Invasive Plant Species

See also Controlling Invasive Plants

The invasive species listed below pose some of the greatest challenges to the preservation and restoration of High Park's natural environment. To learn more about how City staff and volunteers are dealing with these species, see Controlling Invasive Plants and High Park Stewards.


Shrubs & Trees

Common/European Buckthorn


Asian Bittersweet

Tartarian Honeysuckle

Tree of Heaven

Norway Maple


Dog-strangling Vine (Cynanchum rossicum and C. louiseae)
DSV Seed Pods

Dog-strangling vine, a common woodland invader, forms dense colonies, attaching to and aggressively climbing ground vegetation including tree seedlings and saplings. Shortly thereafter, vegetative growth of the vine overwhelms existing native vegetation, essentially ‘strangling’ understory species. The potential for changes in forest composition and function resulting from the spread and growth characteristics of the dog-strangling vine has been a documented threat to the long-term viability of forests in Ontario.

DSV in its prime, late September
Sharon Lovett

This species is especially difficult for land managers to eradicate due to its extensive root system that re-sprouts following cutting of above ground shoots. Manual removal and/or herbicide application are known management strategies for Dog-strangling Vine. During the first year of growth, manual removal of above and below ground growth can effectively control newly established populations. Careful attention must be applied while weeding, as it can re-sprout from any root fragments left in the soil. Given its robust growth, manual removal is generally not effective following its first year of growth and will require herbicide for successful control.


- “Forest Health Alert – Dog-strangling Vine” Factsheet MNR & Kawartha Conservation, M. Irvine and D. Pridham respectively, published by MNR, 2008


Latest news: You can learn all about dog-strangling vine and the biological control agent “Hypena” on the researchers website .

See also:

- Ontario Nature Video: Identification, Impact & Removal of DSV

- Have you seen this plant? (Bring Back the Don Task Force factsheet)

- Fletcher Garden factsheet (pdf)

- Factsheet about disposal (pdf)

Garlic Mustard (Allaria petiolata)
Garlic Mustard
Linda Read

Early European settlers had the best of intentions when importing this exotic, now invasive herbaceous plant, traditionally used for its medicinal properties and as a garden vegetable, favored for its garlic taste. In addition, it is relatively easily grown and provides good nutrition, high in vitamins A and C. Garlic mustard grows in a variety of disturbed habitats, preferring woodlands and floodplains but generally speaking will grow in moist, rich soils with some shading.

Its best identification traits are the garlic-scented leaves when crushed and white flowers with alternate heart-shaped leaves on the flowering stalk. Like many herbaceous invasive plants, its rapid growth and prodigious seed crops are able to out-compete native flora by effectively shading out competitors. Garlic mustard is in fact a fungicide, destroying vital fungal (i.e. mycorrhizal) relationships which many native tree and herbaceous species depend on for water and nutrients. As a biennial it grows as a basal rosette the first year and throughout the winter, taking advantage of insulating snow and winter sunlight before flowering in early spring, setting seed soon after and dying off by July.

Since humans created the disturbances permitting garlic mustard to thrive, a suitable control or eradication initiative might include harvesting wild populations for use in the kitchen. A delicious edible pesto can be made from its leaves with multiple recipes and how-to guides available on the internet. As an evergreen, it can also be eaten fresh throughout winter.

It is important to keep in mind not to harvest garlic mustard for human consumption from dumps, ditches or roadways as resident plants can uptake and accumulate harmful pollutants discharged into the natural environment. (Collecting in the park is not permitted.)


- from The Monday Garden – publication by Sue Sweeney, “Garlic Mustard: The Invader’s Edge” issue #54, April 6, 2003 article

White Sweet Clover (Melilotus alba)
White Sweet Clover
Sharon Lovett

Chances are you’ve seen or wandered through a patch of white sweet clover given its abundance in Ontario. It is an alternative-leaved biennial wildflower growing up to 5’ tall introduced into North America from Europe as high quality forage for cattle, and still used as animal forage and as a cover crop on farms. As a nitrogen fixer, it increases soil fertility by converting atmospheric nitrogen into usable nitrogen in the soil. Its flowers are favored by bees and other pollinators, historically planted as a nectar host plant by beekeepers. In fact, “Melolotus” in Greek means honey.

White colored flowers with small stalks are found in dense clusters in the top few inches of main stems and bloom June through August in the plant's second year of growth. As a successful invader white sweet clover has a strong and deep taproot, permitting survival in dry soils and drought conditions. Also, seeds remain viable for up to 30 years in the seedbank and will readily establish following a disturbance, further complicating eradication efforts. White sweet clover dominates successional areas including meadows, prairies, savannas, woodlands, roadsides and waste places, having the ability to colonize and thrive in a variety of disturbed habitats, even a barren, nutrient-poor quarry floor!

Left unmanaged, dense thickets form, out-competing and shading-out sun-loving native vegetation, ultimately reducing local biodiversity. At High Park, this invasive is especially problematic, responding positively to fire (i.e. prescribed burning) by scarifying or preparing seeds, thereby encouraging germination.


- Invasive Exotic Species Ranking for Southern Ontario © Urban Forest Associates Inc., January 2002,pdf

- Ontario Wildflower article

- Minnesota State factsheet

- Credit Valley Conservation factsheet

Himalayan Balsam (Impatiens glandulifera)
Himalayan Balsam
Mehdi Zarrei

Like most invasive herbaceous species, Himalayan balsam forms dense colonies in disturbed environments, out-competing and choking out native ground vegetation, thereby threatening local biodiversity. It is similar in appearance to our native jewelweed/touch-me-not (Impatiens capensis), both with large showy flowers; Himalayan balsam has pink/purple flowers and jewelweed has orange. Generally speaking, it prefers woodlands and floodplains but will thrive in any disturbed location with moist, rich soils and some shading. Owing to its invasive profile are explosive seedpods, hurling up to 2500 seeds per plant airborne once fully matured. In fact, once mature, merely touching the seedpods demonstrates this phenomenon. Also, it can reach heights in excess of two meters and easily overwhelms and displaces native flora with its rapid growth. Seeds often end up buoyant in local watercourses where survival is relatively long, facilitating colonization in new environments downstream.

Himalayan balsam is particularly damaging to riparian areas where dense groves form monocultures; following die-back in the fall bare soil is exposed resulting in increased soil erosion and sedimentation of watercourses. Control is achieved through manual weeding, cutting stalks before seed dispersal or by herbicide application.


- Invasive Plants of BC Factsheet Balsam”

- “Invasive Plants of Ontario”Invasive Plants Fact Sheet, Stratford Naturally, City of Stratford

Purple Loosestrife (Lythrum salicaria)
Purple Loosestrife
Mehdi Zarrei

This woody plant species has been nominated as among 100 of the "World's Worst" invaders (GISD, 2010). Purple loosestrife has been a beautiful and aggressive invader in North America since the 1800’s. Originally from Europe and used as an ornamental flowering plant, the seeds escaped and now have spread across much of Canada and the United States. Purple loosestrife flowers from late June to early September, relying on pollination by insects or birds to flourish, and can grow up to 6.5 feet/2m high. It has distinctive large flower spikes made up of many smaller flowers – 5 to 6 pink-purple petals surrounding a smaller yellow center. Plants can go to seed as early as late-July. The leaves are downy with smooth edges, are usually oppositely arranged in pairs that whorl around the stem.

Several control methods have been used on purple loosestrife with varying levels of success. So far, the most successful endeavour has been a biological control using beetles. In High Park, there is purple loosestrife in the wetlands that was brought under control by the introduction of beetles in 2003-2004; Galerucella pusilla and Galerucella calmariensis, eat the leaves and new shoot growth which seriously affects growth and seed production of the plant (OFAH, 2011). This was more evident in the summer of 2010, possibly because it was such a good growing season. The danger to High Park is that mature plants can reproduce vegetatively via underground perennial rootstocks that can spread quickly throughout the year. These rootstocks are extensive and can create a dense structure that chokes out other vegetation. As tiny as grains of sand, seeds are easily spread by water, wind, wildlife and humans. Germination can occur the following season and in many environmental conditions. Seeds are hardy and can lay dormant in the seed bank for several years before sprouting. The best time to control purple loosestrife is in June, July and early August when it is in flower and easy to recognize before it goes to seed.


