Norway Maple – Acer platanoides

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!

Jonathan Billinger / A fine Norway Maple, Acer platanoides

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.

Distribution of Norway Maple in the Greater Toronto Area.
EDDMapS. 2022. Early Detection & Distribution Mapping System. The University of Georgia – Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/; last accessed July 13, 2022.

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].

Norway Maple Samaras.
Homer Edward Price, CC BY 2.0, via Wikimedia Commons

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…

  • Height
    • Norway maples can grow up to 20-30 m tall before ending in a round crown
  • Trunk
    • As a deciduous hardwood, Norway maples are covered in a grey-brown bark lined by criss-crossing grooves
  • Leaves
    • 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.
  • Flowers
    • From April to May, Norway maples can produce large umbel-clusters of small (~8-10mm) greenish-yellow flowers, each with five petals
  • Fruit
    • 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!

Interested in learning more? See also:

European buckthorn – Rhamnus carthartica

What is it?

The European buckthorn, common buckthorn, or just buckthorn, is an invasive flowering tree native to Europe that was introduced in the 1880s to Canada, and has since spread across the country, becoming widespread throughout eastern Canada by the 1900s. They remain common across Ontario, having been naturalized in many regions and are one of the most iconic invasive plant species.  

Unlike our previous plant guest, buckthorn is well named for the sharp thorns at the end of its branches hidden by its lush green foliage. Since the leaves of this prickly tree species tend to stay a little longer than its native colleagues in the forest, they can stick out to hikers planning to enjoy a colourful autumn at this time of year. Watch out though, though their thorns aren’t poisonous, they’re certainly not dull!

Why you should care

Common Buckthorn (Rhamnus cathartica), not to be confused with the similarly named but native Alder Buckthorn (Rhamnus alnifolia), is one of the most iconic and harmful invasive species, and it isn’t just because they can poke your eye out if you fall into one by accident!

Much like other naturalized invasive species, the common buckthorn and its relatives like the Glossy Buckthorn (Frangula alnus) were often deliberately planted and spread by humans to serve as ornamental plants, hedgerows and windbreakers. Additionally, its berries, which are mildly toxic to humans, are sometimes consumed by birds and small mammals that then disperse the seeds that pass through their intestinal tracts and can be spread over a wide area to quickly germinate.

Combined with its hardy nature, being able to repeatedly resprout from just its roots, and tolerance of a very wide range of soils and low light conditions, the common buckthorn can thrive in many environments. As a result, it has become extremely common in Ontario, and can be found along roadsides, fences, the edges of urban forests, and other locales. 

Beyond being the occasional nuisance, the spread of the invasive common buckthorn has had serious negative effects on the landscape. Not only does this prickly bush take up space that could have otherwise housed native trees and shrubs, but its dense thickets, which develop leaves before and last long after most native plants, can crowd and shade out new undergrowth, including important wildflowers and herbs, and even alters soil nutrients to discourage potential native competitors while favoring other invasive species like garlic mustard. This has also diminished local water quality as many native plant species are critical to reducing erosion and runoff. 

Additionally, though buckthorn berries are often consumed by small wildlife, it has little nutritional benefit, has laxative properties, and is toxic in high quantities, being eaten only because, like our prickly enemy’s leaves, they persist even into winter, being a seemingly easy source of food for unsuspecting mammals and small birds. 

Worse, this nasty invader has also had an impact on Canadian agricultural output! The common buckthorn can serve as hosts for at least three agricultural pests. One is the Oat Crown rust (Puccinia coronate spp. avenae), which appropriately harms oat crops (Avena sativa). Another is the spotted wing drosophila (Drosophila suzukii), a fruit fly pest that has damaged many soft fruit and berry crops that Ontario is famous for. The third, and more dangerous pest that the common buckthorn invites is the soybean aphid (Aphis glycines). This nasty critter not only hurts soybeans, but other vegetable crops as well, by consuming them and using them as host plants to survive the winter. Even worse, the soybean aphid itself has facilitated the invasion of the multi-coloured Asian Lady Beetle (Harmonia axyridis) which feed on the aphid and whose presence has led to the decline of native lady beetle species.  

As a result, there should be no surprise that the common buckthorn listed as a noxious weed in Ontario’s Weed Control Act, with growing awareness and efforts in recent decades to stamp it out and restore our native ecosystems!

What can we do?

The best way to help reduce the impacts of the common buckthorn is to help identify and report infestations. Much like other invasive species, early detection is critical in controlling its spread as younger shrubs are much more easily removed compared to older growths.

How to identify Buckthorn

The common buckthorn is one of the more distinctive invasive plants out here in Ontario but can sometimes be confused with the similar glossy buckthorn, which is also invasive, and should also be removed, so in this case, mix-ups between the two are no great loss, though this is unlikely to happen as while the common buckthorn prefers well-drained soils, the glossy buckthorn prefers to grow in wetlands. 

