Identifying Common Invasive Trees of London

Invasive species are those that come from somewhere outside an area (for example, imported accidentally or intentionally from another country or region) and quickly outcompete the plants and/or animals native to the area.

Invasive species are a growing problem in Canada, so much so that in many communities they have actually become the most common types of trees in the area.

Sadly, this is also true of London, where it is now two invasive species that hold the title for "most common tree" - European Buckthorn (by number of stems) and Norway Maple (by total size).

Some Fast Facts About Invasives

  • Invasive plants are responsible for the decline of at least 44 species at risk, and threaten numerous habitats and ecosystems in Canada.
  • Invasives cost the agricultural sector in Canada $2.2 billion each year.
  • On average, a new invasive species is introduced to Canada every two years. And once they're here, they're here to stay.

 How to Identify Them

Unfortunately, there are more invasive species in London than we can cover here, but the following images will help to identify four of our most common invasive tree species: Norway Maple, European Buckthorn, Glossy Buckthorn, Tree of Heaven and Sycamore Maple.

Some invasive species are referred to simply as a trademarked name and it can be difficult to know what species it really is. We recommend not to buy a tree or shrub unless the scientific name of the plant is stated, assuring you know what species it really is.

For a more complete list of invasive trees and shrubs occuring in our area, check out our Resources page which contains brochures for choosing suitable trees and shrubs for use in London yards and parks. The red panels in both these brochures list common invasive species that should never be planted in London (and should be removed whenever possible!)

(Click on any of the below images to expand)

Norway Maple (Acer platanoides)

Norway maples are notorious for their dense shade and shallow roots, which makes them very difficult to garden under. They are also especially afflicted by the unslightly black tar spot fungus which affects Norway maples far worse than our native maples.

This invasive tree has leaves which make it an extremely close lookalike for our native sugar maple, and people readily confuse the two. The important differences between the two lie in their maple keys ("helicopters"), their bark, and in the colour of the juice that comes out of their leaf stalks when plucked.

Common varieties of this invasive species include "Crimson King" (has burgandy leaves and often wrongly called red maple), "Schwedleri" and "Drummondii" (which has variegated leaves). Norway maples may also be known as Harlequin Maple and Emerald Queen.

Key Characteristics:

1) Sugar Maple-shaped leaves 2) Seed keys paired and set at a wide angle (almost 180 degrees) to one another (looking "open") vs. sugar maple keys which point down (looking "closed") 3) White, milky sap exuded from the bottom of leave stems when leaves are plucked. (Note: this is a very easy way to tell apart Norway from Sugar Maple, whose leaves otherwise often look very similar! The juice from a Sugar Maple's leaf stalk will run clear, like water). 4) Bark of parent trees is set into tight "diamond" patterns along the trunk (vs. long, slightly peeling strips on sugar maples)         

Some cultivars of Norway Maple have purple or red leaves throughout the year - the leaves of our native sugar and red maples are always green except in autumn.


Image Sources:

Paul Wray, Iowa State University,

Rob Routledge, Sault College,

John Ruter, University of Georgia,







European Buckthorn (Rhamnus cathartica)

 The invasive European Buckthorn grows along the edge of almost all our natural areas and woodlands throughout London. Because of its invasive properties, this unwanted plant is now the most common tree in London.

Key Characteristics:

1) Very small thorn extending directly from the end of some twigs

2) Many clumps of black berries in late summer through winter.


3) Small, mostly round leaves with small teeth around the margins and veins that curve to run up towards the tip (rather than directly out to the edge).


4) The green leaves of European buckthorn often persist on its branches long after everything else has turned colour and fallen in autumn.

Image Sources:

Richard Webb, Self-employed horticulurist,

Paul Wray, Iowa State University,

Rob Routledge, Sault College,

 John M. Randall, The Nature Conservancy,



Glossy Buckthorn (Frangula alnus)

Key Characteristics:

1) Tear drop shaped leaves with smooth margins/

2) Smooth green or red berries which turn black late in the year.