- Global Invasive Species database

- Ontario Federation of Anglers and Hunters Factsheet

Greater Celandine (Chelidonium majus)
Greater Celandine
Linda Read

Celandine is a biennial (sometime a short-lived perennial) wildflower introduced to North America from Europe in the 1600s for medicinal purposes and is still available in some nurseries as an ornamental. It is commonly found in woodland edges, roadsides, farm edges and waste areas and is becoming increasingly established in Ontario.

Celandine can tolerate most soil types, thriving in its preferred rich, moist soil. Stems grow up to 80cm with the best identification trait being the orange-yellow colored juice or sap contained in leaves, stems or roots that will stain clothing and cause skin irritations in some people. Flowers are yellow, 4-petalled with multiple stamens and appear in May to June. Ants are known vectors of dispersal, attracted by an elaiosome on the tiny black seeds. Elaiosomes are fleshy growths rich in proteins and fats attached to the seeds of many plant species and are favored by ants as larvae feed.

Greater Celandine
Linda Read

Impacts on the natural environment where celandine has established are a result of heavy shading conditions created when dense stands form; native vegetation including tree seedlings are suppressed due to a lack of sunlight penetrating the ground layer.


- Ontario Weeds Factsheet

- invasive.org

- DCNR Factsheet and DCNR Factsheet (.doc)

- Trees Canada

Japanese Knotweed (Polygonum cusipdatum)
Japanese Knotweed
Linda Read

Japanese knotweed is among the most damaging invasive plant species in North America. Originally a garden escapee for use as a screen plant and still propagated at many local nurseries, it can be found in a wide array of disturbed habitats including woodlands, roadsides, lakeshores, fields, meadows, brownfields and other waste places. It is also known to wreak havoc along natural watercourses where seasonal flooding and subsequent erosion disturbs the root systems, dislodging rhizome fragments, transported and deposited downstream to colonize new habitats. In fact only 2cm of rhizome is needed for a new plant to establish.

Flowers are greenish-white, blooming July to September and are found in panicles in the upper leaf axils. Stems can be up to 10’ tall, mottled reddish-purple and mostly hollow. Young shoots are a fantastic edible cooked and served similar to asparagus. Widely spreading and deeply penetrating rhizomes are largely responsible for its success and proliferation, not its infrequent seed crop. These highly aggressive rhizomes can spread up to 60’ from the host, sending up root suckers along its course. Left unmanaged, Japanese knotweed will form dense groves choking out and eliminating native flora in natural areas, threatening biodiversity and impairing local infrastructure in urban areas.


- Article

- Factsheet

- OMAFRA factsheet

Common or Spreading Hedge Parsley (Torillis arvenis)
Spreading Hedge Parsley
Linda Read

Hedge Parsley has come to High Park more recently – in summer 2010 the first efforts were made to pull it. Spreading Hedge Parsley is a parsley-like biennial with a taproot and erect, ridged stems. They grow in a spreading form up to 3 feet/1m in height. Leaves are triangular shape in outline and a fern-like in appearance; alternate, pinnately divided, 2 to 5 inches long and may be slightly downy on the upper surface. The tiny, white flowers are clustered in small, open, flat-topped umbels. Its flowers attract many insects, including bees, flies, wasps, and beetles. The caterpillars of the butterfly Papilio polyxenes asterias (black swallowtail) also feed on the foliage (Illinois Wildflowers, 2011).

Similar to the Japanese hedge parsley (another invasive plant that has not yet shown up in High Park), spreading hedge parsley lacks bracts at the base of the umbel structure. The small fruits are covered in velcro-like hairs, which attach to clothing and fur, or catch the wind, readily dispersing seeds. Hedge parsley matures quickly and re-seeds itself, usually forming dense colonies that can out-compete native vegetation. Pulling manually or mowing prior to flowering, easily controls spreading hedge parsley. Treating foliage with an herbicide is effective if done early in the spring or on re-sprouts after cutting.


- Weed Control Factsheet

- Illinois Factsheet


Common/European Buckthorn (Rhamnus cathartica) & Glossy Buckthorn (R. frangula)

European and glossy buckthorn are highly aggressive invasive shrubs or small trees growing in dense groves, suppressing native tree and shrub seedlings due to heavy shading of its foliage (Tree Canada, 2007). Berries persist on branches throughout winter and are transported by some native birds and mice who favor the berries as year-round forage, facilitating its dispersal and colonization following defecation. It was imported from Asia and Europe as a windbreak species on farms and for erosion control and is now established in many disturbed habitats, especially woodlots. Able to thrive and invade a wide variety of environments, these highly adaptable buckthorns also have prolific seed crops which rapidly germinate.

Glossy Buckthorn resprouting after being cut down

At High Park, invasive buckthorns are of particular concern in threatening the long term integrity of open oak woodlots and savannas where lightly-shaded conditions and canopy openings permit establishment in the understory.


- Tree Canada (2007) “Tree Killers – Common Buckthorn or European Buckthorn” Factsheet

- Unwanted Settlers Common Buckthorn (OIPC, unknown date)

Euonymus: European Spindletree (Euonymus europaeus) & Winged Euonymus (E. alatus)
European Spindletree / Burning Bush
Linda Read

Both euonymus species are known to invade forest understories, with European spindletree preferring the shrub layer while winged euonymus is more commonly found in forest edges.

European spindletree (Euonymus europaeus), also known as burning bush) very closely resembles and is easily confused with the provincially rare eastern wahoo or burning bush (Euonymus atropurpurea). Identification of these closely related shrubs can only be achieved when flowering, as vegetative growth characteristics are too similar. As a result, drupes should be left unless a positive identification has been verified during flowering. European spindletree has greenish white flowers in late May, compared with eastern wahoo with dark purple or maroon flowers appearing in June. Also, the spindletree holds its leaves much later into the fall in comparison to the native wahoo.

Winged euonymus (Euonymus alatus), also known as winged burning bush is distinctly different from Ontario’s rare and native burning bush or eastern wahoo (Euonymus atropurpurea). It is a dense, outward-arching shrub imported from Japan and produces abundant small red seed capsules with orange seeds on short stems which persist into the winter. Flowers are small, greenish-yellow with 4 petals. Stems are distinctively 4-winged, appearing square-stemmed.

Winged Euonymus
Karen Yukich

Winged euonymus was first imported to North America in the 1860s as an ornamental, praised for its bright crimson-red foliage in the fall, and its propagation continues at nurseries for use in roadside plantings and in gardens. It is capable of invading a variety of disturbed habitats and is highly adaptable with a wide range of soil type and pH tolerance. As well, there are no threatening pest associations in North America and is tolerant of full shade conditions. Once established it out-competes native vegetation and forms dense colonies, thereby shading out and destroying the understory flora. Birds are the primary vector for seed dispersal, capable of carrying seeds long distances and generally germinate rapidly following defecation.


- invasive.org

- Growing Trees from Seed - A Practical Guide to Growing Native Trees, Vines and Shrubs by Henry Kock, Paul Aird, John Ambrose, Gerry Waldron, page 123

- “Invasive Exotic Species Ranking for Southern Ontario”, Urban Forest Associates Inc., January 2002, article

- DCNR Factsheet

Asian Bittersweet (Celastrus orbiculatus)
Asian Bittersweet in September
Sharon Lovett

This woody-perennial, vigorously growing vine you will likely see in High Park, climbing over and smothering vegetation. With alternate, tear-shaped leaves that are distinctly toothed and come to a point, Asian bittersweet has small greenish-yellow flowers, which bloom in the early summer months and begin to fruit in the fall. The small greenish-yellow fruits grow in clusters of 3-7 along the stem at the leaf axils, and split open to reveal a bright red inner fruit.

A native of Eastern Asia, Asian bittersweet is beginning to out-compete our native American bittersweet after escaping cultivation. Much like kudzu from the Southern US, it often kills vegetation from excessive shading or breakage – when it climbs high up on trees (up to 60 feet/17.3m), the increased weight can lead to uprooting and blow-over during high winds and heavy snowfalls.