However, you should not confuse either for the native alder buckthorn, though it is more similar in appearance to the glossy buckthorn than the common buckthorn. 

To identify a true common buckthorn, look out for the following characteristics:

  • Height
    • Depending on the site, the common buckthorn can grow as a short shrub two to three metres tall, but under optimal conditions, can grow into a tall six-metre tree.  
  • Stem
    • Woody twigs with sharp thorns at the tip rather than on the sides like a rosebush
    • At its full size at 6 metres, the central trunk can be up to 25 centimetres in diameter
    • Younger shrubs will consist of multiple stems with flakey bark 
  • Leaves
    • Glossy dark green on top, and pale green on the bottom; retains its colour even in late autumn when it becomes more obvious   
    • Short stalk per leaf
    • Opposite, with fine toothed margins, and parallel veins curving into the tip 
    • Leaves generally elliptical ranging from two-point-five to six centimetres long, rounder at the base, and narrowing into a curved pointed tip. 
    • Develop early in the spring, and only loses its leaves in late autumn, sometimes persisting into early winter 
  • Flowers
    • Small, inconspicuous with two – six light yellow-green petals 
    • Flowers during May and June
  • Fruit
    • Distinctive clusters or singular stalked glossy purplish-black round (5 – 6 mm) berries
    • Juicy with stony seeds that can germinate quickly
    • Develop during in July and August, but can persist into the winter despite shriveling 

Can you remove Buckthorn?

If it is on your property, instances of common and glossy buckthorn can and should be removed and replaced with native species like these.

As we stated earlier, because buckthorn shrubs are easiest to remove when young, monitor your property for any emergent saplings. Like other invasive species, any instances of common buckthorn shrubs should be removed in their entirety, from root to stem. This can be accomplished by manually pulling up the buckthorn saplings and is easiest in mid-October when the common buckthorn is not only easiest to identify by its distinctive persistent leaf cover, but when the soils are loose and damp, and collateral effects on other species will be minimal. Stronger tools such as weed wrenches may be used to clamp onto the stems of buckthorn saplings and pull them out. 

When removing the pulled-up buckthorn plants, do not discard them in the compost, or deposit them in natural areas as they are extremely hardy, and may resprout roots, or overlooked seeds in the flowers may germinate, allowing the infestation to persist. 

However, if the stand is too well developed, pruning can be used to slowly cut back the extent of the infestation, until the plant either dies or can be more easily removed. Legal herbicides may also be used, either separately or in conjunction with pruning efforts. 

In any case, the hardiness and extent of common buckthorn infestations in Ontario are generally so serious that you should be prepared to spend several years battling recurring and persistent infestations. While it can be tempting to use limited burning practices, as many city officials and conservation authorities sometimes do, we DO NOT recommend attempting this as it can quickly become a serious fire hazard. 

A more detailed set of management practices for controlling and removing common buckthorn infestations can be found below:

On public properties, many conservation authorities organize expeditions to collect and remove emergent saplings. Check with your local conservation authority to see if you can participate!  

Additionally, you can also help prevent the spread of invasive plants like the common buckthorn by staying on trails and keeping pets leashed, reducing the chances that your clothing or pet’s fur will pick up and spread its berries and seeds. 

If you see any plants that match the description above, please contact the Invading Species Hotline at 1-800-563-7711 or the Natural Resources Information Support Centre at 1-800-667-1940 or 1-800-387-7011 to submit your observations! Also consider visiting the EDDMapS Ontario which has partnered with both organizations above to report sightings!

Additionally, importation of the common buckthorn and other invasive plants is illegal and can be reported to the MNRF TIPS line at 1-877-TIPS-MNR (847-7667) toll-free anytime. You can also call Crime Stoppers anonymously at 1-800-222-TIPS (8477).

Interested in learning more? See also:

Ontario’s Invading Species Awareness Program – Common Buckthorn (Rhamnus cathartica)
Ontario Weeds: European Buckthorn 
Nottawasaga Valley Conservation Authority – Common & Glossy Buckthorn
Ontario.ca – Common Buckthorn
Friends of the Mississippi River – Buckthorn: How can a shrub be so harmful?
Grow Me Instead
Invasive Common (European) Buckthorn (Rhamnus cathartica) – Best Management Practices in Ontario

Dog Strangling Vine – Cynanchum rossicum  (formerly Vincetoxicum rossicum) 

What is it? 

Dog strangling vine, also known as European swallowwort, pale swallowwort or just swallowwort is a perennial non-woody flowering plant, native to southeastern Europe and Asia, and introduced to North America in the 1800s for gardening but which has since become invasive. 

Contrary to its menacing name, this vine like plant cannot strangle man’s best friend but its rapid spread across southern Ontario and parts of the US, makes it a threat to local biodiversity. 

Why you should care 

Dog strangling vine can quickly grow and settle in new areas with tough, thick roots via seeds carried by the wind. Once established, the dog-strangling vine can easily regrow from even small fragments of its root system making it difficult to remove from infestations.  

Although it prefers open sunny areas, dog strangling vine is also common in areas disturbed by humans such as old fields, railways, and a broad variety of urban environments, and will aggressively wrap around native plants and trees to form dense mats of vegetation in the underbrush, literally ‘strangling’ their access to sunlight and suppressing other plants from growing.   

The result is a reduction in habitat and food available to birds and larger animals. Dog strangling vine also poses a threat to insects, being avoided by pollinators, and is often mistaken for native milkweeds by the already endangered monarch butterfly. Since the monarch butterfly needs to lay its eggs on native milkweed plants for its larvae to survive, by being tricked into leaving its eggs on a look-alike, the spread of dog strangling vine could further reduce the population of these beloved butterflies.   

What can we do? 

Early detection is key! – if we can remove instances of dog strangling vine before they become too well established, dog strangling vines can be much more easily managed! 

How to identify dog strangling vine 

dog-strangling vine | As lovely as it is invasive. I've neve… | Flickr
Photo by Steve Begin @flickr (CC)

Dog strangling vine can be confused for many other plants, including the black dog strangling vine (Vincetoxicum nigrum), which is within the same genus and visually similar but has darker coloured flowers.  

It may also be confused for native milkweeds like common milkweed (Asclepias syriaca) which can be distinguished by its warty seeds, butterfly milkweed which has orange flowers and alternate leaves, and swamp milkweed. However, note that all native milkweeds are erect plants that do not twine like true dog-strangling vine.  

Similarly, dog strangling vine may also be confused for dogbane (Apocynum cannabinum), but which has erect reddish stems, droopy leaves, and white flowers.  

 To identify a true specimen of dog strangling vine, look out for these characteristics:

Height

  • Dog strangling vine can grow up to heights of about 0.6 – 2m  

Stem

  • Herbaceous (non-woody) – will twine around surrounding objects, climbing around trees, and even among themselves to form dense vegetated mats  

Leaves

  • Green, opposite, smooth, and wavey margins, ovoid with a pointed tip  
  • Leaves are rounder at the base, largest (7 – 12cm) at the middle of the vine, and smaller (5 – 7cm) and narrower at the top  

Flowers

  • Pointed ovoid buds with twisted petals pre-blooming 
  • Occur at the vine tips  
  • 5 – 7mm individually  
  • Five-fold symmetry  
  • Usually red-brown, maroon, or pink though the undersides may be a lighter pink, orange, or yellow 
  • Blooms in late June and July 
  • Emerge in stalked clusters of 5 – 20 flowers with  

Fruit

  • long pointed chili- like seed pod – 4 – 7cm long and 0.5cm wide 
  • contain milky sap 
  • green when un-ripened   
  • pale brown when ripe – will split open to release multiple fluffy seeds with coma (tufts of feathery hair) – seeds are easily distributed by wind contributing to their invasive status  

Can you remove dog strangling vine? 

Yes! Just like any other plant, if it is found on your property in small quantities, dog strangling vine plants can be physically pruned away to reduce its reproductive potential and ability to spread, but only digging it up or killing it, roots and all, will permanently remove plants already present. When digging it up, avoid tilling the soil excessively to prevent fragmenting and spreading the dog strangling vine’s roots.    

Please see the Landowner’s Guide to Controlling Invasive Woodland Plants or the Toronto Botanical Garden’s guide for more detailed instructions on how to physically remove instances of dog strangling vine, as well as other guides here.  

Chemical control via pesticides can also be effective on a larger scale but different herbicides can have different levels of effectiveness and more often just allows for other non-native species to move in. While you can buy the right pesticides from licensed vendors (see here for further details), it may be better to check with your local conservancy for volunteers they may need to help out! 

After any attempts at removing dog strangling vine, remember to check your clothing, any pets you may have, vehicles, and other equipment to ensure that you are not inadvertently spreading the seeds of this invader. Dispose of the remains in the garbage rather than the compost or in natural areas as discarded flowers may contain seeds.  

If you have removed any dog strangling vine on your property, consider planting native flowers in their place, which are just as beautiful and much more helpful to the local ecosystem.  

If you see any plants that match the description above, please contact the Invading Species Hotline at 1-800-563-7711 or the Natural Resources Information Support Centre at 1-800-667-1940 or 1-800-387-7011 to report your sighting! Also consider visiting the EDDMapS Ontario which has partnered with both of the organizations above to submit observations! 

Additionally, importation of dog strangling vines and other invasive plants is illegal and can be reported to the MNRF TIPS line at 1-877-TIPS-MNR (847-7667) toll-free anytime. You can also call Crime Stoppers anonymously at 1-800-222-TIPS (8477). 

Interested in learning more? See also: