Invasive Plants

Native plants are fighting a battle for survival.  Some combatants are easily recognized...the bulldozer plowing down natural habitat...the industrious gardener poisoning "that patch of weeds" where their property meets the local meadow...  Others are more pernicious.  Alien invasive plants have been steadily infiltrating North America for hundreds of years, arriving with settlers, imported by garden centers, escaping from our gardens...

Some are relatively innocuous...merely occupying space more usefully filled by native species.  A very few even fill a small role in local ecosystems.  The nectar of Queen Anne's lace, for example, is enjoyed by Swallowtail butterflies and dandelions provide flowers for early spring pollinators to feed on.

Many more, however, not only play no part in ecosystems, they out-compete native species to create monocultural deserts.  Phytochemicals secreted by some species actually inhibit the growth of tree seedlings and other plants.

How can you help?

  • Learn about invasive species by reading or visiting websites devoted to the subject
  • Join a restoration effort in your area to weed out invasive plants
  • Remove them from your own properties
  • Teach others to seek out and destroy these invaders
  • Ask local nurseries to stop selling them
  • Contact your Provincial Member of Parliament or State Legislator and municipal Councillor to find out if there is legislation to halt the sale and propagation of these plants...and if not, demand that they do so!

Common Buckthorn

Common Buckthorn

(Rhamus cathartica)

Identification: It is a small tree or large shrub, which grows up to about 7 m (20 feet). It usually has multiple trunks, which become covered in shaggy bark as it matures. Twigs often end in short sharp spikes.

Buckthorn bark  Buckthorn twig

(All photos by John Oyston. Oak Hills Farm and Toronto)

 Inner bark is yellow and the heartwood is orange. It has dark green oval leaves.

Buckthorn leaves

The berries tend to be left on the bushes until late winter, and are only eaten when food has become scarce. It has a laxative effect on birds, which helps distribute the seed widely.


Buckthorn berries



Buckthorn was introduced into North America from Europe in the early 19th century as an ornamental plant, and it was widely planted as a hedge.


Common buckthorn invades forests, prairies and savannas in the Midwestern United States and can form dense thickets crowding out native shrubs and understory plants. It is also a problem in farmlands in Ontario.

It is shade tolerant and drought resistant. It rapidly produces stands which prevent other species growing, and it is a overwintering host for the soybean aphid. Buckthorn has become so widespread that it has taken over many hedgerows.


Elimination will only be achievable on small properties. In larger areas, it is a matter of controlling Buckthorn by eliminating the larger seed-bearing shrubs. Seeds remain viable in the soil for up to five years, so even after apparent elimination of mature Buckthorns, repeated efforts need to be made to remove new seedlings.

Physical methods of elimination include burning, flooding, grazing, and mowing.  Pulling up seedlings may result in vigorous re-sprouting. Girdling is only effective if followed immediately by applying herbicide. Foliar herbicides, applied from leaf out until mid-July. may be effective, but the agent proven to be most effective is triclopyr, which is banned in many jurisdictions.

Cutting down mature specimens with a chain saw must be followed by the application of herbicide such as glyphosate within 24 hours to prevent resprouting.

Buckthorn stumpsPile of cut Buckthorn

Availability and Alternatives:

Common Buckthorn does not seem to be sold in North America. Alternatives recommended by the Invasive Plant Atlas are:

Cornus racemosa (gray dogwood)
Euonymus atropurpureus (burningbush)
Frangula caroliniana (Carolina buckthorn)
Hamamelis virginiana (American witchhazel)
Photinia melanocarpa (black chokeberry)
Ptelea trifoliata (common hoptree)
Staphylea trifolia (American bladdernut)
Viburnum prunifolium (blackhaw)


Not Wanted - A photo gallery of invasive species

Not wanted - a photo gallery of invasive species

A brief and pictorial introduction to some invasive alien species

Read more: Not Wanted - A photo gallery of invasive species

Garlic Mustard

Garlic Mustard 

Alliaria petiolata or Alliaire officinale


First year plants form low rosettes that remain green throughout the winter.  Second year plants grow from 0.3 - 1.3 m (1-4 ft) tall. They produce attractive white flowers in early spring producing long (2.5 - 6.5 cm) narrow seed pods 4 - 8 weeks afterwards that can persist and drop seed throughout the summer.  Seeds can remain viable in the soil for at least five years.

Garlic mustard leaves in winter

Photo: John Oyston Toronto Ravine, Dec 2010

Garlic mustard can be insect or self-pollinated.  A single plant is capable of producing thousands of seeds which are easily transported by animal and human activity over large distances.  

It is found predominantly in rich moist forests and along stream banks but can also tolerate full sun.   It rapidly colonizes disturbed sites. Garlic mustard prefers calcareous soils and does not tolerate acid conditions. 


Garlic mustard was likely imported by settlers from Europe and Asia for use as a medicinal and cooking herb.  It was first recorded in 1868 in Long Island NY.


It is a biennial that can rapidly overtake native vegetation.  In addition to directly competing for light, moisture and nutrients, it secretes a phytochemical that suppresses other plants, including tree seedlings. Since garlic mustard most readily colonizes disturbed areas, try to maintain a healthy native plant ecosytem.

Control Measures:

Hoeing first year rosettes and hand pulling is most effective before populations become widespread.  Garlic mustard can re-sprout from root and stem fragments.  Pull when the soil is moist to aid in removing the entire root.

Second year growth can also be cut close to the ground before releasing seed.  New flowers can grow from leaf axils, so care must be taken to cut below the first axil.   

Remove all plant material, particularly seed pods, from the site and dispose of in tightly sealed garbage bags.  To ensure that seeds are not dispersed from landfills by seagulls and other means, try to solarize or burn them before disposing of the residue.  DO NOT COMPOST.   

Burning and chemical intervention can in some cases be effective, but extreme care should be taken around existing native vegetation.  Glyphosate-based biocides can be taken up by woody species.  Since garlic mustard prefers moist areas, care should also be taken to avoid contaminating water courses.  

Whichever method is used, it is essential to monitor the site annually to ensure that new seedlings are not emerging. 

Biological control involving several types of European weevils (Ceutorhynchus spp) including two stem-feeders, a seed-feeder, and a root crown feeder, are under study.   While these insects do help to control garlic mustard in its native Europe environs, it is essential to ensure that the weevils will feed exclusively on garlic mustard before releasing them in North America. 

Garlic mustard in flower

Garlic mustard in flower (Photo:  John Oyston)