What is it?
The Norway maple is an invasive deciduous tree native to Eurasia, the hardwood of which is commonly used in furniture and other products. Music lovers may be interested to know it may have been one of the woods used to produce the famous Stradivari violins!
First introduced to North America in the mid-1700s as an ornamental tree, the Norway maple’s popularity exploded following the second world war after American elms (Ulmus americana), the then predominant ornamental native tree, largely disappeared due to disease. It remains favoured among gardeners and horticulturists to this day. Norway maples can be found from coast to coast across Canada, from British Columbia, through Ontario, and Newfoundland in the east, and as far north as the Yukon.
Chances are, readers of our blog have sat under the shade of these trees at least once in their lives, in the park, or by the sidewalk. The most noticeable feature of these alien Maples are their five lobed leaves, with many different cultivars famous for their rich colours: vibrant green, and purple in the summer, that become a rich red or yellow in autumn.
However, on closer inspection, you’ll find that the Norway maple is nowhere near as pleasant as our native Sugar maple (Acer saccharum). Instead of a sugary sap, when broken, the stems of the leaves of Norway maples release a white sap and the toothy leaves are more sinisterly jagged. Lastly, the most noticeable evidence of a Norway maple is its helicopter blade like seeds, called samaras, which fall in summer to coat your driveway. Yes, those oddly shaped seeds belong to them [the Norway Maple].
Why you should care
Due to its horticultural popularity, the Norway maple is rarely recognized as an invasive species; many even celebrate the autumn colours they bring to local parks. Nevertheless, they are gaining notoriety for damaging local infrastructure and increasing cleanup costs in municipalities. Being fast growers, these trees need much more regular pruning than other trees. Additionally, their brittle branches can easily snap in storms, leaving countless debris.
Worse, these trees are everywhere. Due to its popularity. for the last century, Norway maples have been planted in most cities across Ontario as an ornamental tree. Although its main means of spreading in North America is through human action, Norway maples can easily invade surrounding enclaves of native plants, by releasing its winged seeds, called samaras, that can be carried far by the wind and germinate quickly making it one of the few invasive species that can make inroads against the established forest canopy. This, along with its ubiquity has allowed other invasive flora and fauna to overrun reserves of native species making the Norway maple especially threatening.
The primary means by which the Norway maple damages Canadian forests is by outcompeting native species, usually through its higher tolerance to poor growing conditions. For example, it can survive in polluted urban areas and has a longer growing season. Additionally, at least one study found native sugar maples invaded by Norway maples were more vulnerable to insect and fungal damage, suggesting Norway maples can outlast their native competitors. There is even some evidence their leaves and roots may release toxins to kill off competitors. Once planted, their seedlings can grow into dense mats, draining the soils of water. As they continue to grow, an expanding shallow root system and thick canopy can choke out competing native species such as sugar maples, and other plants.
What can we do?
Unlike many invasive species, Norway maples is an established species across North America. Therefore, unlike other invasive species, merely keeping an eye out for them won’t be much help, except when identifying newer infestations as it is always easier to remove new seedlings rather than established trees. Instead, efforts should be made to replace this alien with native trees and plants. However, to do so, it is crucial to recognize and eventually remove them. Indeed, some native maples such as the red maple (Acer rubrum) can look very similar to the more colourful non-native varieties of Norway maple, so be careful!
How to identify Norway maples
Although similar to many other native maple trees such as sugar maple, Norway maples can be distinguished by subtle differences on closer inspection. Consider these characteristics to help you figure out if what you’re looking at is a Norway maple…
- Norway maples can grow up to 20-30 m tall before ending in a round crown
- As a deciduous hardwood, Norway maples are covered in a grey-brown bark lined by criss-crossing grooves
- Like other deciduous trees, Norway maples first turn yellow or red in autumn, before losing their leaves in winter, eventually sprouting new blunt rounded buds in the spring
- The leaves have the distinct five lobed shape unique to maples with fine hairs at the tip of each lobe
- Each lobe has 2 – 3 jagged points but otherwise has a smooth edge
- The leaves grow in opposing pairs, each 10 – 18 cm wide, and 8 – 16 cm long
- When damaged at the stems, a milky white sap may leak
- Depending on the species, the leaves can be maroon (ex. Crimson King), purple (ex. Schwedleri), or green.
- From April to May, Norway maples can produce large umbel-clusters of small (~8-10mm) greenish-yellow flowers, each with five petals
- Although Norway maples do not produce fleshy fruit, each of the thousands of female flowers can play host to a pair of paper-like winged seeds called samaras
- These samaras also have larger and broader angled wing blades compared to native maple seeds
Can you remove Norway maple?
It is highly recommended that, if possible, any and all Norway maples on your property should be removed in favor of a native tree or plant. Saplings can be manually dug out, while larger trees must be cut down, preferably when seeds are not present.
Please note many municipalities require an application/tree removal permit prior to the removal of larger trees. Be sure to check the bylaws of your local government prior to any removal!
More developed trees are tougher to remove, requiring peeling back the bark with 4-inch-wide cuts in the spring and early summer to weaken it before cutting. Even leftover stumps cannot be allowed to persist as new growth can develop from these hardy maples. While powerful chemical herbicides can be used, always contact experts and consult with product labels while remaining within provincial and federal laws, and it is likely that several years of control will be required to completely remove Norway maple in this manner, with such herbicides most effective on seedlings or young saplings.
As always however, the easiest method of impeding the spread of this alien maple is early detection, whether that is on your property or at any gardening centre still selling Norway maple saplings. Seedlings are easiest to remove when young, and in wet soils. As Norway maples are most often spread by intentional planting of this attractive plant, you can refrain from buying and planting it. Don’t be fooled! Norway maple can be sold under differing trade names such as ‘Crimson King, ‘Emerald Queen’, ‘Schwedleri’ and many more.
Instead, plant native trees and plants. Excellent native trees that are similar in appearance to the attractive Norway maple include the Manitoba maple (Acer negundo), red maple (Acer rubrum), silver maple (A. saccharinum), and their hybrid Freeman’s maple (Acer freemanii), as well as the sugar maple (A. saccharum). The Freeman’s maples may be found under trade names like ‘Celebration’, ‘Autumn blaze’, and ‘Scarlet sentinel’.
Other alternatives include hackberry and serviceberry, both of are just as hardy and tolerant of urban conditions, with the latter even providing fruit!
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