3) Lenticels (little dots) on the bark of young twigs and branches

4) Brown fuzzy buds in winter, lacking bud scales


Image Source:

Gil Wojciech, Polish Forest Research Institute,

Rob Routledge, Sault College,

Leslie J. Mehrhoff, University of Connecticut,



Tree of Heaven (Ailanthus altissima)

This tree gets its name from its long, stout branches and appear to be reaching up into the sky. Extremely tolerant of urban conditions (and in particular pavement), it will grow in just about any nook or cranny where it can set seed. It is also known as "stinking sumac" because of the unpleasant smell that can come from its leaves and twigs when bruised.

 Key Characteristics:

1) Large, compound leaves, thats leaflets have distinct, unusual projections right at their base

2) Stout whitish twigs with extremely large, triangular leaf scars (created where the leaf is plucked or falls off from the stem)

3) Small white diamond pattern in the bark (see image above). 4) Unusual seeds, which are papery (resemble maple or ash) but have the seed in the middle of the "paper"

Image Sources:

Annemarie Smith, ODNR Division of Forestry,

Paul Wray, Iowa State University,

James H. Miller, USDA Forest Service,

Beschreibung: Götterbaum (Ailanthus altissima), Wikipedia






Sycamore Maple (Acer pseudoplatanus) 

The name sycamore maple is derived from the similarity of its leaves to sycamores, though it is not part of the same genus -- hence the ‘pseudo’ in its Latin name.

Originally from Europe and western Asia, sycamore maples thrive in areas with disturbed soil and their tolerance of pollution and road salt made them a popular choice for planting in cities. However, when planted in Canada, it can become invasive, out competing native trees and reduce our forest diversity. According to the USDA Forest Service, sycamore maples can produce "dense strands of trees with the potential for crowding out native plant species." 

Many cultivars of sycamore maple exist, including ‘Esk sunset’, ‘Puget pink’, ‘Leopoldii’ and more. Like Norway Maple, some cultivars have leaves that are quite red, pink or purple.

Key Characteristics:

1)  Leaves have a leathery texture with five lobes and blunt edges. They range from 9-15 cm in length and have thick veins on the undersides. The undersides are lighter in colour. Leaves grow opposite each other. Each lobe of the leaf tends to look a bit more oval than you would see in its closest lookalike, the sugar maple. 2) Bark is gray and mature trees develop scales that flake away, revealing a reddish brown or orange inner bark. 3)  Greenish flowers in clusters up to 13 cm long emerge in the spring after the leaves. The developing maple key wings are visible among the flowers.

4)  Terminal bud is 6-8 mm long and its twigs are hairless, with green or brown pores.

5) Pairs of winged seeds grow in clusters and each wing is 20-40 mm long. The spread between the wings is about a 60-degree angle.

 Image Sources:

Figure 1: "Acer-pseudoplatanus" by user:JoJan - Own work. Licensed under CC BY-SA 3.0 via Commons -

Figure 2: "Acer pseudoplatanus textura del tronco". Licensed under CC BY-SA 3.0 via Commons –

Figure 3: Jenny Bull

Figure 4: "Acer pseudoplatanus buds 01" by © El Grafo / CC-BY-SA-3.0 (via Wikimedia Commons). Licensed under CC BY-SA 3.0 via Commons -

Figure 5: "Acer pseudoplatanus MHNT.BOT.2004.0.461" by Didier Descouens - Own work. Licensed under CC BY-SA 4.0 via Commons -

Sycamore Maple - Article written by Lauren McVittie

A Cool Tool for Removing Invasives

On of the things that makes "invasives" so invasive is that they are hard to kill. Often, if you cut them, they simply re-sprout at the remaining trunk. One solution is to pull the entire tree or shrub out of the ground.

ReForest London has five Weed Wrenches. A Weed Wrench is an easy-to-use tool that grabs the base of the tree or shrub and allows you to pull it out of the ground, roots and all. It uses leverage so that you can remove trees up to about 1 inch in diameter without having to cut them.

Londoners are welcome to borrow a weed wrench to clean up their own properties. Contact us to arrange to borrow a weed wrench so you can make your yard invasive-free!

Additional Resources

To learn more about invasive species and the impacts they have on our forests and other natural ecosystems, visit: