 6-7 feet and a branch spread of 70 feet when growing in the open. It can be confused with many maples species, especially sugar maple, because of similar looking leaves, but can be distinguished by the presence of a milky white sap that oozes out of leaf veins and stalks when broken; however, late in the season this characteristic may be hard to detect.

Article Photos

Norway maple branch
(Photo provided)

The leaves are dark green, palmate (hand-shaped) with 5-7 lobes and about 6 inches wide and 4-5 inches long. Having leaves wider than they are long is another good distinguishing characteristic; sugar maple leaves are generally longer than wide. Yellowish-green flowers form clusters in the spring leading to abundant amounts of seeds in the fall. Wind distributes the winged fruits and the seeds germinate readily, even in dense shade. Trees also expand locally by vegetative reproduction.

Introduced from Europe in 1756, Norway maple was cultivated soon after and became a popular street tree frequently planted in urban and suburban areas. Like many invasive non-native plants, Norway maple did not emerge as a problem until many years after its initial introduction. It wasn't until the early 1900's that plant identification manuals began to note that it "occasionally escaped."

Today, Norway maple is a frequent invader of forests and fields. Its extreme shade tolerance, especially when young, has allowed it to penetrate beneath an intact forest canopy. Forests invaded by Norway maple suffer losses in diversity of native wildflowers compared with forests in which the canopy is dominated by native species such as sugar maple. It can eventually out-compete native trees and create monotypic stands that change the structure of forest habitats. This is at least in part due to the dense shade cast by Norway maples and the shallow roots, which compete with other vegetation.

To control existing stands, one can use manual, mechanical and chemical methods. Seedlings can be pulled by hand and small to large trees can be cut to the ground, repeating as necessary to control any regrowth from sprouts. Glyphosate and triclopyr herbicides have been successfully used.

Unfortunately, Norway maple is still widely available at nurseries. Consider the following native trees as alternatives for landscape use: red maple (Acer rubrum), sugar maple (Acer saccharum), American basswood (Tilia americana) and red oak (Quercus rubra).

This eye on invasives will take a respite and start looking again next spring.

'Eye on Invasives' is a biweekly column that spotlights a top invader when it is easiest to identify. The Adirondack Park Invasive Plant Program is a partnership program housed at the Adirondack Chapter of The Nature Conservancy in Keene Valley, NY. Find out more about this award-winning program online at www.adkinvasives.com.

 
 

 

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