A Thicket of Asian Bittersweet

Manual, mechanical and chemical control methods are all effective in removing and killing Asian bittersweet. In cases where vines are climbing up trees or buildings, a combination of cutting followed by application of concentrated systemic herbicide applications to rooted or living cut surfaces is likely to be the most effective approach. For larger infestations on the ground, a foliar herbicide may be the best choice, as manual or mechanical means could result in high soil disturbance.


- New Invasive Plants of the Midwest Factsheet

- Enature Factsheet

- Celastrus orbiculatus (Oriental bittersweet) - Invasive Plant Atlas of New England (IPANE)

Tartarian Honeysuckle (Lonicera tatatica)
Tartarian Honeysuckle
Mehdi Zarrei

Tartarian honeysuckle is a deciduous, opposite-leaved shrub (up to 3m) among the first to leaf out in spring with white, pink or red flowers developing in leaf axils. This open woodland, old field and shoreline invader was first imported from East Asia as an ornamental and historically planted by government agencies as wildlife shrubs. Birds and small mammals are known consumers of its bountiful seed crop and are agents for its dispersal in disturbed habitats. Also, the fused together pairs of red-orange berries persist throughout winter, effectively aiding in its proliferation once consumed by local fauna.

Tartarian honeysuckle is highly effective in out-competing native ground and understory vegetation, aggressively invading disturbed habitats. It threatens the long-term biodiversity of natural environments; invasion often forms a dense understory thicket which not only prevents growth of the herbaceous ground layer but tree seedlings as well. In fact, it is now among the most undesirable invasive species in central and eastern Ontario when considering its ability to displace native flora.

- Ontario Nature, “Natural Invaders,” written by Kim Gavine (unknown date) article

- invasive.org

Tree of Heaven (Ailanthus altissima)

Tree of heaven, closely resembling native sumac (Rhus spp.) is a fast growing, aggressive and adaptable tree originally imported to North America from China. The leaves carry an unpleasant odor and are a good identification tool if bruised or crushed. Owing to its success in disturbed environments are spreading rhizomes (i.e. roots and shoots in the soil layer), abundant production of winged seeds and secretion of allelopathic chemicals, thereby preventing the growth, or sterilizing the soil for many native species. Tree of heaven is also very tolerant of urban pollutants and as a result are often found in waste places such as ditches and roadsides where native trees aren’t able to establish. With a life span of 25 to 50 years, it can reach heights of 15 to 30 meters. Due to the rhizomatic spreading, root suckers often form groves or thickets and persist for up to centuries while providing a seed source for nearby disturbed areas.


- Ontario Trees & Shrubs article 1

- Ontario Trees & Shrubs article 2

- factsheet

Norway Maple (Acer plantanoides)
Norway Maple creates dense shade early in spring
Karen Yukich

Considered by many botanists an invasive, the exotic Norway maple is widely planted as a lawn and street tree, many of which are the full growing season red-leaved variety distinctly different from our native maples. It closely resembles sugar maple (Acer saccharum) and is easily identified by the milky white sap contained in the leaf petioles when removed from the branch. Owing to its success as an invader are results of dense canopies providing deep shading, a thick blanket of leaves which smother ground flora in the fall, highly viable winged seeds (called samaras) which readily blow into nearby natural areas and the root secretion of allelopathic chemicals preventing other native plants from establishing.

Having high shade tolerance, Norway maple produces abundant seedlings which are able to out-compete native flora in heavy shading conditions ultimately forming monocultures over time. Tolerance to harsh conditions including heavy shade and dry soils are known and favored in urban settings. Norway maple has no specific habitat preference and will thrive in any disturbed habitat. Control is achieved simply by cutting to or digging out of the ground, girdling and/or cutting any resprouts; herbicides are generally not required for successful eradication.


- Ontario Trees

- ISA Ontario

- Article

Authors: Andrew Dean & Emily Stairs

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Content last modified on August 02, 2018, at 05:27 PM EST