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As I look out of the window, this is the tree I see.  It is on these limbs that I see the crows, ravens, doves, and blue jays landing.  I was not aware that I may be harboring (and loving) an enemy.
Norway maple or érable de norvège is a shade-tolerant deciduous tree in the soapberry (Sapindaceae) family, formerly in the Aceraceae family. Its broad native range is from Norway southeast to the Caucasus and northern Turkey.

John Bartram of Philadelphia first introduced Norway maple from England to the U.S. in 1756.
In Canada after World War II Acer platanoides were planted in cities and towns as a reliable and rapidly growing replacement for white elms (Ulmus americana) when they were largely eliminated as an urban street tree by Dutch elm disease.(1)
It has many desirable characteristics as a street tree’ tolerance to urban impacts, resistance to insects and fungal disease, low maintenance, ease of propagation, and an attractive appearance with a variety of cultivars.
The Norway maple escaped cultivation and spread out into fields, edge habitats, disrupted habitats, and mature forests. It quickly shades out other trees, shrubs and herbaceous species and establishes itself as the primary canopy cover. Since its seedlings and saplings grow well in the shade of the bigger seed-source tree, it forms monoculture stands that perpetually expand.  It is widely established in temperate North-Eastern America (USA and Canada) including Ontario, Nova Scotia and Newfoundland as well as in British Columbia (2).
Identification The Norway maple tree is a medium- to large-sized deciduous tree, with a straight trunk, spreading symmetrical crown, and a rounded appearance.  Mature trees (12-18m) can be up to 22m tall and diameter at breast height (DBH) ranges from 50-80 cm. (1)
Leaves are opposite, simple, 5-lobed, lustrous dark green above; lobes wide and sharp pointed, dentate with pointed teeth; (2) bright yellow in autumn (4).
Flowers: Yellow to yellowish-green, 7 mm in diameter, in clusters, flowers start before leaves in spring. (2) Photo: Jan Smaaneck, Phytosanitary Administration, via bugwood.org
Buds are very large, much larger than the buds of sugar and red maples (Acer saccharum & Acer rubrum). They are glabrous, and stocky and usually turn green to red with large scales surrounding them. The terminal bud is especially large and is an easy distinguish Acer platanoides from native maples in the winter (but not to be confused with the sycamore maple, Acer psuedoplatanus, another invasive species, which has a large terminal bud that remains green).
The petiole exudes a milky sap when pinched/picked in spring and fall and this is unique to the Norway maple: similar-looking maples do not have this sap.
Fruit: Another characteristic is the widely spread winged samaras.  These ripen in the fall and are widely dispersed by wind and water.  Both halves contain seeds. It is possible that some seeds are eaten and dispersed by birds and small mammals as well. (4)
The typical “coat hanger” shape of the Norway maple samara.Photo: Paul Wray, Iowa State University, via bugwood.org
Stem and Bark:  Twigs are stout, smooth, and hairless. Branch tips are often forked, the result of a terminal flower cluster and opposite arrangement. The terminal bud is purplish-green or reddish-purple, plump, and blunt with three to four pairs of fleshy scales. The young bark is grey and smooth. Older bark is dark grey with a regular, shallow intersecting, furrowed crisscrossing pattern. (1)
Roots: The root system is fibrous and shallow, with the potential of exposed surface roots as well as girdling roots causing the absence of plant species underneath the crown and soil erosion (5)
Habitat:   Acer platanoides is a shade-tolerant, prolific seed producer and, and easily grows in a variety of soil types (2). It is very tolerant of temperature extremes and so performs well in open, hot areas such as lawns, municipal parks, and along streets. They grow quickly and without much care.
Invasion It has many competitive advantages over native maple and other tree species. The mechanism of spreading is the effective dispersal of the samaras by wind and water.   Seeds are produced in abundance and germinate quickly.
They deprive native competitors of adequate light for normal functioning.   A field experimental study on the impact of Acer platanoides on forest ecosystem processes confirmed this with Acer platanoides. The formation of monospecific Acer platanoides stands, for example, greatly altered the vertical structures and light environment of the forests. Under the dense canopies of Acer platanoides trees, there were in this study almost no sub-canopy saplings or seedlings growing (6).  Acer platanoides is very shade tolerant and can out-compete native trees, such as Sugar Maple and Red Oak. Leaf out is early in spring and leaf drop is late in the fall giving it a long growing season compared to native species (1)
Leaf herbivory on Acer platanoides is three times lower in North America than in its native Europe (7).
A shallow root system prevents native tree seedlings from establishing (7.)
Leaves release toxins that affect soil fungi and microbes (5).
Other research suggests that, after the invasion of Acer platanoides trees, there would be less inorganic N available for plant uptake, and a greater likelihood of inorganic N being lost from the plant-soil systems because of the higher relative nitrification.(6)
In summary: dense shade, competitive networks of shallow roots, and toxic chemicals – all are detrimental to other native species.
The Greatest Threat is ultimately the domination of forest canopy and the subsequent loss of biodiversity and native species, both in the canopy and understories.
Because of its hardiness, ability to grow in a variety of soils, rapid growth, and copious seed production, the Norway maple long ago spread into the mature and second-growth forests of the northeast U.S. and Canada.  Here, it outcompetes native canopy and keystone species such as red oak (Quercus rubus), pin oak (Quercus palustris), tulip tree (Liriodendron tulipifera), red maple (Acer rubrum) and bitternut hickory (Carya cordiformis). Because it creates large amounts of shade it has inhibited the growth of mid-layer species such as hackberry (Celtis occidentalis), flowering dogwood (Cornus florida), redbud (Cercis canadensis), and black cherry (Prunus serotina).
Once it has established itself as the dominant canopy species, the Norway maple vigorously propagates its own seedlings and shades out other species’ seedlings. In a study it was determined that even the sugar maple (Acer saccharum) and American beech (Fagus grandifolia), which are the only two canopy species in the northeast U.S. that are not shaded out by the Norway maple as they reach for the canopy, did not fare well as seedlings under a Norway maple canopy. Norway maple seedlings comprised up to 98% of all seedlings found under Norway maple canopy.
The Norway maple is also a primary host to the Asian longhorn beetle, fast becoming a danger to eastern hardwood forests. This is a large beetle which lays its eggs within the bark of the tree. The larvae tunnel into the wood and create extensive galleries in the heartwood and cambium, resulting in damage to the tree structure and nutrient flow. Adults chew their way out to infest other nearby trees.
Removal and control -
Don’t plant Norway Maple!Methods of removing or halting the growth of the Norway maple vary and can be determined by the severity of the invasion, the type of environment, and the resources available.Best practice would include consultation with an arborist.
  • Pulling by hand or a weed wrench, (a long-handled device that grips a sapling at its base) uses leverage to pull Norway maples out of the ground with most of their roots intact.
  • Pruning: Eliminating seed-bearing limbs of large Norway maples may is an option in a garden or a forest.  More of the tree/forest structure is allowed to remain by leaving a standing dead tree available for habitation by natural residents of the forest such as woodpeckers, insects, and rodents.
  • Remove the large seed-source trees first where many Norway maples are already established and compose all or some of the canopy-cutting trunks followed by herbicide application to the stump.
  • Girdling large trees by cutting into the cambium layer around the trunk in a continuous ring is effective in killing them, typically within a couple of growing seasons.
  • Herbicide application to seedlings or young saplings: several years of control may be needed since the seeds are unaffected by the above methods and seedlings can recolonize an area
Small saplings can also be snipped using pruning loppers or machetes (could be followed by applying herbicide to the exposed stump.) Herbicides are effective in speeding up the killing process by applying to both cut stumps and girdled trees. Tryclopyrs and glyphosate agents are effective.Basal bark herbicide treatment is a control method in which an oil-soluble herbicide is mixed with an oil carrier instead of water and applied directly to the bark of woody plants less than 6 inches in diameter, but it has been used on bigger Norway maples. (8)
If large trees are removed, plant native plants so sun-loving invasive vines do not take over the space beneath the newly opened canopy.
Herbicides must be applied in accordance with the federal Pest Control Products Act, the Ontario Pesticides Act, Ontario Regulation 63/09 and in accordance with all label directions
Plant instead:  Native alternatives are available that provide as much shade and aesthetic presence as the Norway maple. Consider planting keystone species: At the top of the list for woody species are the oaks, followed by members of the Prunus family (our native Black Cherry, Chokecherry, Canada Plum being good examples), the Willow family (which also supports 14 species of native bees), and fourth, our native Birches. These trees support hundreds of species of caterpillars (oaks support approx. 450 species). Oaks have other advantages for the climate-resilient garden: they are the best watershed managers and the best carbon sequesters. For a smaller tree or shrub plan Alternate-leaf dogwood, or the Red osier dogwood. The dogwoods support approximately 115 species of caterpillars, and 4 species of specialist bees. (13)
If your heart is set on maple, the Sugar Maple (Acer saccharum) and Red Maple (Acer rubrum) are excellent native Maple trees. Both have vibrant fall colours and produce maple syrup. For a smaller, shrubby native Maple, try Striped Maple (Acer pensylvanicum) or Mountain Maple (Acer spicatum).
Elsabe Falkson, April 2023
Resources & References:Acer platanoides
  1. Norway Maple, Ontario Invasive Plant Council https://www.ontarioinvasiveplants.ca/invasive-plants/species/norway-maple/  Accessed April 9, 2023.
  2. Acer platanoides Dalhousie University-Faculty of Agriculture: Master Gardener Plant Identification course: Deciduous Trees Information Sheets p3     Accessed February 2, 2023.
  3. Tree Killers: Norway Maple https://treecanada.ca/resources/tree-killers/norway-maple
  4.  Introduced Species Summary Project, Norway Maple (Acer platanoides)    Shakespere G http://www.columbia.edu/itc/cerc/danoffburg/invasion_bio/inv_spp_summ/Acer_platanoides     Accessed April 9, 2023.
  5. FACT SHEET Norway Maple VASCULAR PLANT Acer platanoides ,   Nova Scotia invasive Species Council https://nsinvasives.ca/norway-maple/  Accessed April 9, 2023.
  6. A field experimental study on the impact of Acer platanoides, an urban tree invader, on forest ecosystem processes in North America | Fang et al. https://ecologicalprocesses.springeropen.com/articles/10.1186/s13717-020-0213-5 Accessed April 10, 2023.
  7. A cross-continental test of the Enemy Release Hypothesis: leaf herbivory on Acer platanoides (L.) is three times lower in North America than in its native Europe  Adams et al. https://link.springer.com/article/10.1007/s10530-008-9312-4
  8. Norway Maple -Woody Invasives of the Great Lakes Collaborative https://woodyinvasives.org/woody-invasive-species/norway-maple/ Accessed April 10, 2023.
  9. Basal Bark Herbicide Treatment for Invasive Plants in Pastures, Natural Areas, and Forests Enloe et al  https://www.aces.edu/blog/topics/forestry-wildlife/basal-bark-herbicide-treatment-for-invasive-plants-in-pastures-natural-areas-and-forests/  Accessed April 10, 2023.
  10. Norway Maple https://www.ontarioinvasiveplants.ca/invasive-plants/species/norway-maple/
  11. Norway Maple https://www.invasive.org/alien/pubs/midatlantic/acpl.
  12. Understory influence of the invasive Norway maple (Acer platanoides). P. Wyckoff, S. Webb. https://www.semanticscholar.org/paper/Understory-influence-of-the-invasive-Norway-maple-Wyckoff-Webb/
  13. https://rideau1000islandsmastergardeners.com/
  14. Photo: Leslie J Mehrhoff, UConn, via bugwood.org